We are receiving initial reports of white mold in soybean (causal agent: Sclerotinia sclerotiorum) in different areas of Pennsylvania (Figures 1 and 2). According to USDA-NASS (report date: August 20, 2017), over 60% of the soybean fields in Pennsylvania are setting pods (R3), which means it is too late to consider a fungicide application to manage this disease.
Therefore, our focus should be on obtaining accurate information about the impact of white mold by assessing the incidence of the disease as we move into harvest. This information is important to document correctly the impact of the disease in terms of yield loss, as well as to help in making decisions regarding the best management options to reduce the impact of white mold in the future.
For fields where white mold has been noted, it will be very important to assess the incidence of white mold at maturity (growth state R7). This assessment will help to quantify the potential yield loss due to white mold, not only the field level, but also by integrating the field data to look at the yield loss at the local, area, and state level.
Disease incidence can be estimated by examining different areas of the field, whereby we quantify the number of plants with white mold divided by the total number of plants assessed. For example, if 100 total plants were assessed in the different areas and the number of plants with white mold was 42, the incidence would be 42%.
Yield loss can be calculated by considering the following: for every 10% increase in the incidence of white mold observed at R7, yield can be reduced by two to five bushels per acre. Additional information is available at the “Crop Protection Network” website.
Before we focus on management options to reduce the risk of white mold in the future, it is important to mention that fields with white mold should be harvested last. It is probable that sclerotia of S. sclerotiorum will end up in the combine, and without proper cleaning, it will be very easy to move this inoculum from one field to another. You may need to clean the combine several times during the harvest, especially if you are moving from one farm to another in your operations.
Looking towards the future, by knowing and documenting the incidence of white mold for each field, we can consider several different management options.
First, variety selection is important and working with your local seed person will be important to consider pathogen-free seed, selecting varieties with the best available level of resistance, and using the most appropriate maturity group for your production region. No variety is completely resistant to white mold, but several varieties with partial resistance are available and these should be discussed with your seed rep (Figure 3).
Furthermore, cultural practices like reducing plant populations and increasing row width, rotating with nonhost crops, using alternate tillage practices, controlling weeds that can be hosts of S. sclerotiorum, and using cover crops that help to reduce inoculum density may help to minimize the impact of white mold in the future.
For example, forage legumes like alfalfa and clovers, can be infected with S. sclerotiorum, while small grain cover crops like oat, wheat or barley may stimulate early emergence of apothecia, which enables the soybean crop to escape the period of infection. Depending on the field history, a longer rotation may be required with nonhost crops like corn or small grains.
If you will consider applying fungicides in the future, and if conditions warrant such an application, it is important to understand how different products work, since they will inhibit infection, or modify the soybean canopy, differently. Depending on the product selected, a second application may also be required.
Futhermore, and depending on field history, biological control has potential as part of a long-term strategy to reduce inoculum density.