White mold, also known as Sclerotinia stem rot, is one of the major fungal diseases that affect soybeans. White mold has become a major concern over the last few years, especially in Eastern South Dakota. Based on remote sensing and ground-based white mold observations, some counties had over 30% of soybean acres affected by white mold during 2017 growing season. Higher incidence of white mold typically leads to greater yield loss. For example, every 10% increase in white mold incidence may result in a corresponding 2-3 bu/acre loss.
Source of Inoculum
The white mold inoculum comes from the overwintering hardened fungal mass called sclerotia from previously infected plants (Figure 1). In the spring, if soil is shaded and moist, the sclerotia produce small mushroom-like structures called apothecia (Figure 2). The apothecia produce spores and when these land on senescing soybean flowers. Under conducive weather conditions (<85° F and >42 hours of continuous wet stem surface or >12 hours of surface wetness on a daily basis), infection is initiated.
Figure 1. A soybean stem infested with sclerotia, a black hardened white mold fungal mass. These can be inside and outside the infected stem. During soybean harvest, the sclerotia fall on the ground and become the source of inoculum for the following seasons.
Some of the soybean fields that had white mold last year already have apothecia beginning to develop (Figure 2) Most of the soybean producing areas have had good rainfall amounts and also overcast days which are highly conducive weather conditions for white mold spores production. Although early planted soybeans may be starting to flower (R1), the majority of soybeans in SD have not reached R1. However, the fact that white mold apothecia are starting to develop, this means there is a risk for white mold to develop. Producers are encouraged to assess the risk factors for white mold and plan a timely fungicide to protect soybeans from white mold.
Figure 2. Small mushroom-like structures called apothecia germinating at a soybean field with high white mold incidence last season.
Risk Factors for White Mold
The extent of white mold development in a field is dependent on: field history of white mold, narrow row spacing/high planting population (>160,000 seeds per acre), susceptible cultivar, high soil fertility environment and favorable weather conditions. Field history is important because white mold sclerotia can remain active in the field for a long time especially where tillage is practiced. Narrow row spacing and high planting population provide early canopy closure, shading the soil surface and providing cool tempratures which stimulate the sclerotia to start producing spores.
White Mold Management
White mold is best managed through a combination of various tactics. These include planting a resistant variety, crop rotation, wider row spacing (>20 inches), proper soil fertility management (avoid excessive high N fertilizers such as animal manure), and timely fungicide application. Fungicide application is the only in-season tactic for white mold management. To obtain the maximum benefits from a fungicide application, the time of application and type of fungicide are important. The best timing is beginning flowers (R1) to protect initial flowers where the infections take place. Upper flowers do not get infected because of unfavorable microclimate (good air movement and less free moisture. For fungicides effective against white mold, see this guide under the white mold column.
Some herbicides with lactofen active ingredient such as Cobra can reduce white mold development by opening up the plant canopy. Use of these herbicides should be weighed against side effects on the plants such as crop injury.