SOYBEANDISEASES
Plant Protection ProgramsCollege of Agriculture, Food
and Natural Resources
IntegratedPestManagement
Published by MU Extension, University of Missouri-Columbia
$3.00 IPM1002
CONTENTS
Seed and seedling diseases . . . . . . . . . . . .3Pythium seed decay and damping-off . . . . .3Phytophthora seedling blight . . . . . . . . . . . .5Rhizoctonia seedling blight . . . . . . . . . . . . .6Fusarium seedling blight . . . . . . . . . . . . . . .7Charcoal rot . . . . . . . . . . . . . . . . . . . . . . . .8Phomopsis seedling blight . . . . . . . . . . . . .9
Foliage diseases . . . . . . . . . . . . . . . . . . . . .9Septoria brown spot . . . . . . . . . . . . . . . . . .9Bacterial blight . . . . . . . . . . . . . . . . . . . . .10Bacterial pustule . . . . . . . . . . . . . . . . . . . .11Frogeye leaf spot . . . . . . . . . . . . . . . . . . .12Downy mildew . . . . . . . . . . . . . . . . . . . . .13Powdery mildew . . . . . . . . . . . . . . . . . . . .14
Virus diseases . . . . . . . . . . . . . . . . . . . . . .14Bean pod mottle . . . . . . . . . . . . . . . . . . . .14Soybean mosaic . . . . . . . . . . . . . . . . . . . .15Tobacco ringspot . . . . . . . . . . . . . . . . . . .15Bean yellow mosaic . . . . . . . . . . . . . . . . .16
Root and lower stem diseases . . . . . . . . .16Soybean cyst nematode . . . . . . . . . . . . . .16Late-season Phytophthora root
and stem rot . . . . . . . . . . . . . . . . . . . . . .17Rhizoctonia root rot . . . . . . . . . . . . . . . . . .18Fusarium root rot . . . . . . . . . . . . . . . . . . .18Sudden death syndrome . . . . . . . . . . . . . .19Charcoal rot . . . . . . . . . . . . . . . . . . . . . . .20Sclerotium blight (southern blight) . . . . . . .21Sclerotinia stem rot (white mold) . . . . . . . .22Brown stem rot . . . . . . . . . . . . . . . . . . . . .22
Stem, pod and seed diseases . . . . . . . . . .23Pod and stem blight and Phomopsis
seed decay . . . . . . . . . . . . . . . . . . . . . . .23Anthracnose . . . . . . . . . . . . . . . . . . . . . . .24Cercospora blight, leaf spot and
purple seed stain . . . . . . . . . . . . . . . . . .25Stem canker . . . . . . . . . . . . . . . . . . . . . . .26
Authors
Laura E. SweetsCommercial Agriculture Crops Focus TeamDepartment of Plant Microbiology and PathologyUniversity of Missouri-Columbia
Allen WratherDelta Research CenterDepartment of Plant Microbiology and PathologyUniversity of Missouri-Columbia
Photo credits
Tom D. Wylie, University of Missouri (retired)provided photographs 47 and 49.
Ward C. Stienstra, University of Minnesota(retired) provided photograph 50.
All other photographs are by Laura E. Sweets.
Production
Extension and Agricultural InfomationDale Langford, editorDennis Murphy, designer and illustrator
© 2000 University of Missouri
This publication is part of a series of IPMManuals prepared by the Plant ProtectionPrograms of the University of Missouri. Topicscovered in the series include an introduction toscouting, weed identification and management,plant diseases and insects of field and horticul-tural crops. These IPM Manuals are availablefrom MU Extension at the following address:
Extension Publications2800 Maguire Blvd.
Columbia, MO 652111-800-292-0969
College ofAgricultureFood andNaturalResources
conditions favorable for development offoliage diseases or stem diseases, scoutingwill provide information necessary to makedecisions on the use of foliar fungicides.Finally, recording information on incidenceand severity of soybean diseases in all fieldsover the course of the growing season willprovide information that can be used tomake decisions on variety selection, croprotation and other cultural practices to pre-vent or reduce these diseases in future years.
The soybean diseases most likely tooccur in Missouri include early-season seedand seedling diseases, foliage diseases, virusdiseases, root and lower stem diseases andstem, pod and seed diseases. This publicationcovers the common diseases and manage-ment strategies for each of these categories.
SEED ANDSEEDLINGDISEASES
The early-season soybean dis-eases include those that causeseed decay, seedling blights and
root rots of soybeans. Most of these early-season soybean diseases are caused by fungiin the soil that are found wherever soy-beans are grown. Pythium, Phytophthora,Rhizoctonia and Fusarium are the most com-mon of these early-season pathogens,although Phomopsis (pod and stem blightfungus) and Macrophomina (charcoal rotfungus) may also cause early-seasonseedling problems. Symptoms of theseearly-season soybean diseases may rangefrom seed decay to preemergence or post-emergence damping-off to wilt and deathof established seedlings.
Pythium seed decay anddamping-off
Pythium seed decay and damping-offare caused by as many as six different
Soybean diseases can and do occureach year in Missouri. Problemswith germination and stand estab-
lishment that are related to seed decay,damping-off and seedling blights are oftenencountered in the field. These losses canbe costly, especially if replanting is neces-sary. Diseases may cause leaf spots or leafblights, wilts or premature death of plants.Soybean diseases also can affect the qualityof the harvested crop and cause storagelosses. The extent of the damage due tosoybean diseases in a given season dependson a number of factors, including the sus-ceptibility of the soybean variety to the spe-cific disease, the level of pathogen inoculumpresent and the environmental conditionsduring that season.
To minimize losses due to field cropdiseases, it is important to correctly identi-fy the disease or diseases present so thatappropriate management steps can betaken. This bulletin is designed to aid in theidentification of the soybean diseases mostcommonly occurring in Missouri. In somecases different diseases may have similarsymptoms, so it may be necessary to submitsamples to a plant diagnostic laboratory foran accurate diagnosis of the disease.
Since there are few rescue treatmentsfor soybean diseases, the importance ofscouting fields for soybean diseases might bequestioned. Scouting fields early in the sea-son to determine the extent and cause ofstand establishment problems can providevaluable information for making replantdecisions, particularly in relation to the useof resistant varieties or the use of fungicideseed treatments. In seasons with weather
SOYBEANDISEASES
Integrated Pest Management4
50–59 degrees F. Because of this lower opti-mum temperature range, these species aremore common problems in northern areasor on early-planted soybeans. The otherPythium species prefer soil temperatures inthe range of 86–95 degrees F and are morecommon in southern areas, in late plantedfields or on plants later in the season.Crusting, deep planting, compaction, her-bicide injury and similar factors that delaygermination and seedling emergence maylead to an increase in incidence and severi-ty of Pythium seed decay and damping-off.
species of the fungus Pythium. ThesePythium species are present in most soils,and they can attack a wide range of hostplants. These fungi survive on organic mat-ter in the soil, in crop residues or as survivalstructures called oospores. Pythium speciesare more likely to cause seed decay, pre-emergence damping-off or early post-emergence damping-off.
Seed may decay before germination.Infected seed becomes soft and rotted.These rotted seeds may have soil adheringto them or may have completely decom-posed, making it difficult to find the dis-eased seed in the soil. Seedlings can also bekilled before they emerge through the soilsurface or just after they emerge. Areas ofbrown discoloration and soft, watery rotdevelop on infected hypocotyls and coty-ledons. Infected seedlings wilt, collapse andshrivel up. Over time, infected or deadseedlings become dry and stringy or shred-ded. Diseased plants are easily pulled fromthe soil because few roots have developedand the hypocotyls are shriveled and deteri-orated (Figures 1 and 2).
Pythium diseases are generally associat-ed with wet soil conditions. At least three ofthe Pythium species involved in these early-season diseases on soybean have an opti-mum temperature range for infection of
Figure 1. Pythium damping-off.
Figure 2. Stand loss due to Pythium damping-off.
Management options for Pythium seeddecay and damping-off
• Plant good quality seed with a good germi-nation rate.
• Plant in good seedbed conditions. Delayingplanting until soil temperatures are above 59degrees F may reduce infection by somePythium species.
• Pythium diseases may be more likely todevelop in low, wet areas or compactedareas of a field. Tiling to improve drainageand taking steps to reduce or prevent com-paction may help minimize problems with thisdisease.
• Fungicide seed treatment or at plantingfungicide treatment may help protectseedlings from this disease. Products con-taining either metalaxyl or mefenoxam as anactive ingredient are particularly effectiveagainst water mold fungi such as Pythiumspecies.
Soybean Diseases 5
hypocotyls. Cotyledons and hypocotylsturn brown to black and have a wet, rottedappearance. Over time, infected or deadseedlings become dry and stringy or shred-ded. These symptoms are similar to thosecaused by Pythium damping-off.
Another symptom of Phytophthora is theseedling blight phase of the disease inwhich young seedlings that appear to beestablished turn off-color to yellow, wiltand die (Figure 4). The stems of theseplants may show a brown discoloration thatbegins at the soil line and extends up thestem (Figure 5). The brown, dead leavesremain attached to the plant, and the deadseedlings remain evident in the field untilthe rows close. Seedlings of highly tolerantvarieties may be stunted but not killed.
Phytophthora root rot is more severe inareas that are low or poorly drained, incompacted areas or in clay or heavy soils,but the disease can appear on plants grow-ing in lighter soils or higher ground if thesoil remains wet after planting (Figure 6).Significant rain after planting favors thedevelopment of Phytophthora in all sites. Adry period after planting drastically reducesthis disease. Phytophthora may occur at soiltemperatures as low as 50 degrees F, butgreatest root damage occurs when soil tem-peratures are 59 degrees F or above.
Phytophthora seedling blight
Phytophthora sojae is another soil-inhabiting fungus that causes seed decay,preemergence or postemergence damping-off and seedling blight of soybeans. Thisfungus produces structures called oospores,which enable it to survive from year to yearin crop residues or in the soil. In the spring,the oospores germinate to produce sporan-gia. When soils are flooded or saturated,the sporangia release zoospores, which areattracted to the growing soybean root tip,where infection occurs.
Phytophthora can rot soybean seedbefore germination. Infected seed will havea soft, mushy, fuzzy appearance similar toseed infected with Pythium species.
Phytophthora can also kill youngseedlings either before they emerge fromthe soil or just after they have emerged(Figure 3). Dark, reddish brown to black,water-soaked lesions are evident on the
Figure 4. Phytophthora seedling blight causing wiltingof older seedlings.
Figure 5. Brown discoloration of stem due to Phytophthora seedling blight.
Figure 3. Young seedlings killed by Phytophthora.
preemergence damping-off of soybeanseedlings. The infected seed or seedlingroot is discolored and decayed. The discol-oration is usually a red to reddish browncolor and the decay may be a dry type ofrot. The hypocotyls and roots may be dry,stringy or shredded. Seedlings can bekilled.
More typically, symptoms of Rhizoc-tonia are found on seedlings, young plantsand even older plants. Localized red to red-dish brown lesions develop on thehypocotyl near the soil line. The red colorof lesions on the hypocotyls (Figure 7) is agood symptom to use in diagnosingRhizoctonia, but it is best observed immedi-ately after plants are removed from the soilbecause the color fades as plants areexposed to air. Lesions are usually confinedto the cortical layer (outer layer) of themain root and hypocotyl. If the lesionsremain localized, the stem usually remainsfirm and plants will survive although theymay be stunted. If lesions girdle the entirestem, plants may be extremely stunted anddie, especially during periods of moisturestress (Figure 8). Infected plants may bestunted or less vigorous than healthy plants,resulting in uneven stands (Figure 9). Thefoliage of infected plants may have an off-color or yellow cast, and the root systemsmay be poorly developed with decayedlateral roots.
Rhizoctonia solani can survive under awide range of soil moistures and soil tem-
Rhizoctonia seedling blightRhizoctonia solani is another common
soil-inhabiting fungus with a wide hostrange that includes field crops, vegetables,fruits and ornamentals. This fungus cansurvive well in the absence of host plantsbecause it grows well in the soil, colonizesall types of plant debris and survives as rest-ing mycelium or sclerotia.
Rhizoctonia can cause seed decay and
Integrated Pest Management6
Management options for Phytophthora seedling blight
• Select varieties with either race-specific resistance, tolerance or acombination of race-specific resistance and tolerance in fields with ahistory of Phytophthora. More than 39 races of Phytophthora sojaehave been identified based on their ability to overcome specific Rpsgenes or combinations of Rps genes in soybean varieties. Race-specific varieties contain a single gene or combination of genes (i.e.,Rps1c, Rps1d, Rps1k or Rps3a) that confer resistance to specific racesof Phytophthora sojae. Tolerant varieties have a non-race-specific,partial resistance and may also be called field-resistant varieties.
• Plant in good seedbed conditions.• Phytophthora is more likely to occur in low, wet areas, poorly drained
areas or compacted areas of a field. Tiling to improve drainage andtaking steps to reduce or prevent compaction may help minimizedisease problems.
• Avoid the application of high levels of manure or fertilizer (KCl) justbefore planting.
• Rotate crops to prevent the increase of inoculum levels in a field.• Use an appropriate fungicide seed treatment or an at-planting fungi-
cide treatment. Products containing either metalaxyl or mefenoxam asan active ingredient are particularly effective against water mold fungisuch as Phytophthora sojae.
Figure 7. Rhizoctonia lesions on soybeanhypocotyls.
Figure 6. Stand loss due to Phytophthora seedling blight.
Fusarium seedling blightBoth Fusarium oxysporum and Fusarium
solani cause seedling blight and root rot ofsoybean. These two fungi can persist in thesoil, colonize various plant residues andsurvive as chlamydospores (fungal survivalstructures) or mycelium.
In fields with Fusarium seedling blight,stands may be uneven, there may be skips inthe rows and seedlings may be stunted andweak. The pathogens cause a rot of lateralroots, taproot and lower stem (Figure 10).Lesions developing on the taproot rangefrom a nondescript brown to a dark purplebrown or black. These lesions may increasein size until they girdle the taproot. Thelower part of the taproot and the lateral
peratures. Populations of the fungus maydecline when soils are flooded or when soiltemperatures are unusually high.Symptoms, especially on above ground por-tions of the seedlings, are usually moresevere during periods of drying winds orwarm to hot weather. During such condi-tions seedlings may wilt, yellow or die.
Crusting, hard pan layers, herbicideinjury, deep planting, poor seed quality, haildamage, insect damage, mechanicalinjuries, poor fertility or other factors thatdelay germination and emergence favor thedevelopment of Rhizoctonia root rot.Rhizoctonia root rot is frequently found incombination with other diseases such assoybean cyst nematode or Fusarium rootrot. Damage from Rhizoctonia may be moresevere when it occurs in combination withother diseases.
Soybean Diseases 7
Management options for Rhizoctonia seedling blight
• Plant good-quality seed with a good germination rate.• Plant in good seedbed conditions.• Minimize or avoid stresses that delay germination or emergence, i.e.,
avoid or prevent herbicide injury and insect injury, correct soil com-paction and hard pan layer problems and reduce injury from soybeancyst nematode.
• Use an appropriate fungicide seed treatment.• Cultivate to ridge soil around the base of plants, thereby promoting
root growth above the area of the stem that is diseased. This may behelpful only if there is sufficient soil moisture after cultivation for rootsto grow.Figure 8. Seedlings killed by Rhizoctonia.
Figure 9. Uneven stand due to Rhizoctoniaseedling blight.
Figure 10. Progression of symptoms caused by Fusarium seedlingblight and root rot (plant on left shows initial discoloration and rottingof taproot; at right, taproot has broken at rotted area).
ty, hail damage, insect damage, mechanicalinjuries, poor fertility or other factors thatdelay germination and emergence favor thedevelopment of Fusarium seedling blightand root rot. Fusarium root rot is frequent-ly found in combination with other diseasessuch as Rhizoctonia root rot or soybeancyst nematode. Damage from Fusariummay be more severe when it occurs in com-bination with other diseases or stresses.
Charcoal rotCharcoal rot, caused by the fungus
Macrophomina phaseolina, occurs worldwide.The fungus is widely distributed in soilsand has a wide host range attacking a num-ber of crops, including soybeans, corn andsorghum. Macrophomina phaseolina pro-duces small survival structures calledmicrosclerotia, which allow it to survive insoil or in host residues for long periods oftime.
Charcoal rot may be more commonlyrecognized as a mid- to late-season diseaseon maturing soybeans (see charcoal rot insection on Root and Lower Stem Diseases),but it can also occur early in the season onyoung seedlings. Infected seedlings tend toshow a reddish brown discoloration fromthe soil line up the stem. The discoloredarea changes from reddish brown to darkbrown or black. Foliage may appear off-color or begin to dry out and turn brown. Ifthe growing point is killed, a twin-stemplant may develop. Under hot, dry condi-tions, infected seedlings may die. Undercooler, wetter conditions, infected seed-lings may survive but carry a latent infec-tion. Then symptoms may reappear later inthe season with hot, dry weather.
Macrophomina phaseolina grows best attemperatures between 82–95 degrees F.Infection of seedlings is most likely tooccur if conditions of high soil tempera-tures and low soil moisture exist during thefirst two to three weeks after planting.
root system may be rotted or destroyed. Ifplants are pulled from the ground, the tap-root may break at this rotted area. A prolif-eration of secondary roots may developabove the damaged main taproot giving theplants a shallow, fibrous root system. Theabove-ground portions of infected plantsmay have an off-color to yellow cast.Foliage may dry and plants wilt and dieduring periods of drying winds or warm tohot weather (Figure 11).
Fusarium root rot can occur at any timeduring the growing season, but is mostcommon on seedlings and young plants.Disease is most severe when the soil is sat-urated and soil temperatures are around 57degrees F. Crusting, hard pan layers, herbi-cide injury, deep planting, poor seed quali-
Integrated Pest Management8
Management options for Fusarium seedling blight
• Plant good-quality seed with a good germination rate.• Plant in good seedbed conditions.• Minimize or avoid stresses that delay germination or emergence, i.e.,
avoid or prevent herbicide injury and insect injury, correct soil com-paction and hard pan layer problems and reduce injury from soybeancyst nematode.
• Use an appropriate fungicide seed treatment.• Cultivate to ridge soil around the base of plants, thereby promoting root
growth above the area of the stem that is diseased. This may be help-ful only if there is sufficient soil moisture after cultivation for roots togrow.
Figure 11. Marginal browning and desiccation of foliage due to Fusarium root rotand environmental stress.
Foliage diseases such as Septoriabrown spot, bacterial blight, bac-terial pustule, frogeye leaf spot,
downy mildew and powdery mildew canoccur on soybeans in Missouri. Generallythese diseases occur in low levels and do notcause significant losses. However, underfavorable conditions for disease develop-ment, losses can be serious.
Septoria brown spotThe fungus that causes this disease,
Septoria glycines, survives in infestedresidues left on the soil surface. Duringperiods of wet spring weather, spores pro-duced on the residues are splashed or blownto cotyledons or unifoliolate leaves on soy-bean plants where they cause infection.Continuous soybeans are more likely toshow damage.
Septoria brown spot causes small, angu-lar to somewhat circular, red to brown spotson the unifoliolate and lower trifoliolateleaves (Figure 13). The individual spots canrun together, forming irregularly shapedbrown blotches on the leaves. Infected uni-foliolate leaves will yellow and drop prema-turely. Brown spot usually starts on the
Phomopsis seedling blight
Phomopsis longicolla and the otherPhomopsis and Diaporthe species that causePhomopsis seed decay and pod and stemblight (see Stem, Pod and Seed Diseases)can survive in infested crop residues and inthe soil. These fungi can also survive on theseed and Phomopsis seedling blight is morelikely to be a serious problem if infectedseed is planted. Infected seed may fail togerminate or it may germinate slowly. Onseed that does germinate, the seed coat mayremain on the cotyledons rather thansloughing off as it does on healthy plants.The seed coat adhering to the cotyledonsmay have a white moldy appearance (Figure12). Severely infected seedlings may col-lapse and die. Phomopsis seedling blighttends to be more severe if weather condi-tions after planting are cool and wet.
Soybean Diseases 9
Management options for charcoal rot
• Rotate to cereals, cotton or other nonhostcrops for one to two years.
• Maintain good crop vigor to reduce lossesfrom charcoal rot.
• In irrigated fields, watering during periods ofhigh temperatures and drought stressbetween the time when soybean plants are inbloom and the time that pods fill may helpreduce charcoal rot.
Management options for Phomopsis seedling blight
• Plant disease-free seed with a good germi-nation rate.
• Plant in good seedbed conditions.• Use an appropriate fungicide seed treatment.
Figure 12. Phomopsis seedling blight.
Figure 13. Septoria brown spot.
FOLIAGE DISEASES
Bacterial blight
Bacterial blight, caused by the bacteri-um Pseudomonas savastanoi pv. glycinea,occurs worldwide and is common duringcool, wet weather. The causal bacteriummay be carried in seed or can survive incrop residues. Bacteria on the seed maycause cotyledon infection. Bacteria canthen be spread from infected cotyledons orinfested residues by wind driven rain orsplashing rain. Further spread occurs dur-ing rainstorms and hail storms or duringcultivation when plants are wet.
Dark brown to black lesions developalong the margins of the cotyledons. Asthese lesions enlarge, the entire cotyledoncollapses and becomes dark brown.Seedlings may be stunted and if the grow-ing point is infected, the seedling may die.
Bacterial blight also produces lesionson the leaves. The lesions usually begin assmall, angular, yellow lesions (Figure 16).Lesions usually have a translucent or water-soaked appearance that may be more easilyseen if leaves are held up to the light(Figure 17). Lesions progress in color fromyellow to light brown and eventually to adark reddish brown. Older lesions have adark center surrounded by a water-soakedmargin and a yellow halo. In cool, rainyweather the small, angular lesions mayenlarge and merge, producing large, irreg-ular dead areas in the leaf. With wind andrain these large dead areas drop out or tearaway, giving the leaf a ragged appearance
lower portion of the plant. Under favorableweather conditions (warm, wet weather),the disease may move up through the plant.When this occurs, the field may have a yel-low or brown appearance (Figure 14). Latein the growing season, infected leaves mayturn rusty brown or yellow and drop pre-maturely (Figure 15).
Warm, wet weather favors infection anddisease development. Symptoms developover a temperature range of 59–86 degreesF with 77 degrees F being optimum forsymptom development. The spread ofbrown spot is restricted by dry weather.Because the pathogen survives in infestedresidues left on the soil surface, the diseaseis more severe in continuous soybean fields.
Integrated Pest Management10
Figure 16. Initial symptoms of bacterial blight.
Figure 14. Yellowing of foliage across field dueto Septoria brown spot.
Management options for Septoria brown spot
• Rotate crops with at least one year between soybean crops.• Use of foliar fungicides from bloom to early pod development may be
warranted in high value fields (e.g., seed production fields).
Figure 15. Premature yellowing and droppingof lower leaves due to Septoria brown spot.
Bacterial pustuleBacterial pustule, caused by the bacteri-
um Xanthomonas axonopodis pv. glycines,occurs in most soybean-growing areas andis common during warm, wet weather. Thecausal bacterium may be carried in seed orcan survive in crop residues. Bacteria arespread from infested residues or infectedplant tissues by wind driven rain or splash-ing rain. Further spread occurs during rain-storms and hailstorms or during cultivationwhen foliage is wet.
Bacterial pustule lesions begin as small,light-green lesions. Initially the center ofthe lesion may be slightly raised, and thismay become more evident on older lesions.Lesions may range from small spots to largeareas of brown tissue formed when smallerlesions merge. With wind and rain, theselarge dead areas drop out or tear away, giv-ing the leaf a ragged appearance. Small,reddish brown, slightly raised pustules mayoccur on pods of susceptible varieties.
Symptoms of bacterial pustule mayappear similar to those caused by bacterialblight. Typically bacterial pustule lesions donot show the water soaking around thelesions that is common with bacterialblight. Also, the small, raised pustules in thecenters of the lesions is characteristic ofbacterial pustule but not of bacterial blight.
Bacterial pustule is favored by wet orrainy weather. Disease outbreaks usuallyoccur five to seven days after wind drivenrains. Bacterial pustule is not slowed byhigh temperatures as is bacterial blight.
(Figure 18). Symptoms typically occur sev-eral days after a rain with driving winds or ahailstorm. If there are alternating periods ofwet and dry weather, plants may showbands of leaves with symptoms, that is,leaves that expanded during wet periodsshow bacterial blight symptoms, and leavesthat expanded during dry periods are free ofdisease.
Pods may also be infected. Initial podlesions are small and water soaked. Theymay enlarge and coalesce to cover much ofthe pod. Pods may turn brown to black.Infected seed may be shriveled, sunken anddiscolored or they may show no symptomsat all.
Bacterial blight is favored by cool, rainyweather. During early to midseason, diseaseoutbreaks usually occur five to seven daysafter wind driven rains. Hot, dry weatherchecks disease development.
Soybean Diseases 11
Management options for bacterial blight
• Plant disease-free seed.• Avoid highly susceptible varieties in areas where bacterial blight is
serious.• Rotate crops with at least one year between soybean crops.• Do not cultivate when foliage is wet.
Management options for bacterial pustule
• Plant disease-free seed.• Select resistant varieties.• Rotate crops with at least one year between soybean crops.• Do not cultivate when foliage is wet.
Figure 17. Water-soaked appearance of bacte-rial blight lesions.
Figure 18. Ragged appearance of foliageresulting from bacterial blight.
stem lesions age, the centers become brownto gray in color. Lesions on pods are circu-lar to elongate, reddish brown in color andmay be slightly depressed. The fungus cangrow through the pod wall to infect matur-ing seeds. Infected seeds may show discol-oration of the seed coat that ranges fromsmall specks to large blotches of light grayto dark gray to brown.
Disease development is favored by
Frogeye leaf spot
Frogeye leaf spot, caused by the fungusCercospora sojina, occurs worldwide.However, the disease is most serious inwarm regions or during periods of warm,humid weather. The fungus that causesfrogeye leaf spot survives in infested soy-bean residue and infected seeds. Spores pro-duced on infested residues or infected planttissues are spread by splashing rain or winds.
Symptoms of frogeye leaf spot occurprimarily on leaves, although the causal fun-gus may also infect stems, pods and seed.Lesions are small, circular to somewhatirregular spots that develop on the upperleaf surfaces. Initially the spots are dark andwater soaked in appearance (Figure 19). Asthe lesions age, the center becomes lightbrown to light gray in color. Older lesionshave a light center with a darker red topurple-brown border (Figure 20). Lesionsmay merge to kill larger areas of the leaf sur-face. These areas may drop out, giving theleaves a very tattered or shot-hole appear-ance (Figure 21). Heavily spotted leavesusually wither and drop prematurely. Fieldswith high levels of frogeye leaf spot maylook brown and desiccated (Figure 22).
Stem lesions can develop later in theseason. Young stem lesions are reddishbrown with a narrow, dark margin. As the
Integrated Pest Management12
Figure 19. Initial symptoms of frogeye leaf spot.
Figure 20. Older lesions of frogeye leaf spot.
Figure 21. Tattering of leaf from severe frog-eye leaf spot.
Figure 22. Browning of foliage across fieldfrom severe frogeye leaf spot.
diseased areas (Figure 24). Severely infectedleaves turn yellow and then brown.Premature defoliation may occur.
Pod infections occur without visibleexternal symptoms. However, the inside ofthe pod and the seed coats of the beans areusually covered with a white, powdery coat-ing of fungal material (Figure 25). Infectedseeds may be smaller in size and lighter inweight than healthy seeds.
Downy mildew is favored by highhumidity and temperatures of 68–72degrees F. Downy mildew seldom causessignificant losses. However, under idealconditions for disease development, it cancause defoliation, reduce seed quality andlower yields.
warm, humid weather. Leaves that expandand develop during periods of warm, wetweather are more likely to be infected thanleaves that expand during dry periods. Dryweather severely limits disease development.
Downy mildew
Downy mildew, caused by the fungusPeronospora manshurica, is reported wherev-er soybeans are grown. The downy mildewfungus survives as oospores in infected leafresidues and on seeds. Spores produced indiseased areas on lower leaf surfaces arewind-blown and serve to infect additionalleaves on that plant or other plants.
Initial symptoms of downy mildew arepale green to light yellow spots or blotcheson the upper surface of young leaves(Figure 23). These areas enlarge into paleto bright yellow lesions of indefinite sizeand shape. Eventually the lesions turn gray-ish brown to dark brown with a yellow mar-gin. During periods of heavy dew or wetweather, a gray to purple fuzz that is visiblegrowth of the downy mildew fungus devel-ops on the lower leaf surface beneath the
Soybean Diseases13
Management options for frogeye leaf spot
• Plant disease-free seed.• Select resistant varieties.• Rotate crops with at least one year between
soybean crops.• Use of a foliar fungicide is seldom warranted,
except on high-value fields (e.g., seed pro-duction fields) or in years when weather isespecially favorable for disease development.
Management options for downy mildew
• Plant disease-free seed.• Rotate crops with at least one year between soybean crops.• If infected seed must be planted, use a fungicide seed treatment.
Figure 23. Initial symptoms of downy mildew.
Figure 24. Downy mildew fungus evident onlower leaf surface.
Figure 25. Downy mildew encrusted soybeanseed.
VIRUS DISEASES
Several viruses commonly infectsoybean plants in Missouri, includ-ing bean pod mottle, soybean
mosaic, tobacco ringspot and bean yellowmosaic. These virus diseases tend to causegreen to yellow mottling of leaf tissue,stunting of plant growth and deformation ofplant tissues. Symptoms of bean pod mottleand soybean mosaic are similar, so the twoare difficult to identify in the field. Also,these viruses may occur in combination.Laboratory tests are the only means of accu-rately identifying which virus or combina-tion of viruses is present in infected plants.
Bean pod mottle Bean pod mottle causes a green to
yellow mottling of young leaves in theupper canopy (Figures 27 and 28). Beanpod mottle symptoms are most apparentduring periods of rapid growth and coolconditions. Symptoms are usually not obvi-ous during periods of high temperatures orafter pod set. This virus is sap-transmissibleand vectored by beetle species such as thebean leaf beetle. The host range of beanpod mottle virus is limited to legumes. Inaddition to the soybean plant, other hostsinclude lespedeza, alfalfa and clover. Thebean pod mottle virus is not considered tobe seedborne.
Powdery mildew
Powdery mildew is caused by the fun-gus Microsphaera diffusa. This species ofpowdery mildew affects other legumes suchas garden bean, pea, cowpea and mung beanas well as members of two other plant fam-ilies (honeysuckle and nightshade families).Ascospores (sexual spores) released in thespring are the most likely source of primaryinoculum. Wind-borne spores start newinfections and repeat the disease cycle untilsoybean plants mature or temperaturesincrease above 86 degrees F.
Powdery mildew begins as small, circu-lar areas of white, powdery mold growth onthe upper leaf surface. These areas enlargeto cover larger areas of the leaf surface(both upper and lower surfaces) and maydevelop on stems, petioles and pods as well(Figure 26). When severe, all above groundportions of the plant appear to be coveredwith white to grayish white, powder-likemold growth. Heavily infected leaves mayyellow and drop prematurely.
Powdery mildew is favored by coolerthan normal temperatures, that is, tempera-tures in the range of 65–75 degrees F.Temperatures above 86 degrees F restrictdisease development.
Integrated Pest Management14
Figure 27. Bean pod mottle.
Management options for powdery mildew
• Select resistant varieties.• Use of foliar fungicides is seldom warranted.
Figure 26. Powdery mildew.
When bean pod mottle and soybeanmosaic occur in combination in the sameplant, symptoms are more severe and yieldlosses higher than those caused by eithervirus alone.
Tobacco ringspotSoybean budblight, caused by the tobac-
co ringspot virus, results in the curving ofthe terminal in the shape of a crook (Figure30). The plant may show axillary prolifera-tion of leaves and bud tissue. Stems andbranches may show a brown discoloration ofthe pith starting at the nodes and extendinginto the internodal areas. A brown discol-oration of petioles and larger leaf veins maybe evident. Plants are usually stunted with abushy appearance, leaves are rugose androlled and pods may be poorly developed oraborted. Plants tend to remain green afteruninfected plants have matured and turnedin color (Figure 31). Tobacco ringspot virus
Soybean mosaicSoybean mosaic virus causes a green to
yellow mottling or mosaic pattern on leaftissue as well as puckering and distortion ofthe leaf shape (Figure 29). On severelyinfected plants, leaves are curled downwardalong their margins and plants are stunted.Symptoms vary with the susceptibility of thehost, strain of the virus, age at which theplant became infected and environmentalconditions. Soybean mosaic virus is sap-transmissible and vectored by several aphidspecies. Soybean mosaic virus may also beseedborne. Infected seed may show a bleed-ing hilum symptom or a brown to blackmottling of the seed coat. However, seedcoat discoloration is not a definite indicationof seed infection because infected seed canbe symptomless and seed coat discolorationcan be caused by factors other than soybeanmosaic virus.
Soybean Diseases 15
Figure 28. Bean pod mottle.
Figure 29. Soybean mosaic. Figure 31. Green tobacco ringspot infectedplant in field of maturing soybean plants.
Figure 30. Tobacco ringspot causing crookingof terminal.
plants or dead plants (Figure 32). Foliagesymptoms may include a yellowing ofleaves from the margin inward or a generalyellowing of leaves. But such foliage symp-toms are also caused by an number of otherfactors including root rot diseases, nutrientdeficiencies, herbicide injury and com-paction, so foliage symptoms should not beused to diagnose SCN. Plants with SCNmay have poorly developed root systems. Ifplants are carefully dug up, females may beevident on the roots. The females appear astiny (smaller than nitrogen-fixing nodules),whitish to yellow to brownish, lemon-shaped structures on the roots (Figure 33).Symptom expression may be more severe ifplants are subjected to other stresses suchas moisture stress, nutrient deficiencies,herbicide injury, insect damage or otherdiseases (Figure 34).
The cysts are the bodies of the deadfemale nematodes. The cysts are actually
is readily sap-transmissible. Although noefficient insect vector has yet been discov-ered, thrips are implicated in the spread ofthe virus. The soybean budblight virus has awide host range, including a number ofweed species. Pastures or uncultivated areascontaining other hosts for tobacco ringspotvirus may serve as sources of inoculum fornearby soybean fields. The virus may also beseedborne.
Bean yellow mosaicSoybean yellow mosaic is caused by the
bean yellow mosaic virus. The leaves exhib-it brilliant yellow mosaic patches. Theseyellow patches tend to occur along themajor veins. In some cases, depending onthe strain of the virus, leaves may showcrinkling and puckering. Symptoms aremore pronounced during periods of coolertemperatures. The virus has a wide hostrange and is transmitted by several aphids.Seed transmission has not been reported.
Soybean cyst nematode
The soybean cyst nematode,Heterodera glycines, is a seriousproblem throughout the state
and in most soybean-producing areas of theUnited States. Symptoms of soybean cystnematode (SCN) range from no obvioussymptoms to subtle differences in plantheight and vigor or unexpected decreases inyield to severe stunting and discoloration of
Integrated Pest Management16
Figure 33. Soybean cyst nematode females onsoybean roots.
Figure 32. Stunting and yellowing due to soy-bean cyst nematode.
Management options for soybean virus diseases
• Plant disease-free seed.• Select varieties resistant to soybean mosaic virus.• Maintain good weed control.• Control of insect vectors may not be practical.
ROOT AND LOWER STEMDISEASES
Late-season Phytophthora rootand stem rot
Phytophthora sojae can cause seed decayand seedling blight as previously describedin the section on Seed and SeedlingDiseases. In some situations plants infectedearly in the season with Phytophthora maynot show symptoms until later in the sea-son. As plants begin to flower and set pods,symptoms of late-season Phytophthora rootand stem rot may develop. Infected olderplants show reduced vigor through thegrowing season or die gradually over theseason. Lower leaves may show a yellowingbetween the veins and along the margins.Upper leaves may yellow (Figure 35). Thestems show a characteristic brown discol-oration that extends from below the soilline upward (Figure 36). Eventually the
protective egg cases that contain up to 250SCN eggs. Eggs in cysts may survive in thesoil for extended periods of time even in theabsence of soybean crops. Anything thatmoves cyst-infested soil can spread SCNincluding machinery, animals, migratorybirds, people, wind, water and soil pedsassociated with seed. Once in a field, SCNmay take several years to build up to dam-aging levels.
For more detailed information onSCN, soil sampling for SCN and strategiesfor managing SCN, please refer to MUpublication G4450, Soybean Cyst Nematode:Diagnosis and Management.
Soybean Diseases 17
Management options for soybean cystnematode
• Employ a program of soil sampling to identifyproblem fields and to determine the extentand severity of the problem in the field.
• Select resistant varieties. It is important torotate sources of genetic resistance to SCNin soybean varieties grown if at all possible.
• Rotate to nonhost crops (the length of timeout of susceptible soybean varieties willdepend on population levels of the soybeancyst nematode in a given field).
• Maintain good plant vigor.• Although several nematicides are labeled for
use on soybeans, economic and environ-mental concerns limit their use.
Figure 34. Severe damage from soybean cystnematode, drought stress and nutrient defi-ciency.
Figure 35. Late-season Phytophthora root rot.
Figure 36. Stem discoloration due to Phytophthora root rot.
developed, lateral roots may be discoloredor rotted and the stem may have a brick reddiscoloration beginning at the soil line andextending in either direction from the soilline (Figure 39). If plants are stressed byhot, dry conditions, severely infected plantsmay die. If cool, wet conditions occur afterplants are infected with Rhizoctonia, a flushof secondary roots above the diseased stemarea may be evident. Management wouldbe as for Rhizoctonia seedling blight.
Fusarium root rotThe two Fusarium species that cause
seedling blight can also cause root rot onolder plants. Older plants may be stunted,have an off-color or yellow cast and appearless vigorous than adjacent healthy plants.When plants are removed from the soil,the taproot will show varying degrees ofdiscoloration and deterioration. Discolor-ation may range from brown to purple-brown to almost black. The taproot mayshow distinct lesions or may be rottedcompletely through. If the taproot is rottedthrough, it may break when the plant ispulled from the ground. If plants arestressed by hot, dry conditions, severelyinfected plants may die. If cool, wet condi-tions occur after plants are infected withFusarium root rot, a flush of secondaryroots above the rotted taproot may be evi-dent (Figure 40). Management would be asfor Fusarium seedling blight.
entire plant may wilt and die. Witheredleaves remain attached even after the plantdies (Figure 37). Management would be asfor early-season Phytophthora seed decayand seedling blight.
Rhizoctonia root rotRhizoctonia solani can cause seedling
blight as previously described in the sectionon Seed and Seedling Diseases. Rhizoctoniacan also cause a root rot of older plants. Onolder plants the lower leaves may begin toyellow. The yellowing may be from themargin in resembling symptoms of potassi-um deficiency or may be a more generalyellowing. Plants may be stunted andappear less vigorous than adjacent plants(Figure 38). When plants are removed fromthe soil, the root system may be poorly
Integrated Pest Management18
Figure 38. Uneven stand due to Rhizoctonia root rot.
Figure 39. Red discoloration due toRhizoctonia root rot.
Figure 37. Death of older plants fromPhytophthora root rot.
(Figure 42). The yellow areas may turnbrown. As the interveinal leaf tissue turnsbrown, it also dries out. Upper trifoliolatesbecome brown and dry out. Severely affect-ed leaflets may drop off the plant, leavingthe petiole attached or they may curlupward and remain attached to the plant(Figure 43). Infected plants may wilt anddie prematurely. Root systems may showdeterioration and discoloration of lateralroots and taproot. When split open, inter-nal tissues of taproot and lower stem mayshow a light-gray to light-brown discol-oration (Figure 44).
Sudden death syndromeIn Missouri, sudden death syndrome
(SDS) has been a problem primarily in riverbottom fields in the central and easternportions of the state. However, thepathogen, Fusarium solani f. sp. glycines,appears to be present in soybean-producingareas throughout the state. In years whenenvironmental conditions are favorable forinfection and symptom development, SDSmay be found in most areas of the state.
SDS has been associated with maximumyield potential soybean production, that is,fields with optimum fertility, irrigation andlime application. Field observations suggestthat SDS is more likely to occur and to bemore severe with high soil moisture,whether that is supplied by rainfall or irriga-tion. High soil moisture during vegetativestages of soybean growth seems to be themost conducive to disease development.The onset of SDS symptoms frequently isassociated with wet conditions and belownormal temperatures at or near bloom.
Symptoms of SDS may appear severalweeks before flowering but are more pro-nounced after flowering. Foliage symptomsbegin as scattered yellow blotches in theinterveinal leaf tissue (Figure 41). Theseyellow blotches increase in size and mergeto affect larger areas of leaf tissue. Veinstypically stay green. The bright yellowblotches between the green veins giveaffected leaves a striking appearance
Soybean Diseases 19
Figure 40. Destruction of the main taproot byFusarium root rot has been followed by a flushof secondary roots above the rotted taproot.
Figure 41. Initial foliage symptoms of sudden death syndrome.
Figure 42. Foliage symptoms of sudden death syndrome.
Charcoal rotCharcoal rot may cause a seedling
infection as previously described in the sec-tion on Seed and Seedling Diseases, but ismore commonly considered a mid- to late-season soybean disease. Symptoms typical-ly begin to develop as plants move intoreproductive stages of growth. Infectedplants are less vigorous and have smallerleaves. Leaves may turn yellow and wilt.Leaves eventually turn brown and have adry appearance (Figure 47). The taprootand lower stem develop a silvery gray tolight-gray discoloration of the epidermis(outer layer of the soybean stem). The epi-dermis may flake or shred away from thestem, giving the stems a tattered appear-ance (Figure 48). Fine black specks ormicrosclerotia may be evident in tissuesbelow the epidermis and eventually in epi-dermal tissues. If the lower stem and tap-
Patterns of symptoms in the field rangefrom distinct oval to circular patches toirregularly shaped bands or streaks acrossthe field (Figures 45 and 46). In severe casesa majority of the field may show symptoms.
Integrated Pest Management20
Figure 43. Wilting of upper trifoliolates due tosudden death syndrome.
Figure 46. Discrete oval patch in field killedprematurely by sudden death syndrome.
Figure 44. Internal discoloration of lower stemfrom sudden death syndrome.
Figure 45. Yellow streaks across field from sudden death syndrome.
Management options for sudden deathsyndrome
• Select varieties that have performed wellwhere SDS has been a problem.
• Improve drainage in poorly drained fields andavoid compacting soils.
• Stagger planting dates and delay plantinguntil soils are warm and dry.
• Rotate crops; avoid continuous soybeancropping.
• Maintain good crop vigor and avoid cropstress, including soybean cyst nematode.
• Harvest fields with sudden death syndromein a timely fashion.
weather, so symptoms usually appear whentemperatures are in the range of 82–95degrees F.
Sclerotium blight (southern blight)
Sclerotium blight, also called southernblight or white mold, is caused by the fun-gus Sclerotium rolfsii. This disease tends tobe a problem primarily in the southeasternpart of Missouri. Sclerotinia rolfsii producessmall, tan to brown sclerotia (small survivalstructures) that allow it to survive forextended periods of time in the soil.
The first symptom of the disease maybe a yellowing or wilting of scattered plantsin the field. Light-brown to brown lesionsmay be present on the lower stem close tothe soil line. A mat of white mold growthmay cover the lower stem and spread outfrom the stem on leaf debris and the soil.Eventually the tan to brown sclerotia beginto form on the plant tissues and soil sur-face.
Sclerotium blight is favored by hot, wetweather; temperatures in the range 77–95degrees F are optimum for fungal growth.High soil moisture levels and high humidi-ty in the plant canopy also favor diseasedevelopment.
root are split open, a reddish brown toblackish discoloration may be seen inwoody tissues of the taproot and stem(Figure 49).
The fungus that causes charcoal rot,Macrophomina phaseolina, is a common soilfungus in Missouri. Corn and grainsorghum are also hosts of the charcoal rotfungus. Charcoal rot is favored by hot, dry
Soybean Diseases 21
Management options for charcoal rot
• Rotate to cereals, cotton or other nonhosts for one or two years; withcorn or grain sorghum, a rotation of three years may be necessary.
• Maintain good crop vigor to help reduce losses from charcoal rot.• Irrigate properly from just before bloom to pod fill.
Management options for Sclerotium blight
• Rotate crops with at least one year betweensoybean crops. If Sclerotium blight has beensevere, a three- to four-year rotation may benecessary.
Figure 48. Soybean stem showing micro-sclerotia (small, black specks) of the charcoalrot fungus in shredded epidermal tissue.
Figure 49. Internal discoloration of lower stemdue to charcoal rot.
Figure 47. Soybean plants killed prematurelyby charcoal rot.
Sclerotinia stem rot or white mold hascaused high yield losses in fields with highyield potential. The disease appears to befavored by moderate temperatures in thecanopy (less than 82 degrees F) and fre-quent rainfalls especially as the crop beginsto flower and set pods.
Brown stem rot
Although brown stem rot has beenreported in Missouri, it is not a widespreador serious problem in the state. Whenbrown stem rot is found in Missouri ittends to be in the northern part of the state.Brown stem rot is caused by the fungus,Phialophora gregata, which survives ininfested crop residues and in the soil.
When foliage symptoms occur, theyusually develop as plants are beginning toset pods. Light green to yellow blotchesdevelop in the interveinal leaf tissue. Theseblotches may increase in size and merge toaffect larger areas of leaf tissue (Figure 52).Over time, the yellow areas may turnbrown. Veins typically stay green. Thesefoliage symptoms resemble those of sudden
Sclerotinia stem rot (white mold)
Sclerotinia stem rot or white mold is aproblem for soybean farmers in Michigan,Wisconsin, Minnesota, northern Iowa andnorthern Illinois. It has been reported inMissouri but has not been a widespread orserious problem in the state’s soybean crop.Sclerotinia stem rot is caused by the fungusSclerotinia sclerotiorum. This fungus has awide host range including dry beans, pota-toes, canola, sunflower, peas and manybroadleaf weeds. Sclerotinia sclerotiorum pro-duces small, black survival structures calledsclerotia. These sclerotia can survive in thesoil for years.
In the field, white mold may first be evi-dent as a wilting of leaves in the uppercanopy. Leaves may have a gray-green oroff-color and wilted appearance. Cankersmay be evident on stems at the nodes.Initially, these cankers are gray-green andwater soaked. The cankers eventually turnbrown to tan or even a bleached white withreddish brown borders. White mold growthmay be present on the stems and may mattogether infected leaves or even plants(Figure 50). Later in the season, the blacksclerotia may be found on the outside ofstems, in the center pith of stems and evenin pods (Figure 51). The fungus may moveto the pods, and infected pods and seedsmay be covered with white mold growth.
Integrated Pest Management22
Management options for Sclerotinia stemrot
• Select resistant varieties.• Rotate crops with at least one year between
soybean crops and do not plant soybeanafter common bean, sunflower or other sus-ceptible crops.
• Plant disease-free seed that is free of sclerotia.• Maintain good weed control.
Figure 50. Sclerotinia white mold of soybean.
Figure 51. Sclerotia of Sclerotinia white moldforming on soybean stem.
Pod and stem blight andPhomopsis seed decay
Phomopsis longicolla and the otherDiaporthe and Phomopsis species that causepod and stem blight and Phomopsis seeddecay can survive in infested crop residues,in the soil and in seed. Symptoms usuallydevelop on stems of plants during laterreproductive stages of growth.
Pod and stem blight infected plants maybe stunted and their stems discolored. Blackpycnidia or fruiting bodies of the causalfungi develop on the lower portion of themain stem, branches and pods as plantsreach maturity. The pycnidia may be limit-ed to small patches near the nodes or may
death syndrome. Upper trifoliolates maybecome brown and dry out. Brown stem rotcauses a brown discoloration of the vasculartissues and center pith of the soybean stemthat is evident when stems are split open.Initially the brown discoloration may befound in stem tissues close to the soil lineand near nodes higher up on the plant(Figure 53). Later in the season, the browndiscoloration may be almost continuouswithin the stem (Figure 54).
Development of brown stem rot isfavored by temperatures in the range of59–81 degrees F. As air temperaturesincrease above 81 degrees F, both incidenceand severity of brown stem rot decrease.Leaf symptoms are most pronounced ifcool weather occurs as the crop enters thereproductive stages of growth. Internalbrowning of stem tissues is greatly reducedat higher temperatures.
Soybean Diseases 23
Figure 52. Foliage symptoms of brown stemrot.
Figure 53. Brown stem rot causing discol-oration of vascular tissues and center pith ingreen soybean stem (discoloration at nodes).
Figure 54. Brown stem rot causing discoloration of vascular tissuesand center pith in mature soybean stem (discoloration almost contin-uous up stem).
Management options for brown stem rot
• Select resistant varieties.• Rotate crops with at least one year between soybean crops. Longer
rotations may be necessary in fields with established brown stem rotproblems.
STEM, POD AND SEEDDISEASES
Anthracnose
Colletotrichum truncatum and severalother Colletotrichum species cause anthrac-nose of soybean. Typically, anthracnosedevelops as a stem and pod disease on soy-bean plants during later reproductive stagesof growth. However, in some seasonsanthracnose may cause a tip blight onplants in early pod filling stages of growth.
On stems, anthracnose causes symp-toms that are somewhat similar to thosecaused by pod and stem blight. Irregularlyshaped brown areas may develop on stems,petioles and pods. The anthracnose fungusalso produces small black fruiting bodies ininfected tissue, although the pycnidia tendto be scattered across diseased tissue ratherthan in rows as are the pycnidia ofPhomopsis species.
The tip blight phase of anthracnosecauses a yellowing or browning of theuppermost leaves and pods. The blightedtips may dry up and die prematurely.
Anthracnose is favored by warm, wetweather. Symptoms on stems will be moresevere if wet weather continues throughharvest. The tip blight phase tends to occurwhen warm to hot weather in midseason isfollowed by a period of rainy weather.
cover dead stems and pods. On stems, thepycnidia are usually arranged in linear rowswhile on pods they are randomly scatteredacross the pods (Figure 55). The fungi maygrow through the pod walls and infect theseed, causing Phomopsis seed decay.Infected seed is usually oblong in shape,somewhat shrunken or shriveled and cov-ered with a white mold growth (Figure 56).
Prolonged periods of warm, wetweather during flowering and pod fill favorthe development of pod and stem blight. Ifwet weather continues through harvest, lev-els of Phomopsis seed decay may be high.
Integrated Pest Management24
Management options for anthracnose
• Rotate crops with at least one year betweensoybean crops.
• Plant disease-free seed (pathogen may sur-vive on infested seed and cause seedlingblights if that seed is planted the next year).
• Use an appropriate fungicide seed treat-ment.
• Use of a foliar fungicide during the growingseason is seldom warranted except in seed-production fields in seasons favorable for podinfection.
Figure 56. Phomopsis seed decay.
Figure 55. Pod and stem blight (note pycnidiadeveloping in lines up the stem).
Management options for pod and stem blight
• Rotate crops with a least one year out of soybeans.• Use disease-free seed (pathogen may survive on infested seed and
cause seedling blights if that seed is planted the next year).• Use an appropriate fungicide seed treatment.• Use of a foliar fungicide during the growing season is seldom warrant-
ed, except in seed production fields in seasons favorable for podinfection.
development. Yields are not usuallyreduced, but a high percentage of seed stainmay be evident at harvest. Heavily infectedseed, if planted, could produce diseasedseedlings resulting in stand problems.
Cercospora blight, leaf spot and purple seed stain
Cercospora kikuchii can infect soybeanseeds, pods, stems and leaves but is mostcommonly found on the seed. Thepathogen survives in infested crop residuesand on seed.
In years in which dry conditions duringpod fill are followed by a period of rainyweather, Cercospora blight or leaf spot maydevelop. Infection occurs primarily on theuppermost leaves and begins as reddishpurple to reddish brown, angular to some-what circular lesions (Figure 57). Theselesions may coalesce to kill larger areas ofleaf tissue. The entire uppermost trifolio-late leaf and petiole may be blighted andbrown. The most striking symptom of thisdisease is the premature yellowing and thenblighting of the youngest, uppermost leavesover large areas of affected fields (Figure58). Symptoms typically do not progressdown the plants more than one to twonodes. Pods at the uppermost node maydevelop round, reddish purple to reddishbrown lesions.
When this fungus infects the seed, thedisease is called purple seed stain. Infectedseed shows a conspicuous discolorationvarying in color from pink to pale purple todark purple. The discoloration may rangefrom small specks to large blotches thatcover the entire surface of the seed coat(Figure 59).
Warm, humid weather favors disease
Soybean Diseases 25
Figure 59. Purple seed stain.
Figure 57. Initial symptoms of Cercosporablight.
Figure 58. Yellowing across field due to Cercospora blight.
Management options for Cercospora blight,leaf spot and purple seed stain
• Rotate crops with at least one year betweensoybean or other legumes.
• If infected seed must be planted, use anappropriate fungicide seed treatment.
or browning of leaf tissue may also develop.These foliage symptoms are similar tothose caused by sudden death syndrome orbrown stem rot.
Stem canker
Although stem canker can occur inMissouri, this disease is usually not wide-spread or serious in the state. Diaporthephaseolina, the fungus that causes stemcanker, survives in infested residues.Infection by the stem canker pathogen isfavored by extended periods of rainyweather during early vegetative stages ofsoybean growth. However, symptoms maynot be evident until later in the season.
Initial symptoms of stem canker aresmall reddish brown lesions on stems near aleaf node. Over time the lesions expand toform larger, sunken cankers that are brownto black in color (Figure 60). Since thelesions tend to develop around leaf nodes,the discoloration of the stem is not contin-uous over the entire stem. The stem nearthe soil line may be green with bands of dis-colored tissue at infected leaf nodes separat-ed by green healthy tissue at healthy nodes.Foliage symptoms of interveinal yellowing
Management options for stem canker
• Rotate crops with at least one year betweensoybean crops.
• Select resistant varieties.
Figure 60. Stem canker causing reddishbrown lesion on main stem.
IPM1002 New 9/00/5M
■ Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Departmentof Agriculture. Ronald J. Turner, Director, Cooperative Extension, University of Missouri and Lincoln University, Columbia, MO 65211.■ University Outreach and Extension does not discriminate on the basis of race, color, national origin, sex, religion, age, disability or statusas a Vietnam era veteran in employment or programs. ■ If you have special needs as addressed by the Americans with Disabilities Act andneed this publication in an alternative format, write ADA Officer, Extension and Agricultural Information, 1-98 Agriculture Building, Columbia,MO 65211, or call (573) 882-7216. Reasonable efforts will be made to accommodate your special needs.
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