identifying and distinguishing seedling and root rot diseases of sugar beets

8/12/2019 Identifying and Distinguishing Seedling and Root Rot Diseases of Sugar Beets

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ImpactStatement

2006 Plant Management Network.Accepted for publication 7 June 2006. Published 15 September 2006.

Identifying and Distinguishing Seedling and Root

Rot Diseases of Sugar Beets

Robert M. Harveson,Panhandle Research and Extension Center,

University of Nebraska, Scottsbluff 69361

Corresponding author: Robert M. Harveson. [email protected]

Harveson, R. M. 2006. Identifying and distinguishing seedling and root rotdiseases of sugar be ets. Online. Plant Health Progress do i:10.1094/PHP-2006-0915-01-DG.

IntroductionSugar beets are susceptible to a number of seedling and root rot

diseases that are primary constraints to profitable sugar beet production

(5,11,1 4,15,33,43). The majority of the seedling diseases are caused by

soilborne pathogens, butPho ma b eta eis a seedborne fungal pathogen that

can cause both seedling andmature root problems during the season

(19,20). Since numerous other environmental and cultural factors may

also cause symptoms that could be easily confused with symptoms c aused

by these pathogens, it is impor tant to be able to co rrec tly identify and

differentiate between the various diseases and problems in order to most

effectively manage them.All o f the seedling patho gens can addit ionally be inv olv ed with roo t ro t

diseases throughout the season; however only the most prevalent and

important root rot diseases will be addressed. For example, two of the

seedling pathogens (Pyth ium andPho ma) can cause ro ot rots later in the

season (21,23,33,43), but will not be discussed furtheras their impact isgenerally minor in comparison with the other more c ommon roo t-rotting

pathogens. The two most important pathogens causing both seedling and

root ro t diseases areRhizo ct onia so laniandAphano myce s coc hlio ides

(16,33,43). Three additional root ro t diseases not generally considered to

cause serious seedling problems will also be discussed, including Fusarium

y ello ws, Fusarium ro ot ro t, and rh izomania. The go al o f this publicatio n isto help others learn to identify and effectively differentiate the various
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Fig. 2. Aphanomyces seedling diseasesymptoms (right) consisting of thinthread-like stems without wilting o f

seedling and root ro t diseases common to sugar beet.

Seedling Diseases

Disease: Damping-off, Black Root ComplexPrimary hosts. Sugar beet (Beta v ulgaris L.).

Pathogens.Rhizo ct onia so laniKuhn, teleomorph: Thanatephorus

cucumeris(A. B. Frank) Donk,Aphano myce s coc hlio idesDrechs.,PythiumultimumTrow,Pyth ium aphanidermatum(Edson) Fitzp.,Pho ma

betae(A. B. Frank), teleomorph: Pleo spo ra bjoerlingiiByford.

Symptoms.R. solanicauses both pre-and post-emergence damping-

off, but is most often observed causing disease on emerged seedlings

(16,20,33,41,43). Evidence of infection begins as dark brown lesions

be low the s oil sur face and pr ogressing up the hy poco ty ls, o ften r esulting

in wilting and complete collapse of cotyledons and death of plants (Fig. 1).Infection does not occur in soil temperatures below 15C, but can occ ur at

anytime abov e 20C (20).

Fig. 1. Permanent wilting and death ofseed lings, characteristic of both

Rhizoctonia s olaniand Pythiumspp.

A. coc hlio idesdoes not generally

affect seedling emergence or initial

stand establishment. The damage it

causes is primarily post-emergent

and is favored b y warm soil

temperatures ranging from 20 to

30C (20,29,30). Symptoms begin

near the soil line as water-so aked

lesions that progress from gray to

black. Stems be co mecharacteristically dark, thin, and
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cotyledons. thread-like with an absence o f

cotyledonary wilting (Fig. 2), which

are the major criteria for distinguishing damage fromR. solaniinfections

(7 ,16,33,43). Plants may reco ver, but stands may still be impacted later as

affected plants are often more susceptible to breakage and death from

wind damage due to the weake ned, de licate stems.

P. ultimu mandP. aphanide rmat um are thosePyth ium species most

often associated with seedling problems (20).P. ultimu mis considered to

be a coo l-weathe r patho gen, re spo nsib le for se ed r ot and p re-emergencedamping-off at temperatures below 20C (16,20).P. ap hanidermat um, in

contrast, is a warm-weather pathogen favored by soils ranging from 30 to35C (16,20,23,42). Post-emergent damping-off symptoms of either

Pythiumspecies are indistinguishable from those o fR. solani, and consist

of wilting, lodging, and death of seedlings (Fig. 1).

Altho ugh p re-emergence damping-off due t oPho ma b etae can occur if

infested seeds are planted into co ol wet soils (4 to 12 C) (19), the majority

of damage occurs after emergence in soils at temperatures of 16 to 20C

(19,20). Symptoms consist of dark brown to black necrosis of hypoco tyls

(Fig. 3), with the diseased tissue often containing small black py cnidia

(200 to 325 m). Little infection occ urs above 25C (16,19,20). A ffectedseedlings may often survive and reco ver to vary ing degrees, but the

pathogen is also capab le of persisting in the crowns and causing leaf spots,

root rots, and storage rots in harvest piles later in the season (19,20,21).

Fig. 3. Symptoms of post-emergentdamping-off due to Phoma betae.

Host range.Rhizo ct onia so laniand bothPythiumspecies have wide

host ranges, consisting of numerous crop and weed species (2,9,23). The

anastomosis gro ups (AGs) ofR. so laniassociated with sugar beets include

AG2-2 and A G4 (2,3,35 ). See dling disease has mo st o ften b een att rib ute dto AG4, but isolates belonging to AG2-2 have also been isolated from
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affected seedlings (35).Aphano myce s coc hlio idesandPho ma b etae are

primarily limited to infecting plants in the Chenopodiaceae, and the

closely relatedAm arantha ce ae (19,29).

Geographic distribution. All pathogens are found worldwide

wherev er sugar b eet is gr own.

Pathogen isolation. All pathogens are easily isolated from diseased

seedlings and grown in pure culture. Several selective media are available

forAp hanomyce s(30,31) andPythiumspp. (23,25); however the

pathogens can also b e rapidly identified from diseased plant tissue inwate r c ultures o r lo w-nutrient m edia suc h as water agar o r -stre ngth

potato dextrose agar (PDA). After 24 to 48 h, the characteristic vegetativemyc elium along with spores or fruiting bodies c an be rec ognized under

low magnification (50-100), even though multiple pathogens may be

simultaneously colo nizing seedlings (Fig. 4) (20). The plant pathology

progr am at Scottsbluff, NE routinely uses -strength PDA amende d with

streptomy cin sulfate (0.5g/liter). Best results are seen after washing

seedlings in water before pro ceeding. Surface sterilizing diseased seedlings

has often been shown to prohibit or reduce the likelihood of pathogen

isolation at this stage because o f the limited amount of tissue being

disinfested.

Fig. 4. Infected seedling in waterculture. NoteAphanomycesencystedzoospores (clusters) and lobatesporangia of Pythiumsp.

Pathogen identification.A. coc hlio idesgrowth in water cultures

consists only of unbranched, cy lindrical tubes (6-8 400-1000 m)(29,43). These hyphal tubes serve as zoosporangia, and zoospore initials


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emerge from the tips of the hyphae arranged in a single row. They collect

as spherical, ency sted spores (usually 7 to 1 0 m in diameter) at the tips

of the sporangia (Fig. 5) and later (1 to 3 h) emerge as secondary

zoospores from the primary zoospore cy sts and swim in the surrounding

wate r (2 0,29,30,45 ). This ent ire proc ess will o cc ur within 1 2 to 14 h at

room temperature, and is diagnostic forA. coc hlio ides.

Fig. 5. Encysted zoospores ofA.cochlioidescollecting around tips ofsporangia. Secondary zoospores are

released (left), leaving empty cysts.These zo ospores subsequently serve asnew inoculum and infect sugar beetroots.

Within the c or tex of y oung se edlings or in c ultu re,A. co ch lioides

produces abundant nonseptate, but mo rphologically distinctive my celium(Fig. 6) (29,30). Macroscopically this distinctive hyphal growth on PDA

appears cur ly at the edge of colo nies (Fig. 7 , left) and grows more slowly

(20 to 22 mm/day) at room temperature thanR. so laniandPythiumspp.

On theAphano myce s-selective medium, metalaxy l-benomy l-vancomyc in

agar (MBV) (31), the hyphal growth is not curly, but grows flat and

be neath the agar s urface wit h litt le o r no aer ial myce lium .

Fig. 6. Morphologically distinctive,coenocytic hyphal growth ofA.

Fig. 7. Characteristic macroscopic growthof A. cochlioides (left) and R. solani
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cochlioides. (right) growth in culture on PDA after 2days. Infected seedlings were platedsimultaneously.

In water culture, colony growth ofR. solaniprimarily floats and

spreads widely across the surface (20). No spores are formed, but the

myc elium is distinctly septate, with cross-walls located just abov e and on

the hyphal branches (2,41) (Fig. 8). A similar rapidly spreading pattern of

growth is observ ed in culture (Fig. 7 , right), and most pathogenic isolates

grow rapidly (27 to 30 mm/day ) on all substrates.

Fig. 8. Characteristic growth of R. solani.Note septations and right anglebranching of hyphae.

Fig. 9 (A) Spherical, terminal spo rangia,characteristic of Pythium ultimum. (B)Aplerotic oospore characteristic of P.ultimum(courtesy of G. Abad).

In water c ultures,P. ultimumgrows from seedling tissue and produces

abundant branched, nonseptate myc elium 1.7 to 6.5 m in diameter

(20,23,42), primarily b elow the surface o f the water, as opposed to that of

R. so lani. After 24 or 48 hr, spherical terminal sporangia (12 to 28 m in

diameter) (Fig. 9A) and barrel-shaped intercalary sporangia (17 -23 28 m) are produced. This species is characterized by mo noclinous

antheridia, most originating immediately below terminal oogonia, and

thick walled, aplerotic oospores, 1 4.7 to 1 8.3 Fm in diameter (Fig. 9B)

(42).

In water culture, tissue infected byP. ap hanidermat um yields

nonseptate my celium 2.8 to 7 .3 m in diameter, and later comparatively

large, inflated filamentous or lobed zoosporangia of varying lengths (50 to

1000 m) (Fig. 10) (20,42). Reniform zoospores (7 .5 1 2 m) are formed

within v esicles attac hed to the lob ed sporangia. Zoo spo re s ex it the v esicle(Fig. 10 inset), swim for a period, and then encyst and germinate by the

production of germ tubes.P. aphanide rmatum is additionallycharacterized by terminal, smooth oogonia (20 to 25 m), mostly


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intercalary antheridia (Fig. 11), and aplerotic oospores (Fig. 12) 17 to 20

m in diameter (23,42).

Fig. 10. Lobate sporangia characteristicof P. aphanidermatum. Note emptysporangium (inset).

Fig. 11. Terminal, smooth oo gonia andintercalary antheridia characteristic of P.aphanidermatum .

Fig. 12. Aplerotic oospore andintercalary antheridium characteristic ofP. aphanidermatum(courtesy of J.Taylor).

Oospores, in some instances, will form in diseased tissues (Fig. 13) and

culture media. However, the formation of other reproduc tive structuresneeded for identification of bothPythiumspecies (sporangia, oogonia,

antheridia, and oospores) are more consistently induced through the use

of grass blade culture s. Briefly, this entails using grass blades cut into 1 -cm

pieces, sterilized by boiling in water for 10 min, and incubating in petri

dishes with deionized water and agar blocks of desired cultures at room

temperature (23,42). Cultures are then viewed at desired intervals toobserve formation of v arious structures.


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Fig. 13. Oospores of Pythiumspp.formed in sugar beet root tissue.

BothPythiumspecies additionally grow very rapidly (40 to 45

mm/day) on c ommon media such as PDA, and produce branched,

coenocy tic my celium (Fig. 14). Initially, the myc elial growth is beneath

the agar surface, but after colonies reach edges of plates, growth results in

thick cottony aerial my celium. After several weeks, this dense mycelial

growth collapses and cultures tend to die readily unless continually

transferred or stored in some manner. Growth on Pythium-selectivemedia such as Mircetich medium is inhibitory (25), and similar to that of

A. coc hlio ideson MBV (31). Hyphal growth is closely appressed to the

agar surface with little aerial mycelial growth.

Fig. 14. Hyphal growth typical of Pythiumspp., consisting of branched, coenocyticmycelium.

Pho ma b etae is not as easily identified in water culture as the above-

described pathogens, although pycnidia may sometimes be o bserved o n

some seedlings (19,20). However, growth of P. betae on water agar results

in distinctive structures resembling holdfasts or appressoria on the

bo tto m of a plastic Petr i dish (20 ,22). By e xamination of th e inv erted dish


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under a low-power microscope, the presence or absence o f the pathogen is

v ery easily co nfirmed (Fig. 15).

Fig. 15. Holdfasts observed on bottomof Petri dish, characteristic of Phomagrowth on water agar.

Growth ofP. betae on -PDA c ultures is easily distinguished from the

previously described fungi by producing grayish-black colored aerial

colonies as seen from above, and black when viewed from the underside ofplates (40).P. be tae also produces py cnidia (225 to 325 m in diameter)

in concentric rings in culture (19,20,40). One-celled, hyaline

pyc nidiospores (2.6-4.3 4.1-7 .0 m) exude from mature pycnidia.Pycnidia and spores vary greatly in size due to env ironmental influences

(20,40).

Pathogen storage.Aphano myce sandPythiumare short-lived in

culture and are thus most effectively stored as oospores in soil samples.

Fresh isolates should be subcultured from the leading edge of colonies and

incubated in oatmeal broth (23,37). Oatmeal broth is made by boiling 5 g

of rolled oats in 150 to 200 ml water for 5 min. Oats are removed with

cheesecloth, and the remaining volume of bro th is brought up to 5 00 ml.Fifty ml of broth is aliquoted into 125 ml flasks, autoclaved, and

inoculated with the pathogen after cooling. After 4 to 6 weeks, contents of

flasks are ground in a blender, oospores enumerated with a

hemacytometer, and put into sterilized soil. These samples serve as stock

cultures, and can be diluted as needed to reco ver isolates, or included as

known concentrations in experiments. Samples can be kept at room

temperature for several years (R. M. Harveson, unpublished).

For thosePythiumisolates from which oo spores are unable to be

produced, c ultures may also b e stored o n PDA-mineral oil slants, or ascolonized hemp seeds, wheat leaf pieces, or agar blocks in deionized water

(23).Ap hanomyce scultures may also be maintained on PDA or oat meal

agar (OMA) slants at 12C with periodic hyphal transfers (ev ery 3 to 4


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months) (30).

Pho ma isolates may also be stored by transferring water cultures to

sterilized soil. After cultures have dried, they may be stored at 4C

(21,40).Rhizo ct oniaisolates can be grown on various autoclaved seeds,

and stored at 4C. Barley, corn, or sugar beet seeds have been successfully

used to store isolates (2,35). Additionally, dried c ultures can be cut into

pieces and stor ed in vials at 4C for at least 4 ye ars (R. M. Harv eson,

unpublished).

Pathogenicity tests. Inoculum production of individual pathogensfor pathogenicity testing consists primarily of the abov e-described

storage methods.Pyth ium andAphano myce sare most efficiently testedby infesting soils with o osp or es and planting susc ept ible sugar beet

cultivars into them (23,29). However, A. co ch lioidesoospores will not

germinate in culture, so recov ery or quantification must occur through

baiting with liv e plants (8,30). Patho gens can also be use d for

pathogenicity testing through production (via water, grass blade cultures,

or minimal salts solutions) and inoculation with zoospores (23,26,30,42).

For optimal disease development, both pathogens must also be incubated

within v ery moist so ils and at appro priate soil te mpe rat ures.

Because of a lack of spore production, inoculum ofRhizo ct oniacannotbe easily quantified (2,30 ,32,4 3). Therefore it is gene rally incre ased on

substrates such as seeds o r co rnmeal and sand, and then quantified as

grams of infested material or number of seeds for inoculation purposes

(2). The colonized substrate can be incorporated into test soils or placed

in contact with sugar beet plants (32). Incubation conditions should also

coincide with warm, moist conditions.Pho ma cultures likewise may b e

grown on autoclaved seeds for inoculation purposes, or c onidial

suspensions from sporulating pycnidia may also be utilized (21).

Root Rot Diseases

Disease: Rhizoctonia Root Rot, Root and Crown Rot

Primary host. Sugar beet (Beta v ulgaris L.).Pathogen.Rhizo ctonia so laniKuhn, teleomorph: Thanatephorus

cucumeris(A.B. Frank) Donk.

Symptoms. Two different phases of root disease have been reported,

one being infections beginning in the crown, accompanied by petiole

blacke ning and deat h (cro wn rot) (Fig. 16), o ften asso ciated wit h so il

thrown up into crowns during early cultivations (32,38). The other phase

involves earlier infections occurring on taproots as a tip rot (root rot), and

progressing toward the root cro wn (Fig. 17 ) (14,15,1 6,33). Rootsymptoms begin as circular to oval, localized, dark lesions that coalesce to


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form larger rotted areas of the root as disease progresses (Fig. 18). The

extent of rotted tissue often is restricted to ex ternal layers of the root

(Fig. 19) and does not generally penetrate into the interior until very

advanced stages of disease (11 ,15,1 6,33). Both phases involv e the same

type of foliar symptoms, including sudden, permanent wilting and

complete co llapse and death of leaves and petioles (11,15 ,16,32,33,38)

(Fig. 20).

Fig. 16. Crown rot phase of Rhizoctoniaroot rot. Note infection in bla ckened

petioles progressing ou tward fromcrown.

Fig. 17. Root rot phase ofRhizoctonia root rot showing tiprot symptoms a nd diseaseprogression from the distal tiptoward the crown.

Fig. 18. Circular to oval lesion scoalescing on tap root,characteristic of Rhizoctonia rootrot. Note lack o f evidence o finfection in crowns.


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Fig. 19. Severely affected root infe ctedwith R. solani, showing rotting ofexternal cortical tissues, with limitedinternal penetration.

Fig. 20. Perma nent wilting, collapse ofleaves and petioles, and death ofplants infected with R. solani.

Host range. The host range for R. so laniis very wide (see above), but

AG 2-2 is the one mo st o ften associate d wit h ro ot rot infe ct ions of mature

roots (33,35). Members of this AG have also been reported causing diseasein dry-edible beans (3).

Geographic distribution.R. so laniis found worldwide wherever

sugar beets are grown.

Pathogen isolation. Small root or c rown pieces on the margins

be tween he alth y and r otted tissues can be surface-st erilize d and plated

onto common media (water agar or PDA). Observe cultures and watch for

distinctive, spreading hyphae with right-angle branching.

Pathogen identification. See pathogen identification procedu res

forR. so laniseedling disease above.

Pathogen storage. See pathogen storage procedures forR. so lani

seedling disease abov e.

Pathogenicity tests. Testing procedures for Rhizoctonia root rotwill b e the same as for the see dling diseas e.

Disease. Aphanomyces Root Rot

Primary host. Sugar beet (Beta v ulgaris L.).

Pathogen.Apha nomyce s coc hlio idesDrechs.

Symptoms. Foliar symptoms consist of stunted, yellowed leaves with

non-vigorous growth (Fig. 21) (7 ,11,1 4,16,29,33,43,45). Wilting may also

occur during the day, particularly o n warm days, but plants often recov erat night. However, permanent wilting is not common, in contrast with


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Rhizoctonia root ro t. Leaves may also take on a scorched appearance and

be co me b rit tle . Roo t sy mpt oms be gin as y ello wish-br own, water -soaked

lesions on taproots (Fig. 22) that later become dark and necrotic as they

dry (7 ,14,15 ,16,33,43). These lesions can occur any where on the taproot,

but often o cc ur toward the dist al end as a tip ro t (Fig. 23 ). I f

environmental conditions become more favorable for plant growth, plants

may reco ver to produce a relatively healthy crop. However, many roots

may still exhibit vary ing degrees of root distortion and/or scarring

(Figs. 24 and 25) (7 ,14,16,33,43,45). Severe disease can c ompletelydestroy taproots leaving little except for crowns (Fig. 26), y et often

maintain deceptively healthy-looking tops. Economic loss may also o ccurin several different ways. Field loss occ urs when severely diseased roots

are easily dislodged from soil and thrown into furrows during the

defoliation process and are not subsequently retrieved by the harvester

(Fig. 27 ) (14). Furthermore, infected roo ts also continue to rot in storage

piles after harvest, which interferes with optimal extraction of sucrose.

Fig. 21. Foliar sym ptoms characteristicof Aphanomyces root rot.

Fig. 22. Young actively-expanding,

yellowish-brown, water-soaked lesionson tap root by A. cochlioides.

Fig. 23. Root infection by A. cochlioides

occurring as a tip rot, accompan ied byproduction of secondary rootlets ascompensation for damaged taproot.


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Fig. 24. Mild scarring and d istortion(inset) of roots due to prior infection byA. cochlioides.

Fig. 25. Severe scarring a nd disto rtionof roots due to p rior infection byA.cochlioides.

Fig. 26. Severe rotting of roots byA.cochlioidesleaving little tissue exceptcrowns. Root in the middle is unaffectedas a comparison.

Fig. 27.Aphanomyces-infested field atharvest (irregularly distributed rootswith crowns of varying height) and theseverely scarred a nd distorted rootsbroken off at ground level afterdefoliation (inset).

Host range. Economic problems fromA. coc hlio idesare primarilylimited to plants in the ChenopodiaceaeandAm arantha ce ae ; however,

several other plant families (mostly weed species) have been repo rted

from greenhouse studies as hosts, includingAioace ae , Caryophyllaceae,

Hydro phy llac eae,Linac eae,Pape ravac eae, andPort ulacace ae (29).

Geographic distribution.A. coc hlio idesis found worldwide

wherev er sugar b eets ar e gr own.

Pathogen isolation. If root lesions are actively growing (Fig. 22),

successful isolations may be made on various media, including MBV

(30,31) or PDA. However, the pathogen will not grow from old lesions orthose from dried, scabby-scarred ro ots (Figs. 24 and 25), as o ospores are

formed in tissues and will not germinate in vitro. Shav ings from the scabby

lesions can readily be used in the greenhouse to bait the pathogen out on
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sugar beet seedlings (8). The shavings are plac ed into soils with the seeds

and then isolation is performed as described earlier from emerged,

diseased seedlings.

Pathogen identification. See pathogen identification procedu res

forA. co ch lioidesseedling disease above.

Pathogen storage. See pathogen storage procedures forA.

cochlioidesseedling disease above.

Pathogenicity tests. Testing procedures for Aphanomy ces root rot

will b e the same as for the see dling diseas e.

Diseases. Fusarium Yellows and Fusarium Root Rot

Primary host. Sugar beet (Beta v ulgaris L.).

Pathogens.Fusarium o xysporum Schlect.f. sp. betaeSnyd. & Hans.

andF. oxysporum f. sp. radici-betae .

Symptoms. Foliar symptom s of both Fusarium yellows and Fusarium

root rotdiseases are similar, including interv einal yellowing (Fig. 28),

general chlorosis, wilting (Fig. 29), and scorched, brittle leaves (Fig. 29

inset) (11,1 2,13,33,43). The leaf scorching may be c onfused with thatcaused byA. co ch lioides. However, bothFusarium pathogens additionally

cause internal nec rosis of vasc ular elements (Figs. 30 A and B), which is

characteristic for both diseases, effectively eliminating the possibility of

either Rhizoctonia or A phanomyces roo t rots (14,15 ,33,43). The primary

difference between the two diseases is the dark external cortical rot of the

distal tip of the taproot associated with Fusarium root rot (Fig. 31)

(5,12,13 ,24,33,43). Fusarium yellows sy mptoms are limited to foliar

wilt ing, inte rv einal y ello wing, and v asc ular discolo rat ion, bu t no ro tting of

the taproot (5,12,13,33).

Fig. 28. Interveinal yellowing of le aves,

characteristic of Fusarium yellows o rFusarium root rot.

Fig. 29. W ilting, s corched, and brittle

leaves due to Fusarium yellows orFusarium root rot.


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Fig. 30. Necrosis o f vascular eleme nts(limited damage on left, severe onright) diagnostic for Fusarium yellows orFusarium root rot.

Fig. 31. Severe external, cortical rootrot characteristic for Fusarium root rot.

Host range. Many isolates are limited to sugar beets, but some have

be en demonstrate d to cause dise ase in other plant spec ies within theChenopodiaceaeandAm aranthace ae , such as spinach (Spinac ia

oleraceae) and red roo t pigweed (Amara nthus re troflexus) (12,24,39).

Geographic distribution. Fusarium yellows is a common problem

throughout the production areas of the western United States (33,39,43).

It has also rec ently b een found in the North Dakota and Minnesota

growing regions (Red River V alley) (17 ), and can cause stalk blights of

seed crops in Oregon (39). Before 2006, the known distribution of

Fusarium root rot had been limited to sugar beet production fields in

Texas (12,13,24). However, isolates causing root ro t in sugar beets have

recently been identified from Colorado and Montana (6).

Pathogen isolation. Both Fusarium pathogens are readily isolatedfrom host tissue, plant debris, or soil (12,13,24,44). Isolation frominternal root tissue is easily accomplished with surface disinfestations

using 10 % ETOH and 1 to 5% NaOCl, followed by plating on numer ous

different med ia. Several v ariations of pepto ne PCNB agar are useful for soil

isolations (27 ). Komadas medium (18) is also selective for F. oxyspo rum

from soil or p lant tissue, but these media do not differentiate between the

pathogenic isolates and the ubiquitous saprophytes.

Pathogen identification. The two pathogens are similar

morphologically. The Fusarium yellows pathogen produces sickle-shaped

macroco nidia measuring 3.5-5.5 21-35 m, and straight to slightly

curved microco nidia (2.5-4.5 6-15 m) (39). Globose chlamydospores

can be either intercalary or terminal measuring 7 to 11 m (39). The


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Fusarium root rot pathogen generally produces sparse numbers of

macroco nidia, but does pro duce microc onidia (3-5 8-10 m) in false

heads and chlamydospores (4-7 .5 20 -30 m) (24).

Pathogen storage.Long-term storage ofF. oxyspo rum isolates is

be st ac co mplishe d by ly ophilization o f single-sp ored cultur es in 50%

glycerol or on colonized carnation leaf pieces in sterile skim milk

(1,28,44). Cultures may also be stored for shorter periods of time on

co lonized dried filter paper c ut into pieces and kept at 4C (12). Cultures

may also be stored as c hlamydospores in sterile soil, but some isolatesmay mutate or lose viability over time (44).

Pathogenicity tests.Pathogenic isolates ofFusarium oxysp orumare indistinguishable morphologically from saprophytic ones; therefore

pathogenicity determinations are particularly important for these

pathogens. Mass production of isolates may be accomplished with liquid

culture, solid agar media, or various substrates such as grain chaff, seeds,

or c ornmeal and sand (1,28,44). Inoculations may then be made, but

should correspond to the age of plants, or env ironmental conditions when

natural infections are usually observed. Inoculum should also be applied

into the natural infection court, either through soil or placed next to hosts

(12,13,24,44). Infections generally appear when soil temperaturesapproach 25 to 28C, thus these conditions would be optimal for

pathogenicity testing.

Morphological differences between the two Fusarium pathogens are

minimal, but they can be distinguished based on several different criteria.

In addition to symptom expression in sugar beets (root rot vs. y ellows

symptoms) (12,13 ,24), Fusarium root rot isolates from Texas were also

demonstrated to be distinct genetically from y ellows isolates by analyses

utilizing RAPD PCR, isozym es from at least 3 enzy mes, and ve getativecompatibility groupings (VCGs) (4,12,24). Unfortunately, these types of

tests are often impractical to co nduct, as they would require more

sophisticated equipment and advanced training to effectively differentiatebe tween pathogens. Ther efore , patho genicity testing, alt hough d ifficu lt to

standardize, is one of the more common methods for characterizing F.

oxysporumisolates.

Disease: RhizomaniaPrimarily h ost.Sugar beet (Beta vulgarisL.).

Pathogen. Beet necrotic yellow vein virus (BNYVV ).

Symptoms. Foliar sympto ms of rhizomania in the field consist of

wilt ing (Fig. 32) and v ary ing degr ees of yellowing o r c hloro sis with anerect, upright posture (Fig. 32 inset) (11,14,15 ,33,36). The chlorosis may


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sometimes be co nfused with nitrogen deficiency, but no interveinal

chlorosis or leaf scorching (typical of Aphanomyc es root ro t, and

Fusarium yellows or Fusarium root rot) are observ ed (15,1 6,33,36).

Systemic infection results in foliar symptoms consisting of yellow vein

clearing (Fig. 33), which may later turn necrotic (16,33,36). This rarely-

seen symptom is the source for the name of the pathogen (BNYVV ). Root

symptoms begin as a light brown discoloration of the central stele within

the taproot. Classical root symptoms following early infection include

small, stunted taproots with masses of secondary roots, giving the roots a"bearded " appearance (Fig. 34) (11,1 4,15 ,33,36 ). This is the origin of the

name "rhizomania," meaning "crazy roo t." Later infections often cau seroots to be constricted, resulting in a wine-glass shaped appearance

(Fig. 35) (34,36).

Fig. 32. Field sym ptoms characteristic ofBNYVV infection, consisting of wiltingand chlorotic, upright growth (inset).

Fig. 33. Systemic symptoms ofrhizomania, exhibiting vein clearing andyellowing.

Fig. 34. Roo t symptom s characteristic ofearly BNYVV infection, showing severelystunted plants with massive formationof secondary rootlets (bearding).

Fig. 35. Roo t symptom s characteristic oflater BNYVV infections showingconstricted, wine glas s-shap ed taprootsand m asses of secondary rootlets.

The virus is transmitted by the soilborne protozoanPolymyxa be tae ,
http://www.plantmanagementnetwork.org/elements/view.aspx?ID=3192http://www.plantmanagementnetwork.org/elements/view.aspx?ID=3191http://www.plantmanagementnetwork.org/elements/view.aspx?ID=3190http://www.plantmanagementnetwork.org/elements/view.aspx?ID=3189


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whic h sur viv es in soil or roo t de bris as t hick-walled surv iv al struct ures

(Fig. 36) (cy stosori), which also house the virus. Under conditions of high

soil moisture, the cystosori liberate zoospores that carry the virus into

plants as they infect roots (34,36). Without the vector, the viral disease is

a non-issue.

Fig. 36. Small secondary rootlet packedwith clusters of cystosori of Polymyxabetae, the vector of BNYVV.

Host range. The pathogen is restricte d to plants in the

Chenopodiaceae,Aizoaceae, andAmara nthac eae(34,36).

Geographic distribution. The pathogen is now widespread

throughout all sugar beet growing regions in the world.

Pathogen isolation. As obligate pathogens, neither the viru s nor the

v ector c an be tr uly iso late d or gr own in pure culture . Howe ver, the v irus

can be transmitted mechanically to induce local lesions on hosts (36),

proving the ex istence of the pathogen without serological or genetic

diagnostic methods. Both the vector and virus can also be transmitted to

new plants by coating untreated seeds with powdered, dried sugar beet

roots infested with viruliferousPolymyxa betae (10).Pathogen identification. The presence of the vector is readily

confirmed by observing the thick-walled cy stosori within roots (10,36) in

wet mounts und er t he micro scope (Fig . 36). The v iral pathogen canno t be

formally confirmed without the use of other mo re co mplex tests such as

electron microscopy , ELISA, dot blots, or PCR assays.

Pathogen storage. Viral isolates can be stored as viruliferous

cy stosori within dried roots, as described previously.

Pathogenicity tests. Isolates can be tested either as mechanical

inoculations for local lesion development or infestation of new seeds with

v irulifer ousP. beta e, as previously described abov e.


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AcknowledgmentsThe author wou ld like to thank Gloria A bad (North Carolina State

University) for contributing Figure 9B, Jeremey Taylor (Monsanto

Chemical Co) for c ontributing Figure 1 2, and Charlie Rush (Texas A&M

University) for contributing Figure 31 .

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19 94. Laboratory Manual for FusariumResearch, 3rd Ed. University ofSydney, Australia.

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8. Har v eson, R. M. 2 000. First Report of Aphanomy ces root rot of sugar beet in

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identifying and distinguishing seedling and root rot diseases of sugar beets

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