Watch for white mold in soybeans
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Soybean canopies have grown very lush from the recent hot weather and rain. Now that the weather has cooled off, particularly if you are in the areas that received heavy rainfall, be on the lookout for white mold. White mold is caused by the fungus Sclerotinia sclerotiorum. Cool temperatures (less than 85°F), rainy weather, moist soil and high humidity in the crop canopy, particularly as plants are flowering provide a favorable environment for the disease. Foliar symptoms of white mold are not very distinct; foliage wilts and the leaf tissue between the veins turns a faded, grayish green. Leaves eventually turn yellow and drop, but stems remain attached. Stems develop white, bleached lesions in the lower part of the canopy. Infections may spread up and down the stem from a node where a flower was infected. Cottony, white mycelium may be present on the stem or other infected plant parts. Sclerotia develop on or in infected stems and pods.
The fungus survives in crop residue as hard, black lumps of dormant mycelium (sclerotia) or on infected seed. In spring and summer, the sclerotia germinate and form mushroom-like fruiting bodies that produce large numbers of spores, which spread by wind and splashing rain. Infection takes place mainly through soybean flowers. The spores germinate using dead and dying soybean flowers as a food source to grow mycelium. The mycelium infects the stem near the node and spreads to other plant parts. White mold also infects dry and snap beans, peas, potatoes, alfalfa, cabbage, canola, carrots and others, including many broadleaf weeds. Corn, sugar beets and small grains are not susceptible. This disease is difficult to manage. Even if you are rotating to a non-susceptible crop, this fungus can survive for years in the absence of a susceptible host. Control broadleaf weeds during the non-host crop rotation – many are hosts for this disease. Plant partially resistant varieties. See MSU soybean variety trials web site at: http://www.css.msu.edu/varietytrials/soybean/whitemold.htm
Use planting rates and row widths that promote air circulation and rapid drying of plants and soil surface. Irrigate only as required for optimum plant growth. Using fungicides successfully is difficult, as soybeans often re-flower low in the canopy, providing additional entry points for the fungus later in the season. Fungicides labeled for white mold control in soybean include Topsin and Thiophanate methyl. We are well past the flowering stage where application is generally recommended (R1-R2 and 7-14 days later), and fungicides are likely to be of limited value at this stage of development.
The University of Wisconsin had developed an excellent guide to managing white mold based on field history and the susceptibility of the soybean variety you are planting.