corn diagnostic guide

7/28/2019 Corn Diagnostic Guide

1/60


2/60


3/60

Table of Contents

Introduction ........................... ............. .. .. ........................... ........................... .......1

Examining a eld ......................... ........................ .... .... ... ...... ............................ .... 2

Tips for shipping to diagnostic laboratories ... ....................... . . . . . . . . . . . . .......................... 2-3

Stage I: Scouting from emergence to knee-high ... ... ... ... ... ... ... ... ... ... ... .. .. ... ... ... ... ... ... ... ... 4

Stage II: Scouting from knee-high to tasseling ... ... ... ... ... ... ... .. .. .. .. .. ... ... ... ... ... ... ... ... ... ... . 11

Stage III: Scouting from tasseling to maturity ...... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... . 16

Disease symptoms ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .... .... ..... ..... ..... ..... ..... ..... . 24

Leaf diseases ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .. ... ..... ..... ..... ..... ..... . 24-26

Smutting diseases ............................................................................................... 26

Virus and virus-like diseases ...................................................... . . . . ......................... 27

Fungal systemic diseases ..... ..... ..... ..... ..... ..... ..... ..... .. .. . ..... ..... ..... ..... ..... ..... ..... .. 27-28

Stalk and root rot diseases ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... . 28-29

Ear and kernel rots ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .... . . ..... ..... ..... ..... ..... ..... .... 29-30

Insect injury symptoms and management recommendations .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . .. .. .. .. . 31-39

Herbicide injury symptoms ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .. .. ..... ..... ..... ..... .. 40-42

Herbicides that may be used or trigger symptomology in corn .... .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. 43-47

Herbicides listed by active ingredient and mode of action .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 48-51

Nutrient deciency symptoms ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .. 52-53

Index .................... ............... ............................ ............................ .................... 54


4/60

IntroductionThis manual is prepared and distributed by Monsanto Company to help farmers, seed dealers, and companypersonnel determine the likely causes of abnormal corn plant appearance. While the primary target is the North

American Region, the principles and many of the situations described in this publication extend to the regions of theworld where corn is grown.

Symptoms may be due to a single cause or the result of two or more interacting factors. Also, because of theirdifferent genetic backgrounds, different hybrids may not have identical symptoms in response to the samecause. Routine eld examinations are crucial in spotting problems or potential problems. Some growers employprofessional crop scouts rather than perform this function themselves. Weekly examinations are generallysufcient.

Once a problem has been identied, its extent and severity must be determined to decide whether correctiveaction is necessary. Many pest management recommendations include threshold levels when control measureswill return a prot. Consult Cooperative Extension Service and chemical company recommendations for control

measures.

1 Corn Diagnostic Guide


5/60

1Diagnosing FieldProblems in Corn

TIPS FOR SHIPPING TODIAGNOSTIC LABORATORIES

Trained, experienced agronomists, crop protection,

research, and sales personnel stand ready to assist you

in diagnosing eld problems. Local seed dealers are your

rst contact when questions arise. Positive diagnosis

often requires identication or conrmation of causes

by a diagnostic laboratory. Most states have some

arrangement, usually through their Cooperative Extension

Service ofce, for accessing expert diagnosis.

Some of these are more formally conducted than others,and cost of the service varies. Also, there are a few private

laboratories that are equipped to provide such services.

Your local Cooperative Extension Service ofce can

suggest companies and provide contact information.

When preparing plant or soil samples for a diagnosis,

follow these instructions offered by the laboratories.

2

EXAMINING A FIELD

Carry the appropriate tools to help effectively scout or monitor elds. Some basic tools to have on hand include:

Tape measure

Knie

Trowel (6 or 8 inch)

Magniying glassClipboard and record keeping materials

Small plastic bags

Water source

Paper towels

CalculatorField marking ags

A spade and a set of nesting pails may be useful if considerable digging is expected. Digital cameras can behelpful in getting a record of insects or disease symptoms.

Be very careful when making area-to-area or eld-to-eld comparisons. Many factors can inuence appearance,including: soil type, slope, and drainage; previous crops; fertility practices; seedbed preparation; date, depth,and rate of planting; pest control; seed lot; and hybrid.

Look for positives, not just problems. Observe and note hybrid differences, as well as the effects of fertilizer andcultural practices.

The following diagnostic key separates plant growth into three primary stages of crop development:

Stage I Emergence to knee-high

Stage II Knee-high to tasseling

Stage III Tasseling to maturity

In the eld, onset of symptoms might occur earlier or later than indicated, and may be observed during more

than one growth stage.


6/60

Provide representative samples of each problem1.observed, as well as a healthy sample from unaffected

plants or plant parts.

If sending leaf tissue, place sections of leaves showing2.disease symptoms between pieces of dry papertoweling or notebook paper.

If sending the whole plant, remove excess soil from3.the roots and wrap the roots in moist paper toweling.Put roots and towel in a plastic bag. Do not place theentire plant in a plastic bag. Wrap stem and leaveswith paper, foil, or cardboard.

Do not add water or crush specimens unnecessarily.4.

Use a sturdy envelope or box for shipping.5.

Avoid shipping at times that are likely to result in the6.parcel lying in a post ofce or freight depot over aweekend or holiday. Overnight delivery, early in theweek, is strongly recommended.

Information will need to be provided with the plant7.specimen. Often, laboratories have specic forms thatare required when submitting a sample. Information

requested on such forms often includes the following:

Variety (hybrid number) of crop.

Location where sample was taken (county,township, and town)..

Date of planting, date problem was rstobserved, and date sample was collected.Indicate whether the problem is better or worsethan when rst observed. Visit a problem

area twice, on dates about a week apart, todetermine whether the problem is intensifying,

spreading, or disappearing.

Crop symptoms as observed in the eld, suchas plants were wilted or leaves appearedspotted. Describe the size of area affected:spots, strips, or the entire eld.

Percentage of plants affected.

Soil type (clay, sand, muck, etc.).

Topography around affected plants, such ashigh ground, low ground, or gently sloping.

Fertility level (include a soil test report, ifavailable) and the amount, kind, and timing offertilizer application.

Pesticides applied (fungicides, herbicides,insecticides, application rate, and date).

Soil moisture situation at and since planting. Ifthe eld was irrigated, indicate the amount anddates of water application.

Unusual recent air temperatures or humidityconditions.

Previous cropping and tillage history.

Types of weeds in the eld.



7/60

2Stage I: Scouting fromEmergence to Knee-High

4

Determine the extent and severity of any problem identied. Is the problem throughout the eld or spotty andlocalized? Has emergence been completed or are there seeds sprouted, ready to emerge? Take accurate standcounts and determine percent of stand achieved.

If replanting is necessary, take steps to prevent recurrence of the cause for poor emergence. Verify that theplanter is operating properly and that fertilizer or pesticide issues have been corrected.

GENERAL SYMPTOM POSSIBLE CAUSE REMARKS

No seed1. Planter

Rodents or birds

Improper adjustment; row unit drive notengaged; worn parts; clogged spout; emptybox or tank; wrong plates, disks, or drum;excess or wrong seed treatment.

Digging and partly-eaten kernels.

Normal seed2.

appearance; not

swelled

Unfavorable soil conditions

Poor seed-soil contact

Cold, dry soil.

Inadequate press wheel pressure; improperclosing wheel adjustments; inadequate residuemanagement; dry or cloddy soil.

Normal seed3. appearance; swelled

but not sprouted

Unfavorable soil conditionsFertilizer or pesticide injury

Cold, wet soil.Phytotoxic pesticides or too much fertilizer tooclose to the seed.

Seed dead, rotted4. Seed rots or seedling blights These are accentuated when soil conditionsare unfavorable for germination and seedlinggrowth. Many species of fungi and/or bacteriamay be involved. Fungicide seed treatmentprotects the seed, not the seedling.

% stand achieved = X 100number of plants establishednumber of seeds planted

I the stand is uneven or i there are skips down the row,

dig to fnd the planted seed and its distribution.


8/60


Seed dead, rotted4.

(cont.)

Fertilizer injury

Insecticide injury

Dead seed planted


Fertilizer salts, nitrogen, and potassium drawmoisture and may leave seed in soil too dryto support growth. In-furrow applications aremore likely to cause fertilizer injury than starterfertilizers placed at least 2 inches from the seed.Ammonia toxicity is caused when plantingfollows anhydrous or aqua ammonia applicationtoo closely or where application was tooshallow. This can kill or stunt seedlings. Rootsappear sheared off. Boron and some othermicronutrients impair germination if they are tooclose to the seed.

Some soil-applied organophosphateinsecticides can impair germination if placedin furrow with the seed. Check the label andapply only as directed.

Cold, dry, wet, or crusted soil.

Seed hollowed out5. Insects Seed corn beetle, seed corn maggot, orwireworm (see pp. 36-38).

Sprout twisted or6.

leaves expanded

below ground


Seed planted too deep

Mechanical injury to seed inhandling or planting

Chemical injury

Crusted, cold, or cloddy soil. A cloddy surfacecan allow light to reach the sprout and trigger

leang too soon. In the case of crusting, rotaryhoeing may be benecial.

Fertilizer (see p. 5); insecticides (see p. 5); orsome herbicides such as acetanilides anddinitroanilines (see pp. 41-42).

Slow, uneven7.

emergence

Planter


Seed planted too deep

Seed injury due to improper operation oradjustment, including planting depth.

Cold, dry, wet, or crusted soil. In the case ofcrusting, rotary hoeing may be benecial. Properlybanded fertilizer at planting may help seedlingsovercome unfavorable soil conditions.



9/60

6


Seedlings pulled or1.

dug up, seed eaten

Bird or rodent damage Chemical repellents may help.

Slow, uneven plant2.

growth

Unfavorable growing conditions

Low fertility

Insects attacking roots

Nematodes attacking roots

Chemical injury

Non-uniform planting depth

Failure of secondary roots todevelop (rootless corn syndrome)

Cold, dry, wet, or compacted soil. Properlybanded fertilizer at planting may help minimize

the effects of some unfavorable growingconditions.

If a nutrient deciency due to inadequateamounts in the soil, is conrmed, considersidedressing or foliar application, depending onthe nutrients involved (see pp. 52-53).

Corn root aphid, corn rootworm, grape colaspis,webworm, white grub, or wireworm (see pp.31-38).

Requires microscopic analysis.

Fertilizer (see p. 5); insecticides (see p. 5);

herbicides such as Balance

; Command

orScepter carryover (see pp. 41-42); or liquidmanure.

Dry, loose soil is not conducive to normal rootdevelopment. This condition is accentuatedby shallow planting and whipping by wind.Cultivation may help by throwing soil around thebase of plants.

Discolored leaves3. Nutrient deciency


Magnesium, nitrogen, phosphorus, or sulfur(see pp. 52-53). Nitrogen deciency results in

a yellow discoloration of leaves. Phosphorusdeciency results in a purpling of leaves due tothe accumulation of anthocyanins. If nutrientdeciency is conrmed, consider sidedressingor foliar application, depending on the nutrientsinvolved.

Waterlogged, cold, or compacted soil. Theseconditions can also affect nutrient uptake andtranslocation.

I plants are abnormal in appearance,

try to identiy one o the ollowing specifc symptoms.


10/60


Discolored leaves3.(cont.)

Insects attacking roots


Chemical injury

Wind damage

Frost or freeze

Cold (not freezing) temperaturestress

Anhydrous burn

Mechanical injury

Hybrid differences

Most observed leaf discoloration is due tonutrient deciency induced by damaged orinadequate roots (see nutrient deciency,above).


Fertilizer (see p. 5); insecticides (see p. 5);herbicides such as Balance; Commandor Scepter carryover (see pp. 41-42); liquidmanure.

Abrasion by sand or soil particles. Difcultyestablishing secondary roots.

Check growing point for damage. Seedlingsoften recover. Most pronounced in low-lyingareas.

Cool nights and warm days promote above-ground plant growth at the expense of rootdevelopment. This leads to increased demandby the above ground tissues for more nutrientsthan the roots can deliver. The result can beshort-term deciency symptoms until the rootsystem becomes more developed.

Uneven Corn Plant Growth



11/60

8


Leaves rolled or4.

puckered, may be

wilted

Drought

Insects attacking roots or stalks


Mechanical root pruning

See Section 6, Insect Injury Symptoms (p.31-38), plus chinch bug, cutworm, Japanesebeetle, stink bug, or webworm.


Leaves rolled or5.

twisted together

(onion lea or buggy

whip)

Herbicide injury

Temperature variation

Nutrient imbalance

Hail damage

Acetanilides, dinitroanilines, or phenoxys (seepp. 41-42).

Alternating hot and cold weather, inducingvery rapid growth spurts followed by little or nogrowth.

Boron toxicity or calcium deciency (see p. 52).

Injury to the growing point at this growth stage canresult in short-term wrapping of leaves.

Shredded leaves or6.

eaten plants

Wind damage

Hail damage

Insects

Livestock or wild animal grazing

Armyworm, common stalk borer, corn earworm,cutworm, European corn borer, grasshopper,slug, or webworm (see pp. 32-37).

Look for tracks.

Leaves spotted,7.

striped or dead

Wind damage

Low soil pH

Nutrient deciency

Insects

Disease

Fertilizer or herbicide injury

Sunscald or cold

Hybrid differences

Abrasion by sand or soil particles.

Beaded streaking of leaves, which turn reddish-purple and may die.

Boron, copper, magnesium, potassium, sulfur, orzinc (see pp. 52-53).

Flea beetle, leaf miner, or thrips (see pp. 35-37).

Anthracnose, bacterial wilt, eyespot, Gosss wilt,holcus spot, seedling blights (favored by cool, wetsoil), virus or virus-like diseases (see pp. 24-27).

Anhydrous burn; spray drift; foliar-appliedherbicides such as Buctril, Basagran,Gramoxone, or Blazer; Classic, Scepter, orReex carryover; premix acentanilides post-applied (see pp. 40-42).

Cold nights followed by clear, bright, fast-warmingdays.


12/60



Rows o holes across8.leaves

Insects Billbug, common stalk borer, corn borer,cutworm, or stink bug (see pp. 32-37).

Plants wilt and die9.suddenly

Insects

Wind damage

Disease

Herbicide injury

Frost or freeze

Lightning

Anhydrous burn

Flooded, water-logged soil

Billbug, chinch bug, cutworm, stink bug, whitegrub, or wireworm (see pp. 32-38).

Seedling blights, bacterial wilt, or Gosss wilt(see p. 26).

Triazines or misapplications of glyphosateherbicides (see pp. 41-42).

Check growing point for damage. Seedlings

often recover.Kills everything, usually in circular area.

Buggywhip

Corn plant two days ater rost


13/60

10


Plants twisted or10.

broken o

Herbicide injury

Insects

Especially 2,4-D followed by wind (see pp. 41-42).

Billbug, cutworm, lesser cornstalk borer, orstinkbug (see pp. 32, 36-37).

Inhibited root11.

development or

malormed roots

Nematode injury

Insects

Fertilizer injury

Herbicide injury

Soil conditions


Corn rootworm, grape colaspis, white grub, orwireworm (see pp. 34-38).

Phenoxys, Banvel, carryover dinitroanilines, andScepter or Classic carryover (see pp. 41-42).

Planting when soils are too wet can causesidewall compaction that can arrest or severelyrestrict corn root development. Look for at-sided or abruptly arrested root systems.

Wind Damage in Corn Field

Hail Damage in Corn Field


14/60


3Stage II: Scouting fromKnee-High to Tasseling

This is the period of most rapid plant growth. Nutrient and moisture demands are high; deciencies will likelyreduce crop yield potential. Problems must be evaluated for economic damage potential before controldecisions can be made. Observe differences due to hybrids and management practices.

I plants are abnormal in appearance,

try to identiy one o the ollowing specifc symptoms.


Uneven height (tall1.

plants, short plants)

Emerged at different times

Varied nutrient or moistureavailability under droughtconditions

Low soil pH

Herbicide drift

Nutrients concentrated at dry soilsurface

Fallow (idle land) syndrome

Uneven planting depth; uneven soil moisture.

Use of burndown products adjacent to croppedarea.

Nutrients are unavailable to the plant.

Phosphorus is unavailable to the plant (see p.53).

Numerous tillers2. Growing point injury

Favorable early-season growingconditions

Low plant population

Adjacent to open spaces(population gaps) in the row

Hybrid differences

Disease

Mechanical or insect damage.

Optimum moisture, high soil fertility, or both.

Crazy top (see p. 27)

Discolored or dead3.

leaves

Nutrient deciency Nitrogen, phosphorus, or potassium (see p.53).


15/60

12


Discolored or dead3.

leaves (cont.)

Fertilizer or herbicide injury

High temperatures

Sunscald or cold banding

Insects

Mechanical injury

Frost or freeze

Hybrid differences

Barren stalk

Fertilizer or herbicide on foliage. This tendsto be more pronounced at row ends or whereoverlap application was made.

Noted as scalding or bleaching of top leavesrst.

Cold nights followed by clear, bright, fast-warming days.

Chinch bug, spider mite, or root-attackinginsects, which can reduce nutrient uptake (seepp. 32-37).

Purpling or reddening due to anthocyaninexpression in response to accumulation ofsugars that cannot translocate to the initiatingear.

Whorl leaves dead4.

(deadheart)

Disease

Insects

Chemical injury

Excessive heat

Bacterial stalk rot (see p. 28).

Billbug, corn borer, or fall armyworm (see pp.32-33).

More common following a late over-the-topapplication of certain herbicides.

Temperature exceeding 100F, depending onmoisture availability and genetics of the hybrid.

Leaves spotted or5.

striped

Low soil pH

Nutrient deciency

Chemical injury

Disease

Insects

Beaded streaking of leaves, which turn reddish-purple and may die.

Boron, iron, magnesium, manganese, nitrogen,or potassium (see p. 52-53).

Herbicides, additives reactions, or fertilizers;symptoms may be due to spray drift on foliage.

Bacterial leaf blight, eyespot, Gosss wilt, holcusspot, leaf blights, Physoderma brown spot,rust, sorghum downy mildew, virus or virus-likediseases (see pp. 24-28).

Cereal leaf beetle, corn rootworm beetle,corn blotch leaf miner, ea beetle, spidermite, or thrips; root-damaging insectsinduce nutrient deciency or droughtsymptoms (see pp. 32-37).


16/60


Leaves spotted or5.

striped (cont.)

Wind damage

Hail damage

Genetic stripe

Sunscald or cold banding

Abrasion by sand or soil particles.

Observed only on occasional plants.

Cold nights followed by clear, bright, fast-warming days.

Leaves eaten or6.

shredded

Livestock or wild animals

Insects

Hail damage

Wind damage

Disease

Look for tracks.

Armyworm, billbug, common stalk borer, cornborer, corn earworm, cutworm, grasshopper,Japanese beetle, leaf miner, slug, or wireworm(see pp. 32-38).

Determine if the growing point survived. If it didnot, plant will not produce a tassel. Despite this,ear fertilization may still occur.

Sorghum downy mildew (see p. 28).

Plants stunted, leaves7.

close together with

mosaic mottle or

streaks; leaves yellow

or red

Virus or virus-like diseases Corn lethal necrosis, corn stunt spiroplasma,maize chlorotic dwarf, or maize dwarf mosaic(see p. 27).

Gray or black galls8.

growing on plants

Disease Common corn smut (see p. 26).

Stalks spindly,9.

unthrity, yellow

Nutrient deciency

Excess soil moisture

Excessive plant population

Root damage

Stalk damage

See pp. 52-53.

Insects, nematodes, chemical, or mechanical.

Insects, disease, or mechanical.

Plants wilted or rolled10. Drought

Root damage

Stalk damage


Mechanical or insects; chinch bug, common stalkborer, or corn borer (see pp. 32-33).

Top leaves tightly11.

rolled (buggy whip

or onion leafng)

Chemical injury

Cold soil during seedling stage

2,4-D and other phenoxy herbicides; Banvel (seep. 41).



17/60

14


Top leaves tightly11.

rolled (buggy whip

or onion leafng)

(cont.)

Temperature response or rapidgrowth syndrome

Nutrition

Mechanical injury

Hybrid differences

Alternating hot and cold periods, speeding andslowing plant growth.

Boron toxicity or calcium deciency (see p. 52).

Plants growing up in a12.

curved gooseneck

Recovery after root damage

Recovery after early season environmental conditions

Insects (usually, but not limited to corn rootwormlarvae), nematodes, herbicides (especiallyphenoxys), soil compaction, mechanical injury,or wind.

Curled or stubby13.

brace roots

Wind and wet soils

Dry, hot soil surface

Wind action

Plants twisted or14.

growing o at 90angle

Mechanical injury

Plants leaning or15.

broken o

Wind damage

Livestock or wild animals

Root damage

Insects attacking stalk

Chemical injury, especially iffollowed by wind

Soil compaction

Poor secondary root

development in dry soilDisease

Hail damage

Especially if soil is wet. Wind can cause

greensnap at lower nodes of rapidly growingstalk.

Look for tracks.


Common stalk borer or corn borer (see p. 32-33).

Especially Banvel or phenoxy herbicides (seepp. 39-41).

Inadequate root development.

Rootless corn syndrome due to weather and

aggravated by shallow planting.Bacterial or Pythium stalk rots (see pp. 28-29).

Plants wilt and die16.

suddenly

Lightning

Drought

Kills everything, usually in a circular area.

Field areas with low water-holding capacity are rstto show stress.


18/60


Plants wilt and die16. suddenly(cont.) Disease Frost or freeze

Herbicide injury

Bacterial or Pythium stalk rots (see pp. 28-29).

Misapplication of glyphosate, glufosinate,paraquat, or other non-selective herbicides (seepp. 41).

Tassel eeding (while17.

in whorl)

Insects Armyworm, corn leaf aphid, or western beancutworm (see pp. 32-35).

Tassels prolierated18.

into green cluster o

fngerlike branches,

excessive tillering,leaves oten thick and

straplike

Disease Crazy top (see p. 27).

Tassel prolierated,19.

with stringy black

vascular bundles

present

Disease Head smut (see p. 26).

Greensnap at lower nodes o rapidly growing corn stalk



19/60

16

4Stage III: Scouting fromTasseling to Maturity

This period, which commences with the critical pollination stage, includes grain ll, plant maturation, and death.It also includes the critical stage before harvest when standability and ear retention become concerns. Fieldobservations are essential to detect yield-depressing factors during this period.


Eaten1. Livestock or wild animals

Insects

Look for tracks.

Armyworm, corn borer, corn rootworm beetles,grasshopper, or Japanese beetle (see pp. 32-36).

Shredded2. Hail damage

Wind damage

Disease Sorghum downy mildew (see p. 28).

Dead rosted3.

appearance

Frost or freeze

Drought

Disease

Insects

Anthracnose, leaf blights, or stalk and root rots(see pp. 24-26; 28-29).

Corn borer, corn leaf aphid, or spider mite (seepp. 31-33; 37).

Spotted or dead4. Disease

Chemical injury

Numerous leaf blights such as anthracnose,eyespot, Gosss wilt, gray leaf spot,Helminthosporium leaf spot, northern corn leaf

blight, Phaeosphaeria leaf spot, Physodermabrown spot, rust, or southern leaf blight (see p.26).

Especially spray drift.

Discolored sheath5. Insects

Disease

Corn leaf aphid (see p. 31).

Purple sheath spot caused by saprophyticorganisms existing on pollen trapped betweenthe leaf sheath and stalk.

I LEAVES are aected,

try to identiy one o the ollowing specifc symptoms


20/60


Red or purple color6. Mechanical injury

Barren plant

Disease

Insects

Hybrid differences

Anthocyanins develop in response toaccumulation of sugars that cannot betranslocated to lling grain.

Corn rust, corn stunt spiroplasma, high plainsvirus, or maize chlorotic dwarf (see pp. 24-27).

Corn borer (see p. 33).

White striping with7.

white, downy growth

on upper and lower

lea suraces

Disease Sorghum downy mildew (see p. 28).

I SILKS are aected,



Eaten o1. Insects Armyworm, corn earworm, corn rootwormbeetle, grasshopper, Japanese beetle, orwestern bean cutworm (see pp. 32-35).

None visible, or2.

delayed several days

ater tasseling

Heat

Drought

Nutrient deciency or imbalance


Insects

Excessive population for

conditions

Especially nitrogen or phosphorus.

Cold nights occurring just prior to silking.

Corn leaf aphid, fall armyworm, or spider mite(see pp. 31-32; 37).

Trapped, balled within3.

the husk

Drought



Hybrid differences

See pp. 52-53.

Cold nights during early silking.

Red or green color4. Hybrid differences Fresh silk color is genetically controlled.



21/60

18

I TASSELS are aected,



Failed to emerge1. Drought or heat stress

Nutrient deciency

Insects

Especially boron (see p. 52-53).

Aphid stress, corn earworm, or fall armyworm(see pp. 31-32).

A mass o leaves2. Disease Crazy top, head smut, or sorghum downymildew (see pp. 27-28).

One or more small3.

ears ormed

Genotype by environmentinteraction

Most common on tillers.

Kernels develop in4.

tassel

Genotype by environmentinteraction

Most common on tillers.

Broken o, stalk5.

tunneled

Insects Corn borer (see p. 33).

I STALKS are aected,



Red or purple color1. Mechanical injury to plant

Barren stalk

Hybrid differences

Insects

Anthocyanins develop in response toaccumulation of sugars that cannot betranslocated to lling grain.

When corn borers tunnels into a stalk, the areaabove the point of entry may turn red or purple.

Barren (no ear2.

enlargement)

Drought

Heat


22/60


Barren (no ear2.

enlargement) (cont.)


Insects

Silks eaten off prior to pollination

Disease

Excess population for conditions

Tillers

Timing of silk or pollen sheddisrupted

Mechanical injury

See pp. 52-53.

Corn leaf aphid or spider mite (see pp. 31, 37).

Corn rootworm beetle and Japanese beetle(see pp. 34-36).

Head smut, mildews, virus or virus-like disease(see pp. 26-27).

Seldom produce a fully-developed ear.

Environmental stress.

Broken below ear3. Wind

Disease

Nutrient imbalance

Insects

Weather stress

Mechanical injuryExcessive population forconditions

Delayed harvest

Plants just prior to tasseling are especiallyvulnerable to greensnap by wind.

Stalk and root rots (see pp. 28-29).

Excess nitrogen, insufcient potassium (see p.53).

Southwestern corn borer (see p. 33).

Drought, heat, or other conditions limitingphotosynthesis.

Machinery; livestock or wild animals.

Broken above ear4. Wind

Insects

Mechanical injury

Delayed harvest

European corn borer or fall armyworm (see pp.32-33).

Machinery; livestock or wild animals.

Multiple ears at one5.node

Hybrid differences

Mechanical injury

Cold temperature when earsformed

Disease Mildews, virus or virus-like disease (see pp. 27-28).



23/60

20


Multiple ears at one5.

node (cont.)

Insects Results from severe silk clipping by insects suchas corn rootworm beetle and Japanese beetle(see pp. 34, 36).

Leaning, but not6.

broken (root lodged)

Wind and wet soil

Poor root development

Insects

Nematode activity

Disease

Nutrient deciency

Herbicide injury

Mechanical injury

Delayed harvest

Hybrid differences

Drought, fertilizer placement, soil compaction,low soil pH, or poorly drained soil.

Corn rootworm and other root feeders (see pp.34-35).

Stalk and root rots (see pp. 28-29).

Especially potassium (see p. 53).

Especially 2,4-D and other phenoxys (see p. 41).

Machinery, livestock or wild animals

Premature death7. Disease

Insects

Frost or freeze

Severe drought

Lightning

Leaf blights or stalk and root rots (see pp. 24-26, 28-29).

Corn borer (see p. 34).

Kills everything, usually in a circular area.

Black mold8. Saprophytic Buildup on dead stalk tissue under warm, humidweather conditions. Timely harvest will deter.

I EARS are aected,



Numerous ear shoots,1.

leay and barren

Disease Crazy top, sorghum downy mildew, virus orvirus-like diseases (see pp. 27-28).


24/60


Sot, glistening smut2.

galls, black and

powdery when mature

Disease Common corn smut (see p. 26).

Poorly flled tips3. Hybrid differences

Nutrient deciency

Interaction between population,hybrid, and environment

Insects

Disease

Drought

Unusually favorable growingconditions after pollination

Suboptimum light, carbondioxide, or temperature duringkernel ll

Especially nitrogen or potassium (see p. 53).

Silks clipped before pollination. Corn borer, cornearworm, corn rootworm beetle, fall armyworm,or Japanese beetle (see pp. 33- 36).

Foliar disease that reduce photosynthesizingarea (see pp. 24-26).

Silks at ear tips were not present when pollenwas available

Cob elongates, but rarely lls up tip kernel.

Reduced photosynthesis; tip kernels abort.

Small malormed, light4.

weight

Nutrient deciency

Drought or heat stress

Plant damage

Excessive population forconditions

Second or third ear on stalk, oron a tiller

Insect-caused plant stress

Disease

Soil compaction

Especially nitrogen and phosphorus (see p. 48).

Mechanical or disease.

Numerous ear-feeding insects (see pp. 32-38).

Leaf blights, stalk and ear rots, virus or virus-likediseases (see pp. 24-30).

Very short husk,5.

remains tight at

maturity; ear tip

exposed

Weather

Hybrid differences

Usually observed after drought or heat hasstopped husk growth, but later favorableconditions permit more normal ear size todevelop.



25/60

22


Barren (little to no6.

grain)

Disease

Pollination failure

Pesticide injury

Head smut, virus or virus-like diseases (see pp.26-27).

Drought, heat, or other stress interfered withsilk/pollen shed timing; insect (clipped silksor caused tassel to abort); chemical injury(especially by growth regulators like 2,4-D andBanvel).

Following drift or misapplications.

Pinched ear syndrome7. Chilling injury during early eardevelopment

Also called beer can ear. Ear is reduced to3-4 inches. Well lled kernels are on bottom

third of ear, cob tissue on middle third, andundeveloped tissue on top third.

Dropped8. Drought

Nutrient deciency

Hybrid differences

Insect damage to shank

Weakened shank.

Weakened shank.

Ears usually drop free of husk.

Husk usually remains on the dropped ear;European corn borer (see p. 33).

Scattered kernel set9. Lack of adequate viable pollenwhen silks are receptive

Silks eaten off before pollinationHeat or drought at pollination

Nutrient deciency

Herbicide injury

Insect or animals.

Phenoxy herbicides or too-late applications ofglyphosate (see p. 41).

10. Kernel eeding Insects

Birds, animals

Corn borer, corn earworm, fall armyworm, picnicbeetle, or western bean cutworm (see pp. 33-35).

11. Rotten (spots or entireear)

Disease

Insects

Birds

Hail or other mechanical injury

Ear and kernel rots or Helminthosporium leafspot (see pp. 29-30; 25).

Corn borer, corn earworm, fall armyworm, picnicbeetle, or western bean cutworm (see pp. 33-35).Followed by weathering or disease.

Followed by weathering or disease.

Followed by weathering or disease.


26/60



12. Kernels eaten rom

ear tips

Birds

Wild animals or rodents

Insects Corn earworm, fall armyworm, picnic beetle, orwestern bean cutworm (see pp. 33-35).

13. Tunnels in ear, cob,

shank, or stalk

Insects Corn borer, corn earworm, or fall armyworm (seepp. 32-33).

I KERNELS are aected,



Broken seedcoat,1.

popped appearance

Genetic and environmentinteraction

Popped kernels usually occur under hightemperatures.

Horizontally cut or2.

split seedcoat

Silk-cut A genetic and environment interaction. Notvisible until kernels are shelled from the cob.

Pink or red streaking3.

or lengthwise stripes,especially running

over crown

Kernel red streak Caused by toxin secreted during feeding of

wheat curl mite. More pronounced toward eartip. No detrimental effects are known.

Sprouted especially at4.

base o ear

High rainfall and warmtemperature while ear remainederect on stalk

Usually accompanied by mold.

White streaking, also5.

known as starburst

Disease Associated with Fusarium ear rot.

Barren corn Pinched ear syndrome


27/60

24

5This section will help you identify the common corn diseases based on plant symptoms and conditions. It is notuncommon to have symptoms of several different diseases present at the same time. Symptoms of differentdiseases may appear similar, particularly during early stages of disease development. Laboratory culturing andmicroscopic examination may be required to make a positive identication.

Growth stages during which symptoms generally appear are listed following the disease name and are describedas follows:

Disease Symptoms

LEAF DISEASES

Anthracnose (Stage I, III)Oval to spindle-shaped

water-soaked lesions on

youngest leaves turn tan to

brown with yellow to reddish

brown borders. Heavilyinfected leaves wither and

die. Top-leaf die-back may

occur 4 weeks to 6 weeks

after pollination, leaving

the lower stalk green. The

organism thrives in warm, humid weather. The stalk-rot

phase of the organism is of greater concern than the leaf

blight phase.

Bacterial Wilt and Bacterial Lea Blight

(Stewarts Wilt, Stewarts Disease) (Stage I, III)

Young plants exhibit long,green-gray, water-soaked

lesions with wavy margins,

accompanied by stunting

and wilting which lead to

plant death. Cavities may

form in stalk near the soil line.

Bacterial masses ooze from

cut end of infected stalks or

leaves.

The more common leaf blight phase appears after

tasseling. Leaves are streaked with gray-green to yellow-

green lesions, each distinguished by the presence of a ea

beetle feeding scar toward the base of the streak. Streaks

are long and irregular, turning tan as the tissue dies.

Flea beetles (small, oval, black insects) are the primary

carrier. Incidence of the disease is relative to beetle

population. Sweet corn tends to be more sensitive than

eld corn to this disease.

Common Corn Rust (Stage II, III)

Cinnamon-brown, powdery,

circular-to-elongated pustules

(blister-like growths) can

occur on any aboveground

plant tissue, but especially

on both surfaces of the

leaves. In contrast, pustules

of southern corn rust occur

primarily on the upper leafsurface. Pustules rupture

leaf surface and rusty powder can be rubbed off with

ngers. Pustules become dark brown to black late in the

growing season. The organism thrives in moderate to cool

temperatures and high humidity.

Stage I Emergence to knee-high

Stage II Knee-high to tasseling

Stage III Tasseling to maturity


28/60

Eyespot (Stage II, III)

Small (less than inch),circular, translucent lesions

surrounded by a yellow to

purple margin, give a halo

effect. Lesions occur on

leaves (most commonly as

plants approach maturity),

sheath and husk. The

disease is favored by cool,

moist weather.

Gosss Wilt

(Lea Freckles and Wilt, Nebraska Bacterial

Wilt and Lea Freckles) (Stage I, III)

Young plants wilt and

die. Vascular bundles are

discolored. More common

later-season infections

produce dull gray-green

to orange lesions forming

water-soaked streaks with

irregular margins on leaves.

Within developing lesions,

small, irregular shaped water-

soaked freckles appear. Bacterial droplets may oozefrom the leaf surface early in the morning. Plant injury,

such as from hail or wind damage, enhances infection.

Gray Lea Spot (Stage III)

Gray to tan, rectangular

lesions on leaf, sheath or

husk tissue. Spots are

opaque and long (up to 2

inches). Lower leaves are

affected rst, usually not until

after silking. The organism

thrives in extended periodsof warm, overcast days and

high humidity. It has become

more prevalent with increased use of reduced tillage and

continuous corn.

Helminthosporium Lea Spot

(Northern Lea Spot) (Stage III)

Numerous races of the

organism have been

identied. Symptoms vary

by race. Lesions tend to

be oblong to blocky, and

tan to brown in color. This

leaf spot may also cause a

black, charred-appearing

ear rot. It prefers moderate

temperatures and high

humidity.

Holcus Spot (Stage I)

This organism causes small,

circular to oblong, water-

soaked lesions toward tips

of lower leaves. Later, lesions

become creamy white to

tan, then light brown with

reddish margins. Holcus spot

is very similar in appearance

to paraquat damage.

Rainstorms accompanied by

wind splash overwintering bacteria from the residue ontoyoung plant leaves. This organism does not cause serious

loss.

Northern Corn Lea Blight (Stage II, III)

Long (up to 6 inches),

elliptical, gray-green lesions

that become tan-brown

identify infections caused

by this organism. Infection

spreads up the plant

starting on lower leaves. It

is favored by high humidityand moderate temperatures.

Numerous physiologic races

have been described. Host specic races of the organism

may also attack sorghum.



29/60

26

Phaeosphaeria Lea Spot (Stage III)

Lesions are initially small,round to oval, pale green

or yellow, and scattered

on leaves of mature plants.

Spots become light tan with

reddish-brown margins and

may coalesce into irregularly

shaped lesions. This disease

is most prevalent in areas of

high rainfall and moderate

temperatures.

Physoderma Brown Spot (Stage I, III)

Small yellow spots appear

rst at the base of the

leaf. These spots become

brown and combine to form

chocolate-brown to reddish

irregular blotches, sometimes

as bands of infection across

leaf blades. Sheath, husk,

tassel, stalk, and leaves

may exhibit symptoms late

in the season. Infected stalks may break at a node. This

organism is favored by warm, wet weather.

Southern Corn Lea Blight (Stage II, III)The most common race,

named O, produces small,

elongated (up to 1 inch long),

parallel-sided lesions that are

tan with brownish borders. This

blight primarily attacks leaves. It

is favored by high humidity and

warm temperatures.

Southern Corn Rust (Stage II, III)

Small, circular, orange-to-

light-red pustules (blister-like

swelling) occur on leaves,

especially the upper surface,

and sheaths. Pustules rarely

break the leaf surface. (In

contrast, common corn

rust pustules attack upper

and lower leaf surfaces and readily break through the

epidermis.) This organism is favored by warm, humid

weather.

SMUTTING DISEASESCommon Corn Smut

(Boil Smut, Blister Smut) (Stage I, II, III)

Local infection of any plant

part, even below the soil

surface, occurs through plant

wounds or thin-walled cells

of actively growing tissue.

Resulting galls are rst silver-

white, then become gray

to black powdery masses

of smut spores that are

released when the galls

break open. Galls on leaves

seldom develop beyond

pea-size and tend to harden

and dry without rupturing. High fertility and plant injury

favor the disease. Common smut is not toxic to animals.

Head Smut (Stage III)

Seedling infection results

in systemic development

of the disease as the plant

grows and develops. Tassels

of affected plants may

multiply as a mass of leaves

or be replaced by a black,

smutty mass which ruptures

to release spores, leaving

black, thread-like vascularstrands. Ears may be completely replaced by similar, often

triangular-shaped, smut masses. Occasionally, part of an

ear escapes and produces a few kernels. Hot, dry soil at

seedling stage favors infection. One type of head smut

also attacks sorghum.


30/60

VIRUS AND VIRUS-LIKE DISEASES

Corn Lethal Necrosis (CLN) (Stage II, III)Mosaic patterns appear onleaves and husk. Leaves die

from the margins inward and

plants may die prematurely.

The organism generally starts

at the tassel and works

downward. Barrenness

or sharply reduced grain

production is common. CLN

is caused by synergistic

interaction when plants become infected by maize

chlorotic mottle virus and either maize dwarf mosaic virusor wheat streak mosaic virus. Presently, the disease has

been identied only in parts of Kansas and Nebraska.

Corn Stunt Spiroplasma (CSS) (Stage II, III)Corn stunt is caused by a

spiroplasma, not a virus.

Margins of whorl leaf turn

yellow followed by reddening

of older leaves and yellow

striping which runs the

length of leaves. Plants arestunted, have multiple tillers

and produce numerous small

ear shoots. Root systems

are reduced. Corn stunt

symptom development and epidemiological patterns are

similar to those of virus diseases.

High Plains Virus (HPV) (Stage I, II, III)

Symptoms begin as small

yellowish ecks which often

appear as lines running

parallel to leaf veins. Infectedseedlings turn bright yellow

and quickly die. Plants may

be stunted, older leaves

may become red and then

necrotic, and ear and kernel

size may be reduced.

Considerable symptom

variation exists among cultivars. It is spread by the wheat

curl mite.

Maize Chlorotic Dwar Virus

(MCDV) (Stage II, III)

Younger leaves are yellow

and nely striped. There

is a general yellowing

or reddening of leaves

and plants are stunted.

Most affected plants are

barren. MCDV is spread by

leafhoppers from infected

Johnsongrass and other host

species. MCDV is detected

more frequently and is more damaging than maize dwarf

mosaic.

Maize Dwar Mosaic Virus

(MDMV) (Stage II, III)

Mottling of younger leaves

progresses into narrow, light-

green to yellowish streaks

along leaf veins. Leaves,

sheaths and husks may show

symptoms. Plants are stunted

and ear size and seed set are

reduced. Sometimes, multiple

tillers or ear shoots develop.MDMV is most prevalent in

areas where Johnsongrass grows and serves as a winter

host for the virus. It is spread by aphids.

FUNGAL SYSTEMIC DISEASES

Crazy Top (Stage II, III)

Plants have excessive tillering with rolling or twisting of

newer leaves. The tassel

becomes a mass of leaves;

ears, if present, often do

likewise. Leaves are usuallynarrow, thick and appear

strap-like. Occasionally over-

sized plants will develop.

Infection occurs when young

plants are under ooded soil

conditions; therefore, it is

more frequent in low-lying areas. This is one of numerous

downy mildews that attack corn.

UGA1235014

536657



31/60

28

Sorghum Downy Mildew (Stage II, III)

Leaves are yellow with white

stripes; plants are stunted

and ears and tassels multiply

into leafy masses. Often the

base of leaves are chlorotic

and sharply contrast with

healthy green leaf tips. White,

downy growth appears on

either or both leaf surfaces.

Leaves often split or shred.

This is primarily a disease of sorghum.

STALK AND ROOT ROT DISEASESStalk rots are diseases that are most commonly expressed

as plants reach maturity. Stalk rot of corn tends to be a

complex of several disease-causing fungi and sometimes

bacteria; seldom will only one causal organism be isolated

and identied. Plants with rotted stalks almost always have

rotted roots, too. Usually, but not always, the same causal

organisms are involved. Visual identication is very difcult.Typically, wilting is the rst sign of stalk rot in a eld. In a

few days, leaves turn a frosted gray, ears droop and the

outer rind of the lower stalk turns brown. Fields where

stalk rot is developing should be harvested early to reducegrain losses.

Anthracnose Stalk Rot (Stage II, III)

Early infection may kill plants

before pollination, but onset

usually occurs just before

plants mature. Sometimes

a portion of the plant above

the ear blanches and dies

prematurely (top dieback).

Usually, the entire plant is

killed and several nodes arerotted. Late in the season, a

shiny black discoloration develops in blotches or streaks

on the stalk surface, especially on lower internodes.

Internal stalk tissue may become black and soft, starting

at the nodes. Lodging typically occurs higher on the stalk

than with other stalk rots. The same organism causes leaf

blighting earlier in the season.

Bacterial Stalk Rots (Stage II, III)

At least two organisms have

been identied as causing

bacterial stalk rot. With either,

there is rapid development

of a soft rot of stalks,

accompanied by wilting and

plant death. Stalks twist and

collapse while still green,

disintegrating into a soft

mass often accompanied by

a foul odor.

Charcoal Rot (Stage III)

Charcoal rot can be

expected when the crop

has grown under hot, dry

conditions; it also attacks

sorghum and soybeans.

Lower internodes are

affected, causing premature

ripening, shredding, and

crown disintegration.

Vascular strands remain

intact but are black or charred in appearance. Stalks

become gray-black or charcoal color.

Diplodia (Stenocarpella) Stalk Rot (Stage III)

Lower internodes are

straw-brown, spongy, and

dry. Pith disintegrates,

leaving vascular strands

intact. White fungal growth

may appear on the stalk

surface. Minute, dark bodies

embedded just under the

stalk surface are difcult to

remove.

5361254


32/60

Fusarium Stalk Rot (Stage III)

Rotting of roots, crown, and

lower internodes leads to

premature ripening and stalk

breakage. Split stalks may

show whitish-pink to salmon

color. Disintegration starts at

the nodes.

Gibberella Stalk Rot (Stage III)

Affected plants wilt, leaves

turn dull gray-green, and

the lower stalk softens and

becomes straw colored

as plants die. Pith tissue

disintegrates, leaving only

vascular strands. The inside

of a rotted stalk is pink to

red. Small, dark bodies are

supercial on the lower stalk

surface and can be scraped off easily.

Pythium Stalk Rot (Stage II, III)

This organism attacks individual

plants in localized areas,

sometimes before owering. It

is usually conned to the rst

internode above the soil line,

where rind and pith develop wet

rot. Stalks twist and collapse.

Hot, wet weather favors

development of this stalk rot.

Red Root Rot (Stage III)

Red root rot often occursin plants that are also

stalk rotted. Seminal roots

are most damaged and

adventitious root damage

increases with their age. The

root color of affected plants

ranges from pink to royal red,

with higher soil temperatures

resulting in darker pigmentation. The role of this root rot in

stalk lodging is not fully understood.

EAR AND KERNEL ROTSThese rots can affect ears, kernels, or cobs, reducing test

weight and grain quality. Some rots are responsible for

development of mycotoxins that may contaminate grain.

Positive identication is difcult. Rotting observed in the

eld is often due to a complex of causal organisms, not

just one.

Most ear rots are favored by late-season humidity.

Infections are increased by ear damage by birds or insects

and by stalk lodging that allows ears to come into contact

with the soil.

Aspergillus Ear and Kernel Rot (Stage III)

Greenish or yellowish-tan

discoloration occurs on and

between kernels, especially

near the ear tip. Symptoms

are more prevalent if the husk

does not cover the ear tip.

The rot is favored by hot,

dry weather. It may produce

aatoxins.

Cladosporium Ear Rot (Stage III)

Dark gray to greenish black

fungal growth causes kernels

to appear blotched or streaked

Initial discoloration appears

where kernels are attached to

the cob. Infection eventually

progresses upwards and

infected kernels can be seen

scattered over the ear. Ifcompletely colonized, ears

are dark and lightweight.

This disease is often

associated with damage due

to insects, hail, or frost.

Healthy stalk, left. Diseased

stalk, right.



33/60

30

Diplodia (Stenocarpella) Ear Rot (Stage III)

Symptoms include bleached

husks, white mold over

kernels, and rotted ears with

tightly adhering husks. Early

infection (2 to 4 weeks after

silking) is likely to lead to

complete ear rotting. Later

infections may result in partial

rotting, usually beginning at

the base. Since corn is the

only known host, this disease is most severe when corn is

planted following corn in reduced tillage situations.

Fusarium Kernel or Ear Rot (Stage III)

Scattered individual or groups

of kernels show whitish-pink

to lavender fungal growth.

Infected kernels may also

have a starburst pattern of

white streaks on the cap ofthe kernel or along the base.

Infections are more frequent

on damaged ear tips, and

are favored by dry weather.

Fusarium rot may produce mycotoxins.

Gibberella Ear Rot (Stage III)

Symptoms include reddish

kernel discoloration, usually

beginning at the ear tip.

Husks may rot and becemented to the ear. The

organism is favored by cool,

humid weather, particularly

2 to 3 weeks after silking. It

produces several mycotoxins.

Penicillium Ear Rot (Stage III)

Powdery green to blue-green mold develops on and

between kernels. Infection

usually begins at the ear tips

and primarily occurs on ears

with mechanical or insect

damage. Infected kernels

may become bleached and

streaked. Blue eye occurs

when the embryo becomes

discolored due to the presence of blue-green fungal

spores and can occur if infected grain is stored at high

moisture levels.

Trichoderma Ear Rot (Stage III)

Dark green fungal growth

is found on and between

kernels and husks, often

covering the entire ear. The

disease usually occurs on

ears with mechanical or insect

damage. Infected plants tend

to be widely distributed within

a eld.

Photo Citations:

Clemson University - USDA Cooperative Extension Slide

Series, Bugwood.org (viewed 9/22/10); Corn Stunt

1235014

William M. Brown Jr., Bugwood.org (viewed 9//22/10)

High Plains Virus 5366657

Howard F. Schwartz, Colorado State University, Bugwood.

org (viewed 9/22/10) Bacterial Stalk Rot 5361254

Holcus spot and Pythium stalk rot photos courtesy of DonWhite at University of Illinois


34/60

6Insect Identifcation, InjurySymptoms, & ManagemenRecommendations

APHIDS

(1) Corn Lea Aphid

This small, soft-bodied,

bluish-green to gray insect

colonizes on or in the

whorl, upper leaves, and

tassel, especially under

dry conditions. It sucksplant juices and can cause

wilting, pollen shed failure,

and barrenness. The aphid

secretes a sticky honeydew

that may gather dirt and act as a medium for mold.

Predatory insects, like the lady beetle, may be present

and are important predators of aphids. Scouting should

occur during late whorl to early tassel. Aphid infestations

after 50% pollen shed have little effect on grain yield. A

rescue application of insecticide may be warranted if 50%

or more plants have light to moderate infestations and the

crop is under moisture stress.

(2) Corn Root Aphid

This pinhead-sized, soft-bodied, blue-green to gray-green

insect, may be winged or wingless. Corn root aphid groupsare found in clusters on the crown and along the roots of

young plants. They suck plant juices and cause wilting,

discoloration, and stunting. Corneld ants are generally found

in conjunction with corn root aphid colonies. There are no

rescue treatments for corn root aphid. Cultural practices like

crop rotation and tillage can be effective preventative tactics.

Only the most common or the most economically damaging corn insects in the North American region arediscussed in this chapter. Most, but not all, insect identications can be made in the eld by trained individuals.

As insects progress through their life cycles, they change form and features. The following comments

describe the appearance of damaging stages, which may not correspond with their appearance when damageis being investigated. It is not uncommon to investigate crop insect damage, but nd none of the suspectedinsects. At other times, damage from insect feeding may be detected, but the insect is no longer present.

Use economic thresholds whenever possible and seek further pest management guidance if symptoms areunclear. Generalized thresholds for most pests are provided in this guide. These thresholds may changedepending on the cost of a control tactic or the commodity value. Always conrm that pest numbers havebeen suppressed below thresholds following the application of a rescue tactic. Pesticides may perform belowexpectation due to environmental, chemical, or biological situations, or due to application timing.

While this publication is focused on in-season diagnostics and treatment options of insect pests of corn, anumber of Bt traits are available for several of the insect pests discussed in this handbook. The Corn InsectControl with Monsanto Corn Insect Traits and Seed Treatments table (see pp. 39) provides a list of some of

these products as well as the insects they control. These products provide season-long control of the specicinsects and reduce the need for additional pest management action.



35/60

32

ARMYWORMS

Fall ArmywormThe fall armyworm grows

up to 1.5 inches long and

appears greenish-brown in

color with an inverted white

Y on its head. It chews

ragged holes in the leaves

and usually concentrates in

the whorl of young plants.

The fall armyworm also feeds on the tassel and bores into

developing ears. Early detection is critical since this pest

feeds in protected areas of the plant. It lays round, gray

eggs in clusters of 50 or more, which are usually covered

with scales. Eggs hatch in 3 to 5 days and larvae migrate

to the whorl and adjacent plants. Consider using a rescue

application if eggs are present on at least 5% of plants or

whorl injury is detected on 25% or more plants. Because

larvae feed deep in the whorl of young plants, control with

insecticides can be difcult.

(2) True Armyworm

The true armyworm is

greenish-gray with orange

or pinkish lines along thebody. Armies migrate from

maturing small grains or

grass at night. They devour

foliage, sometimes leaving

only leaf midveins, and chew

silks. Infestations are more frequent in no-till or reduced

tillage systems and the true armyworm is usually only an

occasional pest in rotated corn. Consider an insecticide

application if 35% or more plants in the eld are infested.

BILLBUG

This black or gray hard-shelled snout beetle is active

only at night. Leaves are

punctured while still rolled in

the whorl. When they unfurl,

irregular rows of holes appear

across the leaves. The billbug

also chews into plant stems

at or below ground level.

This feeding can damage

the growing point and cause plants to be bent, twisted

or stunted and sometimes die. Billbug incidence is often

associated with nutsedge. Consider spraying for billbug ifstand loss during seedling stages exceeds 5%.

CEREAL LEAF BEETLEThe adult is a 0.2 inch long, hard-shelled beetle with

metallic blue-black head and wing covers, and reddish-

orange legs and thorax. Feeding on corn is rare and

usually limited to eld borders. Though the beetle eats

completely through the leaf between the veins, plants

normally outgrow the injury and rescue applications

are not necessary. Generally infestations are limited to

Michigan, Indiana, and eastward.

CHINCH BUGThe adult is 0.25 inch long,

and is red and black with

white wing covers. When

crushed, this bug emits a

distinct odor. It migrates

from grasses and maturing

small grains, attacks eld

borders rst and is most

destructive during dry years.

The young nymph (juvenilestage) is bright red, but turns black as it develops. Chinch

bugs cluster in groups behind the sheath of lower leaves,

sucking plant juices and causing wilting and eventual

plant death. Management may be necessary if 10 or

more chinch bugs are found on more than 50% of plants.

Insecticide applications may be limited to only the infested

area.

COMMON STALK BORERThis purplish-brown larva may

reach 2 inches in length. It

has white longitudinal stripes

with a distinct purple saddle

near the middle of the body. It

tends to move out of weedy

or grassy eld borders and

attack corn on eld edges (or

throughout no-till elds). The

larva tunnels into stalks or

whorls. Infestation of plants at the 4-7 leaf stage can leadUGA1234011

5364223


36/60

to dead heart. Whorl feeding results in leaves that have

a ragged appearance with injury appearing as a series of

holes across the leaf. Timing insecticide applications priorto borer tunneling is critical. Consider treating if 10% of

V2 plants, 15% of V4 plants, or 30% of V6 plants show

symptoms.

CORN BORER

(1) European Corn Borer

This larvae varies in shades

of tannish-gray and ranges

in size from less than 0.5 to

1.0 inch long. There are two

or more generations per

season. Larvae of the rst

generation feed on leaves

before they burrow into the

stalk. This feeding causes broken midribs and creates a

shot hole appearance as leaves grow out of the whorl.

Consider treating rst generation populations if the moth

ight has already peaked and 5% or more of plants have

shot hole feeding. Second generation larvae feed on and

bore into stalks, tassels, ear shanks, and ear tips causing

top breakage, ear drop (with husk attached), stalk lodging,

and kernel damage. Treatment for second generation

larvae may be necessary if more than 10% of plants have

fresh or hatched egg masses, or young larvae in the

leaf axils. Timing insecticide applications prior to insect

tunneling is critical.

(2) Southwestern Corn Borer

This larvae is white or cream-colored and 0.5 to 1.0

inch long. It has distinct black spots except on the

overwintering form. Two generations feed on corn. The

rst feeds on leaves and creates a shot hole effect

or dead heart in very young plants. Later instars

tunnel in the stalk. The second generation larvae feed

on developing ear tissue, bore into the shank and ear,

and tunnel in the stalk below the ear zone. In the fall,

overwintering larvae migrate to the base of the stalk

and prepare overwintering

tunnels in the stalk below

soil level. These larvae girdlethe plants near the soil level,

resulting in severe stalk

lodging. Treatment may be

necessary for rst generation

populations when greater than

35% of plants hold larvae or

show symptoms. Insecticide

applications for both rst and second generations must

be applied before they begin tunneling. Consider a rescue

treatment for second generation if eggs or young larvae are

found on 20 to 25% of plants.

CORN EARWORMThis larva varies in color from green, yellow, brown to

pink, but all have a yellow-brown head and reach full size

at about 1.5 inches. Since

the insect is cannibalistic,

two or more full-size larvae

are seldom found together.

They may feed on leaves in

the whorl, on

the tassel or

on silks, but

the preferred feeding

site is ear tips and on

developing kernels.

Moreover, ear injury

often leads to ear

molds and rots. The

window for effective

treatment is the few

days after eggs are laid and before larvae tunnel into the

silk channel.

CORN ROOTWORMThis small, white larva feeds on and tunnels into roots

or crowns of young plants. It destroys root systems,

can delay development, and causes root lodging.

Lodging is accentuated by wind storms. Plants may

gooseneck across rows and resume upright growth.

Rescue treatments for control of corn rootworm larvae



37/60

are available, but efcacy

is greatly dependent on the

products ability to move into

the root zone. Continuous

corn or observations of adult

beetles made the previous

year can trigger the use of

control tactics. The adult

beetle may feed on corn

leaves, stripping spots of

the upper surface away. After pollen shed, it feeds on

pollen and fresh silks. Heavy silk feeding can reduce or

prevent pollination and result in partial or complete failure of

kernel set. The beetle can be found in ear tips as maturity

approaches. The general guideline for silk clipping insects

is as follows: control may be necessary if silks are clipped

to less than 0.5 inch and fewer than 50% of plants have

been pollinated. Also, one beetle per plant in late summer

is a useful guideline for justifying whether a control tactic

may be necessary to prevent larval injury in continuous

corn.

(1) Mexican CornRootworm

The adult beetle is light

green and looks similar

to the northern corn

rootworm beetle, but can

be distinguished by a black

stripe on the leg.

(2) Northern Corn Rootworm

The adult beetle is less than0.5 inch long, tan or green

to greenish-yellow without

distinctive spots or stripes.

(3) Southern Corn Rootworm

The adult beetle is 0.25 to 0.5

inch long, yellow or greenish

with six black spots on each

wing cover.

(4) Western Corn

Rootworm

The adult beetle is about0.25 inch long, black and

yellow striped or black

with a yellow tip on the

wing cover. It cannot

be visually identied as

to species.

CUTWORMThere are many species of cutworms that attack corn

and other crops such as cotton, tobacco, and many

vegetable crops. Cutworms are widely distributed; some

species migrate to Corn Belt states from the South and

many overwinter there. Young larvae typically feed on leaf

margins and larger larvae feed below or at the soil surface.

They chew into or completely cut young stalks, causing

34

Corn rootworm feeding


38/60


39/60

36

JAPANESE BEETLE

This shiny, green-bodiedadult beetle has copper- to

bronze- colored wing covers.

As an adult, it grows to about

0.5 inch in length and feeds

on corn leaves and silks. Leaf

feeding by this pest appears

skeletonized or lacy. Larvae

are white with a brown head

and feed unobtrusively on

the roots of plants. They can be differentiated from other

white grubs by the V-shaped pattern of bristles on the

raster. The general guideline for silk clipping insects, suchas an adult Japanese beetle, is as follows: control may

be necessary if silks are clipped to less than 0.5 inch and

fewer than 50% of plants have been pollinated.

LEAF MINERThis small, white larva or

maggot tunnels between leaf

surfaces, leaving long blotchy

tunnels within the leaf. This

pest seldom reaches economic

proportion in corn.

LESSER CORNSTALK BORERThis black larva with white

bands burrows into the stalk

base of young plants. This

borer causes wilting and plant

deformities like twisted, bent

or often barren plants.

PICNIC BEETLE(Sap Beetle,

Scavenger Beetle)(to right)This small, dark beetle

usually has four orange or

cream colored spots on wing covers. It frequently inhabits

ear tips as corn approaches maturity and often appears

where primary insect pests or birds have damaged ear tips.

SEED CORN BEETLEThis small, 0.25 to 0.33 inch long brown ground beetle

damages the germ and hollows seed before it germinates.

The beetle also attacks emerging seedlings, causing spotty

stands. Damage is greatest if germination has been delayed

by the environment. Current seed treatments generally

provide good control.

SEED CORN MAGGOTThis yellowish-white spindle-

shaped larva, is about 0.25

inch long and may eat

the entire kernel before it

germinates. Often times it

leaves only the seed coat

behind. Wet, cold, and heavy

soils are associated with this

pest that can create spotty,

uneven stands. Modern seed

treatment provides good control. Otherwise, there is no rescue

treatment for seed corn maggot and replanting may be the

only option.

SLUGThis soft-bodied, slimy and

legless grayish creature

hides under residue. It is

active at night and leaves a

telltale silver-colored slime

trail on the soil surface. It

feeds using a rasping action

on the lower stalk and leaves

of young plants. This feeding

often removes only onesurface of a leaf and the symptom is more common in no-

till systems. Insecticidal baits are available for slug control,

however, injury seldom warrants this expense.

SOUTHERN CORN LEAF BEETLEAdult beetles are dark brown and can be difcult to nd in

the eld because they are often covered with soil. Beetles

feed on stems and on the edges of leaves of seedlings.

Injured plants appear ragged. When beetles feed in large

UGA2511032


40/60

numbers, plants may die. This beetle is most problematic

in elds that have not been cultivated.

SPIDER MITETwo species can cause

severe damage to corn:

banks grass mite and two-

spotted spider mite. Spider

mite problems are more

prevalent when temperatures

are high and humidity and

rainfall are low. The tiny (about

the size of a pencil dot), eight-

legged creature feeds bypiercing individual leaf cells and sucking out the contents.

Damage is usually rst noted as plants approach tasseling

and continues through grain dent stage. Lower leaves

appear blotched and chlorotic and continued feeding can

cause leaves to die. Damage symptoms progress up the

plant with time. The mite spins a white web on the leaf

surface where it feeds.

STINK BUGSeveral species of green or

brown stink bugs occasionally

attack corn. The adult (shieldbug) has a hard, angular back

and wing covers, and gives

off a foul odor if crushed. It

inserts its piercing-sucking

mouthparts into the base

of young plants and can kill

the growing point or distort

further growth.

SUGARCANE BORERThis tan larvae has

indistinct brown spots on

each segment. It feeds in

whorl-and reproductive-

stage plants similar to

the southwestern corn

borer, except that it does

not girdle the stalk. Yield

losses occur due to

reduced ear weight. The sugarcane borer is found in

Florida, Louisiana, Mississippi, and Texas.

THRIPSSeveral species of this tiny, slender insect occasionally

feed on leaves of young corn plants. As an adult, it

develops wings. With mouthparts tted for rasping and

sucking, thrips remove the green surface layer in tiny

streaks. Individual leaves have a speckled appearance and

elds, or affected areas, may look silvery.

WEBWORMThe garden webworm and sod webworm attack young

corn. It appears as a gray to yellow-green, spotted, bristly

larva about 1 inch long. This pest gets its name from the

ne web it spins from its silk-lined underground nest. The

larva uses this web to travel to plants at night and return

to its nest during the day. Similar to the cutworm and

wireworm, the webworms primary feeding site is at or

just below the surface. Injury becomes more obvious as

damaged leaves emerge from the whorl. Larvae also feed

on the underside of lower leaves.

WHEAT CURL MITE

(Kernel Red Streak)

Kernel red streak is causedby a toxin secreted during

feeding of the wheat curl

mite. It is most common on

yellow corn, but may occur

on sweet corn, popcorn,

and white corn. Streaks are

more pronounced toward ear

tips, especially if kernels are

exposed. Streaks vary from

dark red on yellow kernels to pink on white kernels. No

detrimental effects from the discoloration are known.

WHITE GRUBThe larva has a thick,

soft body with three

pairs of legs just behind

a brown head. It ranges

from 0.125 to 1.5 inches

long depending on



41/60

38

age, and it characteristically curls into a C-shape when

disturbed. There are several species with 1 to 4 year life

cycles. The white grub feeds on developing roots of youngplants, and causes stunting, nutrient deciency symptoms

and death. Heaviest infestations occur where corn is

planted into killed sod. There are no rescue treatments

for grubs and the percent stand loss is the best gauge for

determining if replanting is worthwhile.

WIREWORMThe larva is shiny and slender

with a yellow- to brown-

colored hard body. It ranges

in size from 0.5 to 1.5 inches

long, depending on age and

species. Damage to seed or

young plants reduces stand

and vigor. It damages the

germ, stunts plants by root

pruning or kills the growing

point by boring into the base of the stem near ground

level. Modern seed treatments prevent damage to seeds.

Like grubs, there is no way to combat this pest after

stands have been thinned. Use percent stand loss as a

gauge for determining if replanting is worthwhile.

Photo Citations:

Frank Peairs, Colorado State University, Bugwood.org (viewed9/23/10) Armyworm 5364223

Clemson University - USDA Cooperative Extension Slide Series,Bugwood.org (viewed 9/23/10) Billbug 1234011

University of Georgia Archive, University of Georgia, Bugwood.org (viewed 3/11/2011) Grasshopper 4709020

David Riley, University of Georgia, Bugwood.org (viewed

9/23/10) Lesser Cornstalk Borer 2511032

Alton N. Sparks, Jr., University of Georgia, Bugwood.org (viewed

3/11/2011) White grub (scarab) 1327103

Corn earworm damage


42/60

Below

Ground

Western corn rootworm larvae Trait Trait Trait NC

Northern corn rootworm larvae Trait Trait Trait NC

Mexican corn rootworm larvae Trait Trait Trait NC

Wireworm (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

White grubs (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

Grape colaspis (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

Seedcorn maggot (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

Color Key to Activity Insect activity conferred by:

= Control Trait, Insect icide 250 rate, or multip le

= Suppression Trait, Insect icide 250 rate, or multip le

= No Control NC = No Control

Protection for Above and Below Ground PestsProtection forAbove Ground

Pests

Trait NameGenuity

SmartStaxGenuity VT

Triple PROYieldGard VT

TripleGenuity VT

Double PRO

Trait Logo

Seed Treatment Poncho 250

Insects

AboveGround

European corn borer Trait Trait Trait Trait

Southwestern corn borer Trait Trait Trait Trait

Corn earworm (ear feeding) Trait Trait Trait Trait

Western bean cutworm Trait NC NC NC

Black cutworm Trait/250 Rate 250 Rate 250 Rate 250 Rate

Fall armyworm Trait Trait Trait Trait

Sugarcane borer Trait Trait Trait Trait

Common stalk borer Trait Trait Trait Trait

Lesser cornstalk borer Trait Trait Trait Trait

Corn flea beetle (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

Corn leaf aphid (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

Chinch bugs (seedling stage) 250 Rate 250 Rate 250 Rate 250 Rate

Southern corn leaf beetle(seedling stage)

250 Rate 250 Rate 250 Rate 250 Rate

Corn Insect Control with Monsanto Corn Insect Traits

and Seed Treatments



43/60

40

ACCase InhibitorsAryloxyphenoxy Propionic Acids and

Cyclohexanediones

(Including AssureII, Fusion, Poastand Select Max)

Symptoms of damage from

acetyl coA carboxylase

inhibitors include chlorosis

(yellowing) of newly formed

leaves with possible

reddening or purpling ofolder leaves. Tissues at the

growing point turn brown

and eventually decompose,

a symptom called deadheart.

Sublethal doses result in pale white to yellow streaks

between leaf veins.

ALS InhibitorsImidazolinones, Sulonylureas, andTriazolopyrimidines(Including Accent Gold, Beacon, Classic, FirstRate,

Lightning

, Pursuit

, and Scepter

)

These herbicides alter the

function of the acetolactate

synthase (ALS) enzyme and

affect root and shoot growth

and development. Corn

injury typically shows up as

reduced root systems, often

described as bottle-brush

roots. Roots often grow at

or parallel to the soil surface

and may turn brown. Stems andmidribs can purple, and the stem will be

short and thick below the whorl. Mid

to late-season symptoms include short

internodes, malformed leaves, poor root

systems, and pinched ears.

Herbicides with sulfonylurea chemistry

are labeled for use on both corn and

soybeans. Products in this family may

persist in the soil, particularly if soil

pH is above 6.8 and post-application

rainfall has been limited. Thoughthey have the same general chemistry,

different herbicides are used on corn

and soybeans, and injury of corn

may develop the season following

sulfonylurea herbicide application to

soybean elds.

Most observed herbicide damage is due to misapplication, but hybrid and environment interactions willsometimes cause injury symptoms to appear. Herbicides used to control weeds in the previous crop must betaken into consideration, as some herbicide carryover may affect the following corn crop.

Various tankmixes of herbicides are widely used. Be alert to plant injury that can be caused by individualtankmix components or by an interaction involving two or more components in the tankmix itself.

Corn plants frequently outgrow the effects of herbicide injury and their nal yields may not be noticeably

lowered.

7Herbicide InjurySymptoms


44/60


Synthetic AuxinsBenzoic Acids, Phenoxys, and PyridineCarboxylic Acids

(Including Banvel, Butyrac, Clarity, and Crossbow)

These herbicides, also

known as plant growth

regulators, are translocated

to meristematic tissue and

interfere with cell formation

resulting in abnormal root and

shoot growth. Symptoms

include twisted whorls or

buggy-whipping where

the leaves do not unfurl,

abnormal brace roots,

root proliferation, brittle

stalks, and poor pollination.

Growing conditions

are a factor in damage

expression.

Auxin Transport Inhibitors

Semicarbazones(Including Distinctand Status)

These herbicides are primarily active against broadleaf

plants, but injury symptoms may occur in corn under

certain conditions. Injury is similar to growth regulator

herbicides.

Carotenoid Synthesis InhibitorsIsoxazolidinones

(Including Command)

Shallow planting or stress

conditions that slow seedling

metabolism can increase the

potential for injury. Damage

to corn seedlings the season

following an application may

occur, especially if soil pH is

below 6.0. Affected plants are

distinctly white or bleached.

Most corn plants recover and

new leaves have normal color.

Cell Division InhibitorsAmides

(Including Degree, Dual II Magnum, and Harness)Amides (also known as

acetanilides, acetamides,

or chloroacetamides) are

meristematic growth inhibitors

that are translocated to the

shoot and leaves. These

products can cause seedlings

to leaf underground or trap

seedling leaves so they

cannot unfurl, giving a ladder

effect. Damage is more likely to take place with cool, wetweather occurring immediately before emergence.

EPSP Synthase InhibitorsGlycines(Including Roundup agricultural herbicides and

generic ormulations o glyphosate)

These non-selective foliar-

applied herbicides are

translocated and interfere

with amino acid synthesis.

If spray drift reaches corn,

leaves wilt, turn brown,

and die. Sublethal rates

can cause phenoxy-like

symptoms. Glyphosate applications that occur too late to

corn with Roundup Ready 2 Technology can result in poor

kernel set.

Glutamine SynthaseInhibitorsPhosphinic Acids(Including Igniteand Liberty)

Inhibitors of glutamine synthase

are nonspecic and will kill

plants that have not been

modied to express resistance

to them. Symptomology will


45/60

42

range from light speckling on leaves and yellowing to

plant death,depending on the degree and intensity of d

corn diagnostic guide

Documents