By Mike Staton and Mike Chilvers, Department of Plant, Soil and Microbial Sciences
Soybean growers should use an integrated approach to white mold management, and some of the most effective practices are implemented in the spring. This article discusses these practices and offers specific recommendations.
An overarching concept to keep in mind when deciding which practices to implement and at what level they should be implemented is to select practices that have been proven to reduce white mold when it occurs, but will not reduce yields significantly if environmental conditions don’t favor white mold.
When managing white mold, it is helpful to understand principles of the disease triangle. White mold disease only occurs when all three factors come together, which includes a susceptible host, presence of the pathogen and favorable weather conditions. By altering components of the triangle, which are discussed below, we can influence disease development including variety selection, affecting canopy microclimate through planting rate and row width, and even managing sclerotia of the pathogen.
Varieties vary significantly in their susceptibility to white mold, and planting the most resistant varieties in fields prone to white mold is a key management practice. All seed company catalogs provide relative white mold ratings for the varieties they offer. These ratings provide valuable information when comparing varieties from a given seed company. However, they are less useful for comparing the level of white mold tolerance between varieties from different companies.
Selecting varieties that resist lodging and have a narrower canopy width can also reduce the incidence and severity of white mold. Planting varieties from a range of maturity groups may help some fields avoid severe white mold infestations by staggering the susceptible flowering period. We saw this in 2014 where the early maturing varieties tended to avoid white mold infection and development.
Reducing planting rates can reduce white mold incidence. Reduced planting rates will decrease the potential for lodging and plant-to-plant spread of the disease. Most agronomists agree that a harvest population of 100,000 uniformly spaced plants per acre will maximize economic returns even in the absence of white mold. This would be a reasonable target to aim for in fields having a high potential to develop white mold. There are many factors that determine soybean germination and emergence, and Michigan State University Extension advises growers account for these when reducing planting rates.
Emerson Nafziger and Dennis Bowman at the University of Illinois have developed an excellent soybean planting rate calculator. The calculator allows users to fine-tune planting rates by entering the final stand they want to achieve and adjusting germination and emergence percentages for seed quality and planting conditions. The soybean planting rate calculator is available in the following formats:
Wide rows greater than 20 inches can decrease white mold, but may not always lead to a yield increase. University row spacing trials have shown 30-inch rows reduce soybean yields by approximately 3 bushels per acre when compared to 15-inch rows.
Applications of nitrogen (N) fertilizers or manure have been shown to increase early plant growth and canopy closure, creating favorable conditions for developing white mold. Therefore, N fertilizer and manure applications should be avoided in fields having a history of white mold. Nitrogen fertilizer applications to soybeans are rarely profitable, making this the easiest practice to implement.
Growers may also consider applying a biological control product such as Contans or KONI to fields having a history of severe white mold. These products contain Coniothyrium minitans, a naturally occurring fungus that attacks and degrades sclerotia in the soil.
The products should be incorporated into the soil as uniformly as possible to a depth of 2 inches at least 3 months prior to initial soybean bloom. It is important to note Contans and KONI will attack and degrade the sclerotia only when in contact with white mold sclerotia. Tillage operations deeper than 2 inches deep should be avoided following an application of Contans or KONI to prevent redistributing viable sclerotia into the top 2 inches where they can germinate and infect your soybean crop.
Remember, the most effective white mold strategies incorporate a variety of tactics and many of the most effective tactics are implemented prior to or at planting.