Issue No. 19, Article 6/July 30, 2004
It May Be a Year for White Mold in Parts of Illinois
White mold, also called Sclerotinia stem rot, has been relatively uncommon in most Illinois soybean fields over the past few years, although it was reported in some fields in northwestern Illinois in 2003. This disease may be a greater problem this year. Based on cool weather in many areas and recent reports of white mold in northern Illinois and eastern Iowa, conditions may be set up for greater white mold damage than has been seen in recent years. Soybean fields should be scouted to determine whether and where white mold or other diseases are damaging the crop.
White mold is caused by the fungal pathogen Sclerotinia sclerotiorum. This disease can be very damaging, especially during cool weather (less than 80°F) with heavy morning fogs and dew and when growing conditions favor high yields.
White mold is relatively easy to accurately diagnose. The disease causes soft rot and darkening of stem, leaf, and pod tissues and can usually be recognized by white, moldy growth on infected plant parts. These symptoms are often associated with gray to brown discoloration and wilting of the upper leaves. As the fungus grows and infects soybean tissue, it produces sclerotia, which are gray to black spherical to elongated structures about 1/8 to 1/4 inch in diameter and length.
White mold mycelium (white) and sclerotia (gray/black structures) on infected soybean stems. (Photos from the University of Illinois.)
Understanding the disease cycle of white mold can help with understanding when and where the disease occurs and can give clues for managing it. Initial infection typically occurs during cool and moist weather conditions when plants are flowering. The sclerotia are the resting and overwintering structure of this fungal pathogen and can survive in soil for years.
The sclerotia within about 1 to 2 inches of the soil surface germinate and produce small mushroom-shaped structures, called apothecia, that produce numerous spores, often when soil is moist and after the soybean canopy has at least partially closed. The spores land on senescing flowers and initiate infections that spread on the stem or other plant parts. White mold can spread from infected to healthy plants if they contact each other.
White mold cannot be easily managed, especially when environmental conditions favor disease development and field soil contains large numbers of sclerotia. Soybean cultivars with the highest levels of resistance should be chosen. Many cultivars are rated for resistance to white mold, and additional information on resistance can be found at the Varietal Information Program for Soybeans (VIPS) Web site.
Several cultural practices can also help to reduce white mold under some circumstances. These include late planting, low plant populations, wide row spacing, moderate to low soil fertility, good control of broadleaf weeds, and avoiding rotation with sunflowers and other bean crops. If white mold is noted in irrigated soybeans, irrigation should be applied on a schedule designed to minimize the number of hours a week that plants are wet. Fungicides have shown mixed results. In a recent study with results from Illinois and several other states (Mueller et al., 2002), fungicides applied at the R1 to R3 growth stages reduced severity of white mold when disease pressure was low, but control was inconsistent when disease pressure was high.
For more information on white mold and many other soybean diseases, see the brief summary at the University of Illinois Field Crop Diseases Web site, as well as extensive information at the excellent Plant Health Initiative Web site.--Dean Malvick