Soybean white mold in western Iowa

Growers are beginning to see soybean white mold, which appears as white, fluffy mycelium on the stems of dead plants. In severe cases, growers can spot dead plants without even going into the field. Reports indicate that the disease has caused severe damage in some soybean fields in the western part of state, especially the northwest where spring planting was not delayed. We also have received field reports from central and northern Iowa, some of which had considerable infestation. Delayed planting, a late close in the crop canopy, and the type of weather at blooming have affected the prevalence of the disease this year.

White mold on soybean stem.

If you have seen white mold in any field, assess the damage and the level of infestation right now. You can use this information to help manage the disease in the next soybean crop. Here are common questions growers have about soybean white mold:

  1. I'm seeing soybean white mold. What can I do now? Will a fungicide application help? It is too late to reduce disease damage this season. It is recommended that a fungicide be applied no later than 50 percent of blooming. Applying fungicide this late wastes money.
  2. Will the mold continue to spread in my field? Basically, the disease will not spread, although you may continue to see an increase in the number of dead plants. Those dead plants were infected by fungal spores several weeks ago. Mold can grow from leaf to leaf, but less than 7 percent of dead plants are the result of this type of growth. Some white mold was reported in irrigated soybean fields. Irrigation will not increase the spread but may promote growth of the fungi in infected plants.
  3. What kind of losses can I anticipate? This is a difficult question because white mold occurs in high yield soybean fields and soybean is well known for its ability to compensate for lost foliage. When the kill is light (i.e., 10 percent of all plants), expect a minimum loss. On the other hand, I have seen a loss of about 20 bushels per acre in fields that had about a 50 percent kill from white mold.
  4. How should I harvest infected fields? Will the combine spread the disease to other fields? First, do NOT save beans from fields infested with white mold if you plan to use your own seed. Combines cannot separate white mold sclerotia (a fungal structure that looks like mice droppings) from beans. Use of contaminated seeds will spread the pathogen from one field to another. When you combine soybeans, you may want to first harvest fields free of white mold. If not, you must clean soybean debris from the combine after you harvest a field infested with white mold.

White mold sclerotia with soybean seeds.

Remember, white mold information you gather this season will be useful for management of the disease in the next soybean crop. There is new information available to manage this disease, which I will discuss in a winter issue of the ICM newsletter.

This article originally appeared on page 166 of the IC-476(23) -- September 3, 1996 issue.

Updated 09/02/1996 - 1:00pm