9/22/2015

1

Can You Match Injury Symptoms

with Herbicide Mode of Action?

“Hey Joe,

What happened to your field?”

Plan of Action

• The herbicide classification chart is on

page 98-99 of the Weed Guide

• Approach Site of Action from

symptomology point of view

• Relate to plant physiology

Growth Regulator Herbicides

• 2,4-D, dicamba, Stinger, Curtail, Reclaim

• Activity on broadleaf plants

• Most notable for nastic response

– “Pressed close”

– Caused by unequal growth

• Epinasty

– Plant parts curve downward (2,4-D)

• Hyponasty

– Plant parts curve upward (dicamba)

Dicamba Injury to Corn

• Short brace-

roots

• Fused brace-

roots

Growth Regulator Injury

• Onion-leafing

• Buggy-whipping

• Sleepy wheat with

dicamba

• Deformed small

grain heads with

phenoxys

• Witches broom on

broadleaf plants

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2

Cell Division Inhibitors

• “Yellow” herbicides, trifluralin, Trust,

Prowl, Sonolan

• Activity on germinating grasses and

small-seeded broadleaf plants

• Do not translocate

• Inhibit mitosis

– Chromosomes do not separate

– Cell elongation inhibited

Inhibit Cell Wall Development

• Cell wall strands

develop

perpendicular to

elongation

• Herbicide causes

multidirectional

strand development

Root Injury with Mitosis Inhibitor

• Club-root

• May find

callous ring on

broadleaf

plants at soil

line

• Callous stem

may become

brittle

Trifluralin on Oat

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3

Lipid Inhibitors

• ACCase inhibitors

• Very Long Chain Fatty Acid Inhibitors

ACCase inhibitors

• Puma, Discover, Poast, Select, Assure II

• “Fops”, “dims”, and now “dens” (Axial)

• Activity on grasses

• Prevent production of membranes

• Major activity at growing point

ACCase Inhibitor Injury

• Meristem dies

• Inner whorl can

be pulled from

the plant

Poast Injury from Off-Target Drift

• Bleached tissue

where whorl

opens

Very Long Chain Fatty Acid

Inhibitors (VLCFA)

• EPTC, Harness, Dual, Outlook

• Activity on grasses and small-seeded

broadleaves

• Affect the formation of cuticle waxes

• Seedling shoot inhibitors

EPTC on Oat

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4

Chloroacetamide Injury

• Shortened

midvein

results in

heart-shaped

leaves

Amino Acid Inhibitors

• ALS inhibitors

• EPSPS inhibitor

• Glutamine synthetase inhibitor

ALS Inhibitors

• Wide range of products and selectivity

– Sulfonylureas (Escort, Express, Steadfast)

– Imidazolinones (Raptor, Plateau, Assert)

– Triazolopyrimidines (Python, Firstrate)

– Sulfonylaminocarbonyl-triazolinones (Everest)

• Inhibit branched-chain amino acids

• Synthesis of proteins stops

• Growth ceases quickly

• Chlorosis begins at growing point

Red Coloring with ALS Inhibitor

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5

Steadfast after Organophosphate EPSPS Inhibitor

• Glyphosate

– One chemical, so many products

• Selective to crops and weeds with

resistance

• Inhibit aromatic amino acids

• Growth stops but plants linger

• Shikimate reaches toxic levels, plants die

• Low dose may cause shoot proliferation

or curved growth in broadleaf plants

Glyphosate

Drift

• Chlorotic

streaks near

throat of whorl

Glutamine Synthetase Inhibitor

• Glufosinate, Liberty, Rely, Finale

• Generally non-selective except for

resistant crops

• Non-translocating

• Inhibit nitrogen incorporation into

proteins

• Ammonia toxicity

• Translucent tissue in a day or two

• Wilting in three to five days

Liberty Injury

• Injury progresses to necrosis in a week or less

• May observe strong shadows behind leaves because of translucent symptomology

Photoststem Interactions

Light is necessary for activity

Low light intensity delays activity compared

to bright light

• Photosystem II inhibitors

• Pigment inhibitors

• Photosystem I inhibitors

• PPO inhibitors

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6

Photosystem II (PS II) Inhibitors

• Atrazine, Sencor, Betanex, Basagran,

Buctril

• Main activity on broadleaf plants

• Some products translocate in xylem (water

stream), others do not move

• Stops energy flow through PS II

• Redirected energy destroys chlorophyll

and membranes

Xylem-Mobile PS II Inhibitors

• Intervienal Chlorosis

• Necrosis beginning at leaf margins on older

leaves

Atrazine on Cocklebur Pigment Inhibitors

• Command, Balance Pro, Callisto, Impact

• Main activity on broadleaf plants

• Soil and foliar activity

• Inhibit carotene that protects the plant

from bright light injury

• Plants can’t protect themselves from

normal energy flow

• Excess energy destroys chlorophyll and,

eventually, membranes

Pigment Inhibitor Symptoms

• Injury usually

on young

tissue first

Callisto Injury to Corn

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Callisto Injury to Corn Photosystem I (PS I) Inhibitors

• Gramoxone Inteon, Reglone

• Not translocated

• Non-selective

• Very quick acting, within hours in daylight

• The herbicide harvests energy from PS I

and hands it to molecules that destroy

membranes

• Cells leak fluids so quickly that tissue

gets watersoaked appearance

PPO Inhibitors

• Aim, ET, Cobra, Reflex, Ultra Blazer

• Most non-translocating

• Spartan, Payload, Valor are xylem-mobile

• Herbicide stops production of chlorophyll

• Chlorophyll precursors go where they

don’t belong in the cell

• Molecules generate energy that destroys

membranes

Cobra Aim

• Similar to non-

translocating PS II

inhibitors, non-

lethal injury is

expressed as very

discrete necrotic

lesions

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8

Plumbing Analogies for

Photosynthesis Interactions

• PS II – pipes freeze and the house floods

• Pigment – the bucket underneath a leaky

pipe is not big enough to handle the drips

or it also has a leak in it

• PS I – that pesky neighbor kid comes in

with a pail and dumps water in your living

room

• PPO – you put a water feed into your

bedroom and forget to put a faucet on it

Photo and Movie

Acknowledgement

• Agriliance

• Syngenta

• Oregon State University

• Weed Science Society of America

– http://www.wssa.net