Soybeans scouted last week were found with white mold and sudden death syndrome. These two diseases develop starting at the soybean flowering growth stage and can occur throughout the rest of the soybean growing season. Both diseases do not have in-season rescue treatments once plants have visible symptoms, however, steps can be taken to proactively manage these diseases in future seasons.
White mold develops from spores (ascospores) released from mushroom-like structures called apothecia. The apothecia germinate from winter hardened survival structure, the sclerotia. When ascospores land on senescing flowers under wet (>12 hours continuous wetness for several days) and cool temperatures (<85°F), infection is initiated. The infection spreads rapidly at the point of infection through the stem tissue. Once the lesion girdles the entire stem, the plant wilts and dies (Figure 1). Dead plants can be seen in clusters especially a tree line (if a soybean field has a shelter belt) or high fertility areas of the field.
White mold can be managed through planting tolerant varieties, increasing row spacing (>20” rows), avoiding excessive nitrogen fertilizers (such as animal manure), and applying a fungicide at R1 (beginning flowering). For the fungicide to be effective, the type of the fungicide to be used, timing, and the coverage are important. Only a few fungicides have been found to reduce white mold severity. The list of fungicides against white mold can be found here under white mold column on 2nd page). Research shows that the best timing is R1. In order to increase the chances of the fungicide reaching lower parts of the plant, a high spray volume (at least 15 GPA) is recommended. A biological product called Contans has activity against sclerotia which the source of inoculum but effectiveness of this product depends on the longevity it is in contact with sclerotia. This product should be applied at least three months before the onset of symptoms (i.e, before soybean planting). Repeated Contans applications may be needed.
Figure 1. A wilting soybean plant as a result of white mold infection.
Sudden Death Syndrome (SDS)
Sudden death syndrome (SDS) development is quite different from white mold in that infection actually happens during the seedling stage but the symptoms develop after flowering. The SDS pathogen survives in soil and under wet and cool conditions at planting, the pathogen infects soybean seedlings. It expands to the taproot and lower parts of the stem. Frequent rainfall after soybean flowering promotes the movement of the toxin produced by SDS pathogen from the roots to the leaves. This toxin causes the typical SDS symptoms of bright yellow blotches between leaf veins (Figure 2). To confirm SDS, split the stem longitudinally and examine the lower part of the stem. The cortex should be discolored. If the plant is uprooted gently and the soil is moist, blue mycelia may be observed on the taproot surface.
SDS is managed through planting tolerant varieties, planting in well drained warm soils (>55°F), using fungicide seed treatments (products containing Fluopyram or Thiabendazole), and managing SCN since SCN can increase the risk for SDS.
Figure 2. Sudden death syndrome symptoms: Interveinal blight yellow blotches, with advanced symptoms showing necrotic (dead tissue) between leaf veins (inset picture).