White mold sclerotia on soybean stem
The fungus that causes this disease thrives on the soil surface and lower stems when the environment is humid with a closed canopy that prevents air flow. The stem and lower leaves become infected, and then the disease moves up the plant resulting in yellowing and defoliation. As it progresses, white tufts of fungus are visible on the tissues of the lower canopy, and ultimately hard, black overwintering structures (sclerotia) form that can survive in residue and in the soil for many years.
Where did the fungus come from? Well, most likely it was moved into the field as one of these little sclerotia with tillage operations or on other equipment, but this could've happened years ago since they live so long. The tough sclerotia may have been laying in wait for a long time before the weather was right to start growing and infecting again.
Now what? Survey your fields just before the majority of the leaves start yellowing naturally. If 20% or more of a field is affected by white mold, the variety you are growing is probably not the right one for your situation. Work with your seed reps to select a white mold resistant variety for the future in that field. Not just the next time you grow soybeans--every time you grow soybeans in that location. This fungus can survive in residue and soil for 7 to 10 years. Tillage may help the first year by burying the sclerotia, but the next time you till, you could bring them up again. There are no 100% resistant varieties for white mold, but there are varieties that are better than others, and since once you have the pathogen it doesn't go away, this is one of the strategies you should employ for management.
When you harvest, try to harvest these fields last. The sclerotia can and probably will end up in your combine, so you want to avoid moving them to clean fields. If you have someone come in to custom harvest, warn them that they will probably want to clean the combine as much as possible before moving on to someone else's farm. Apply this same thinking to all your operations in the field that involve soil movement. Anytime you sink equipment into the soil (tillage, planting) there is potential to pick up some fungus and move it to a clean field. Do these operations in your white mold field last, or blow out and power wash your equipment between fields.
When you plant beans in this field in the future, you'll want to consider applying a fungicide at or before R3. There are some effective products available, but you need to get them on before the canopy closes to be sure you can penetrate down to the lower stems. Refer to this fungicide efficacy chart for product selection.
Alfalfa is an alternate host for white mold. Corn and cereals are among the plants that are not affected by the fungus that causes white mold. So if you have grasses in your rotation, they will not increase the fungus in your fields.