July & August: Time to assess white mold risk in soybeans

Dean Malvick, Extension plant pathologist, Angie Peltier, Regional Educator - Crops, & Anthony Hanson, Regional Educator - Integrated Pest Management

Nearly every year white mold develops to damaging levels in some Minnesota soybean fields.  That includes last year in areas that received timely rains in July and early August when it was very dry in most other areas throughout July and August (Fig. 1).
 

Figure 1. Soybean plants killed by white mold.

White mold is favored when soybean plants are flowering, the rows are closing, soil is moist for 10+ days and both stems and leaves are wet and temperatures below 68°F for prolonged periods of time (Fig. 2). But we also see outbreaks of white mold when all of these conditions don't seem to have to been met.

In much of Minnesota recently, the day and night temperatures were too warm to favor white mold. However, with the recent rains in some areas and cooler temperatures in the forecast, white mold could now or soon be developing in soybean fields. As of July 3, 2023, 29% of Minnesota soybeans were blooming based on the USDA-NASS Crop Progress and Conditions report, with much higher percentages occurring in some areas.

Figure 2. White mold infection on soybean stems that has become pronounced later in the growing season.

White mold risk can be reduced in advance with partially resistant soybean varieties, reduced plant populations, and wide rows. At this time of the season, fungicides are one of the few options available that can reduce white mold severity where the disease risk appears to be high. Unfortunately, white mold can persist in the soil for years and will not be removed entirely from a field through rotation in later years.

Risk of white mold in soybean can be assessed in part by the field history of white mold and the crop density, stage, and recent and forecasted weather conditions. A risk assessment tool, Sporecaster, was developed at the University of Wisconsin to help assess risk of white mold in soybeans. It may be helpful to try this tool to determine how well it works in your area. Fields that have had manure applied can also be at higher risk for white mold. Common weed species have also serve as hosts for white mold, such as ragweed species, lambsquarter, and thistles, while also reducing airflow through the canopy that can further facilitate white mold growth.

If the risk of white mold is considered high in a field that is in the early flowering stages and fungicides are to be used, they are most effective when applied to soybeans at the R1/R2 flowering stages when the rows are filling. Therefore, they need to be applied soon. In fields that develop high levels of white mold, two applications of fungicides (R1/R2 and R3) can protect more yield and be profitable compared to one application at the R1/R2 stage, but two applications likely will not be profitable in fields with less severe white mold issues.  One of the obvious challenges though is that it is unknown whether high levels of white mold will develop in any particular field in any given year.

 For more information on white mold and its management visit:

• U of M Extension's Soybean White Mold webpage
• Crop Protection Network: White Mold Introduction
• Crop Protection Network: Integrated Practices for Managing White Mold