Figure 3. A stem with fluffy white fungal growth and black sclerotia following infection by S. sclerotiorum. Image CPN.
- White mold is a stem disease that typically occurs in high production environments
- Cool, wet conditions after canopy closure favor disease
- Fungicide applications between R1-R3 are the most efficacious for suppression
- Product selection is important for white mold management, as efficacy and cost, and ROI differs.
The recent wet cool conditions have caused people in the region to wonder about white mold in soybeans, when fungicides are needed, and what fungicides work best. Below is some general information on this disease and its management that can be used in guiding your management and decisions.
General information (Figure 1)
White mold is a disease caused by the fungus, Sclerotinia sclerotiorum (Ss). This fungus survives harsh environmental conditions as recalcitrant resting spores called sclerotia. Sclerotia in the upper 2-3” of the soil surface germinate under cool wet conditions to produce a fleshy mushroom-like structure that contains infective ascospores (Figure 2). The pathogen then forcibly ejects these spores into the canopy, which land on nearby plants. Interestingly, Ss has evolved such that the environmental conditions that favor spore ejection coincide with canopy closure in soybeans under wet conditions, and filtered light from foliage is needed to properly form the spore bearing structures. Spores only move short distances (75 ft or so) so infections come from within a field, not distant sources.
Spores that land on senescing flower petals and tissues are infected by the pathogen, which then grows into the plant stems, destroying xylem and phloem and reducing water and nutrient uptake, and carbohydrate production. Pods can also be infected. Infected plants may wilt and die, and often are covered with fluffy fungal growth (Figure 3). For every 10 percent increase in the number of plants with white mold at R7, yield is reduced by 2-5 bu / A. Sclerotia are produced in and on dying plants. These sclerotia then are deposited onto the soil surface, where they can survive up to 10 years in the absence of a host.
Figure 1. White mold lifecycle. Image CPN.
Figure 2. A sclerotium with germinating apothecia. Apothecia produce the spores that infect soybeans to cause white mold. Image CPN.
In season management
Fields with a history of white mold and a dense canopy during flowering are at the highest risk for white mold. These are early planted fields with narrow row spacing, high populations, and/or high soil fertility. Wet weather or heavy overhead irrigation from R1-R3 with temperatures below 85°F also favor disease. A good tool for helping you determine white mold risk is to use a free app developed by UW Madison called Sporecaster. This application uses specific environmental data and crop growth stage to deliver a potential risk for white mold in specific fields. If you are at high risk, you have approximately 5 days to make an application of a labeled fungicide to help suppress disease.
Fungicide timing
White mold severity is greatest during flowering and replicated university trials show that applications made between R1 and R3 are the most efficacious and provide the greatest ROI. Under high disease environments, the best fungicides typically reduce white mold by 14-19% and protect 16-23 bu/A of yield (Table 1).
Table 1. Fungicide application timing on white mold disease index (DX) and yield compared to non-treated controls. Different letters within a column indicate statistically significant mean differences. From Wilbur et al. 2020.
Fungicide products
Based on university metaanalyses, Endura® tank mixed with Priaxor®, Aproach®, Endura®, and Delaro® have the greatest overall white mold efficacy in terms of percent yield benefit relative to non-treated controls (Table 2). Although Cobra® provides the best overall reduction in white mold, the yield benefit was negligible, potentially as a result of canopy burn. However, despite lower efficacy, Topsin® and Cobra® actually provided the greatest ROI, due to lower application costs.
One of the benefits of the pre-mix fungicide products and products with a group 7 or 11 mode of action over other single site mode of action products would be the additional potential disease suppression from foliar and seed disease (mostly brown spot, potentially pod and stem blight) that could also be attained in low white mold situations. Products with group 7 and 11 modes of action have excellent residual and spectrum of control and do a nice job of keeping the canopy healthy. Cobra® is not a fungicide and therefore will not provide any foliar health benefits.
Table 2. Effects of fungicides on white mold disease index (DX) and yield relative to non-treated checks. Different letters within a column indicate statistically significant mean differences. From Wilbur et al. 2020.
The following ratings are pulled from the NCERA-137 extension pathology ratings for foliar soybean fungicides.
If you have any questions regarding white mold management or any issues related to field crop production, please contact your local FS Crop Specialist.
References