Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
As mentioned in the vegetable updates, our August and early September weather conditions have favored the development of White Mold caused by the fungus Sclerotinia sclerotiorum. I haven’t yet seen any fields with serious issues, but some scattered plants showing symptoms. This pathogen has a wide host range, and it is more common to see symptoms in peas or other early season crops. This fungus produces black over wintering structures called sclerotia that somewhat resemble mouse droppings. These survival structures can form on the surface or within infected plant tissue and remain in soil for multiple years. Under favorable conditions, cool periods following heavy rains or irrigation, these structures will form apothecia (small, mushroom-like structures) that release spores into the air that infect senescing flowers. The fungus can produce stem lesions that girdle the stem and eventually you may see white hyphae and sclerotia on the outside or inside of the stem (Figure 1). This disease is favored by moderate temperatures, frequent rain or irrigation, high humidity, and dense canopies with limited air flow. Many of the symptoms visible right now are leaf flagging with stem lesions visible as you investigate the canopy (Figure 2). Fungicide applications in soybean are most effective at R1 before symptoms develop. If you are seeing scattered symptoms now or beans are past R3, fungicide application is not likely to contribute to disease control. It is also important to note that many of the fungicides we use for control of foliar disease do not necessarily have high efficacy rankings for control of white mold.
Figure 1. Soybean stem with black sclerotia of Sclerotinia sclerotiorum the causal agent of white mold.
Figure 2. Early symptoms of white mold with soybean plant showing a wilted leaf (top) resulting from stem lesion (bottom) lower in the canopy.