Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Sudden death syndrome was diagnosed on soybean from Sussex County. This was an unexpected find since I associate SDS with cool, wet seasons. The only control for SDS is to plant resistant varieties, or, if susceptible soybeans are to be planted, double crop soybeans are rarely infected. I would not expect this to be widespread but we did see it so be on the lookout for symptoms.
Foliar symptoms of SDS
Infected root on the left with diseased leaf compared to healthy stem and leaf.
Charcoal rot was also confirmed on soybean as well as corn. Soybeans are killed by the fungus resulting in dead, stunted plants in irregular patterns in the field. Symptoms are worse on full season early maturing varieties (Maturity Group III and possibly early Group IV’s). The diagnostic symptoms are the ashy gray stems and lack of roots. Splitting the lower taproot and stem longitudinally with a knife reveal the best diagnostic feature which is the presence of the small black microsclerotia in the pith and often the cortex. Charcoal rot is favored by hot, dry weather and is common in Delaware soils that have been heavily cropped to soybeans over the years. The tiny sclerotia overwinter in the soil and it just takes the right environmental conditions for the disease to occur. There is no usable resistance in soybeans to charcoal rot. The stress during flowering and early pod set triggers development of symptoms if that coincides with hot, dry, stressful weather. Most initial infections occur during the seedling stage and remain latent until flowering and later. Later maturing soybeans typically flower and set pods most years when conditions are less stressful and escape the yield reducing effects of the infection. Double crop soybeans are rarely affected by charcoal rot here in DE. Two year rotation will help reduce microsclerotia levels in the soil but if corn is used at least three years is suggested. Longer rotations would be better if they could be implemented.
Split soybean stem showing the charcoal gray discoloration in the pith with microsclerotia.
Close up of gray pith from the presence of the tiny microsclerotia of the fungus that causes charcoal rot.