Southwest Research and Outreach Center

Fungal Seedling Diseases of Soybeans

Soybeans are susceptible to many species of fungi and bacteria in the seed and seedling stage. Seed rots, damping off and seedling blights of soybeans are generally worse under wet conditions. Poorly drained and compacted soils are especially prone to these diseases. Economic returns most often occur when soybeans are planted under cool, wet conditions. Very early planting, wet or poorly drained soils and minimal tillage are cases where a seed treatment is most likely to pay. The species of fungi responsible for disease in a soybean field depends on several factors: Species complex present, temperature and moisture conditions and the genetics of the soybean variety.

Soybean seed treatments can help protect soybean seed and seedlings from fungal attack early in the season. They do not, however, guarantee that you will not have stand loss from fungal pathogens later. Fungicides for seed treatment, although some are systemic, have a limited time period in which they are effective. This means that soybeans can be protected early in the season but succumb to root rots when conditions are favorable for infection later in the season. In the case of root rot caused by Phytopthora there is an additional option. Genetic resistance is available in the form of Rps genes and field tolerance. Genetic resistance, if matched to predominant races of the fungus in the field, or field tolerance will provide season long protection.

Fungicides are not effective against all species of fungi and bacterial pathogens are unaffected by fungicide treatments. Combinations of several damping off/root rot fungi can attack soybeans. This can also make it difficult to distinguish between primary and secondary invaders on soybean seedlings and roots. The fungicide must be must be matched to the pathogen problem. For example, Metalaxyl and Mefanoxam provide excellent control of Pythium and early season Phytophthora but have little activity on other soybean damping off fungi. Fungicide combinations are used to broaden the species activity spectrum of treated seed. Some seed treatments must be commercially applied. An excellent chart on the effectiveness of seed treatment fungicides against several fungal species is available in Extension Bulletin 472, Soybean Production published by the Ohio State University. This bulletin is also available at http://www.ag.ohio-state.edu/~online/b472/soy.html.

Disease pressure is not consistent from year to year. A fungicide seed treatment does not guarantee an economic benefit. For example, in 1999 studies conducted by Drs. S. Naeve and J. Kurle, the effect of broad spectrum seed treatments varied by location. At Lamberton and Morris a benefit from seed treatment was not observed. In the cooler soils of the Red River Valley locations, Crookston and Ada, yield increases were observed during 1999.

Finally, some fungicide seed treatments may have a detrimental effect on Rhizobia inoculants. Carboxin has a moderate effect on nodulation; Captan and PCNB fungicides can severely reduce nodulation. If using both a fungicide seed treatment and Rhizobia inoculant, apply the inoculant immediately before planting. The table below lists several species of fungi that can cause seed rots and damping off in Minnesota soybeans, their biology and control methods.

Click here to view the table in Adobe Acrobat Reader format.

Disease Organism Overwinters Conditions for Infection Symptoms Late Season Disease/Symptoms Genetic Resistance Cultural and Chemical Control
Phytophthora
Phytophthora megasperma var. sojae Oospores and mycelium in soil and residue Wet soils,compaction

Wet rot, shoestring, "bald head"

Wilted plants with leaves attached

Stunting of plants

Brown-purple lesion above soil

Decayed roots

Wilted plants

Brown-purple lesion above soil

Rps / tolerance

E - Seed trtmnts. containing Metalaxyl/Mefanoxam

G - Seed trtmnts. containing Oxydixyl

Effective on earlyseason damping off only

Resistant (rps genes) or field tolerant varieties

Pythium
Pythium sp. Oospores and mycelium in soil and residue

Cool,wet soils,compaction

< 60 F

Wet rot, shoestring, "bald head"

Wilted plants with leaves attached
--
--

E - Seed trtmnts. containing Metalaxyl/Mefanoxam

G - Seed trtmnts. containing Oxydixyl

Rhizoctonia
Rhizoctonia solani Sclerotia,saprophyte

warm, wet soils

70 - 80 F

Wilting

Rreddish brown lesion on lower stem, stopping at soil surface

 Reddish brown lesions below soil surface
--

G- Seed trtmnts containing PCNB, fludioxonil* or Bacillus subtillus

Rotation

Fusarium

Fusarium oxysporum

Fusarium solani

chlamydospores, mycelium on residue

Cool, wet soils, Flooding, compaction

SCN may increase infection

SDS is caused by a strain of F. solani

Wilting, dark brown discoloration

Reddish- white mycelium

 F. solani foliar symtoms of intervienal necrosis attached petiole
--

Several fungical compounds have fair activity

Downy mildew
Peronospora manshurica Oospores on crop residue and seeds Seed borne or wet weather Stunting with pale green spots on upper leaf

Pale green to yellow spots on upper leaf surface with gray mycelium and spores below leaf

Seed with white mycelium crust

Resistance

Plant clean seed

Resistant varieties

Rotation and bury residue

Pod and Stem Blight(Phomopsis sp.)
Diaporthe phaseolorum var. sojae Mycelia in residue and seeds

Seed borne spores splased to soybean with wet weather

Infection favored by stem injury

Seed infection favored by warm wet fall and delayed harvest

seedling infection resulting in stunting or damping off

Linear rows of black pycnidia on stem and pods

Seed shriveled, moldy or discolored

Varities vary in susceptibility

Plant clean seed

Several fungicidal compounds have Good activity on seed-born phomposis seed rot

Rotation

Stem Canker
Diaporthe phaseolorum var. caulivora Mycelia/perithecia in crop residue and seed Infection at nodes during flower from infected cotyledons (or residue?) Seedling death after emergance

Red brown lesion can girdle stem and wilt or kill top growth

Leaf rolling and cankers on petioles

 Some varieties do not allow disease development

Plant clean seed

Rotation and bury residue?

Anthracnose (Phomopsis sp.)

Colletotrichum truncatum

Colletotrichum destructivum

Mycelium on crop residue and seed

Seed borne and warm moist weather

Delayed harvest

 Reduced germination and damping off of infected seed

Scattered black lesions and pycnidia on stems and pods
--

Plant clean seed

May respond similarly to seed treatments similar to Pod and Stem Blight

Rotation and bury residue

Purple Seed Stain
Cercospora kikuchii Crop residue and mycelium on seed coat

Hot(>80 F) and humid canopy

Spores from infected cotyledons splashed or blown to leaves and stems

Shriveled dark purple cotyledons w/premature drop, stunted or killed seedlings

Purple to red angular lesions sides of upper leaves

Seed with pink to purple discoloration

 Varieties vary in susceptibility Rotation and bury residue
Sclerotinia
Sclerotinia sclerotiorum Sclerotia in soil and seed lots, infected seed Cool wet conditions during flowering Seedling blight

Plant wilting/death with visible mycelia and sclerotia

Small shriveled seed

Varieties vary in susceptibility Plant clean seed Variety selection and moderate populations wide rows

G-seed trtmnts. containing fludioxonil for infected seed; G= Good Control, E = Excellent Control, * = Industry supplied data

References:

Prepared By:
Bruce Potter - Extension IPM Specialist SW MN
Seth Naeve - Extension Soybean Specialist

Return to the SWROC Pest Management Page
Return to the SWROC Home Page
Created on 4/26/00 by M. Werner.