With the soybean crop moving closer (or in some cases at) flowering, now is an excellent time to review factors that may impact the risk of white mold in 2010 (caused by Sclerotinia sclerotiorum). The first two factors that should be considered have already occurred…knowledge of the field history of the disease and the level of resistance in the soybean variety that was planted. These can be used to provide a baseline assessment of risk. If you are unsure of the level of resistance in your soybean variety, I would recommend consulting with your seeds person to obtain that information. In addition, there are several abiotic and biotic factors that need to be considered to understand if there is an increased risk of white mold. If temperatures are in the moderate range (70’s are optimal), rainfall is normal to above normal, soil moisture is at or above field capacity, and there are extended periods of prolonged fog and leaf wetness at or just after flowering, these can increase the risk of white mold in the field. Also, agronomic practices that are used to promote high yield potential in soybean (in particular those practices that encourage early canopy closure) like early planting date, higher plant populations, and narrow row spacing can also increase of the risk of this disease.
Active scouting should commence as we move into flowering for presence of apothecia, which are tan, cup-shaped mushrooms (0.5-2mm in diameter) that can be found on the soil surface (Figure 1). Apothecia produce the spores of S. sclerotiorum that infect soybean plants. Previous research has shown that apothecia production is related to soil moisture and temperature, and the timing and density of the crop canopy closure.
Figure 1. Apothecia of Sclerotinia sclerotiorum. These small (0.5-2 mm in diameter), tan fruiting structures produce spores that can infect senescing soybean flowers. Parting the soybean canopy and inspecting the soil for apothecia is an important way to determine your risk of white mold.
Management options include foliar fungicides (like Topsin M, Domark, and Endura) or herbicides (Cobra). Several questions have been raised about the efficacy of these products and in 2009 results variable when comparing Marshfield, WI and DeKalb, IL. Also, several questions have been raised regarding the use of other herbicides that cause similar changes to the soybean canopy like Cobra. Keep in mind that white mold suppression is listed on the Cobra label, while this is not indicated in other herbicides in the same chemical class. If the decision to spray a fungicide/herbicide is made, make sure that proper application timing occurs – applications should be made at flowering to protect senescing flowers from infection. Also, make sure that there is excellent spray coverage, which means that canopy penetration is essential to protect the developing soybean flowers.