White mold is more correctly called Sclerotinia stem rot. The disease is prevalent in cooler growing regions and can cause significant yield losses, especially during cool, wet seasons. White mold is often recognized by fluffy, white growth on soybean stems. Initial symptoms generally develop from R3 to R6 as gray to white lesions at the nodes. Lesions rapidly progress above and below the nodes, sometimes girdling the whole stem. White, fluffy mycelial growth soon covers the infected area, especially during periods of high relative humidity. Characteristic black sclerotia eventually are visible and embedded within mycelium on stem lesions, and inside the stem as the plant approaches death.
Initial foliar symptoms include tissues between major veins turning a gray-green cast, while vein tissues remain green. This can be mistaken for other diseases like brown stem rot, sudden death syndrome or stem canker. Eventually, leaves die and turn completely brown while remaining attached to the stem. The fungus survives in the soil for several years as sclerotia. Rain, cool temperatures, high humidity and moist soil beneath a closed canopy during soybean flowering and early pod development favor the growth of the fungus. The disease cycle begins when mushroom-like structures called apothecia are formed on the soil surface from sclerotia. Apothecia are typically 1/4 to 1/2 inch in diameter. Shaded conditions favor the production of apothecia from sclerotia. Spores from apothecia infect senescing soybean flowers and the fungus eventually grows to the stem. The disease often develops in areas where moisture collects due to fogs and extended dew periods as well as in pockets of poor air drainage (along tree lines).
The fungus can spread to new fields with improperly cleaned seed and by the movement of infested soil.
Variety selection: No soybean variety is completely resistant; varieties range from moderately resistant to very susceptible. Ratings are based on degree of premature plant death. The incidence of premature plant death is predictive of yield loss, but the rate of plant death is slower for some varieties and allows an acceptable yield even in the presence of disease.
Crop rotation: At least two or three years of a non-host crop such as corn, small grains and forage legumes can reduce the number of sclerotia in soil. Crops that should not be in rotation with soybean in fields with white mold risk are beans, peas, sunflowers and cole crops.
Tillage: More sclerotia are found near the soil surface in no-till systems, but sclerotia numbers begin to decline if left undisturbed. However, viability is maintained if sclerotia are buried 8 to10 inches in the soil. Greater tillage also promotes earlier canopy development, thus increasing the risk of white mold.
Canopy management: Early planting, narrow row width and high plant populations all accelerate canopy closure and favor disease development. However, modification of these practices also may reduce yield potential. The history of white mold in fields should be considered before growers modify practices that promote canopy closure. Bushy soybean varieties and lodging also create a dense, closed canopy which favors white mold growth.
Weed control: Weed control is critical as many broadleaf weeds are hosts of the white mold pathogen. Also, some herbicides (e.g., Cobra®) may suppress the activity of the fungus or disrupt germination of sclerotia.
Biocontrol: Some antagonistic fungi may be applied to the soil to colonize and reduce sclerotia numbers. For example, the product Contans® can reduce white mold sclerotia populations.
Fungicides: Foliar fungicides can effectively manage white mold, or at least reduce disease severity; however, application timing is critical. Fungicides are most effective when applied immediately before infection.
Photo by Daren Mueller