According to the latest Illinois Weather & Crops report (7/17/00), 86% of the Illinois soybean crop is rated good to excellent and two-thirds of the crop is well into bloom. Now, about that price(more about that later). Because much of the crop is in the reproductive phase, it's time to watch for several midseason soybean diseases. For the time being, this advise is particularly true for northern Illinois and parts of southern Illinois, which have received a good amount of rainfall over the last week or so. Considering the fact that as little as 2 months ago many were talking about the "2000 drought," it seems quite odd even to think about white mold. Although I have not heard of any reports of white mold in Illinois at this point, should growers in the northern half of the state experience cooler weather along with continued rainfall, white mold may once again rear its ugly head in Illinois.
White mold is favored by moderate temperatures (less than 85°F), normal or excessive rainfall, and high canopy humidity. Thus, you should monitor field areas that dry slowly due to a dense or lodged canopy or lack of wind, and areas where white mold has appeared in previous years. The first symptoms of white mold generally appear during growth stages R1 through R3 (beginning bloom through beginning pod) and are often aggregated (found in "hot spots") rather than uniform across the field. Infected plants initially have grayish green leaves (similar to the way frost-damaged plants would look) that eventually turn brown but remain attached. Eventually, all or part of the plant dies due to girdling stem lesions typically located several inches above the soil line. Side branches and pods may also be infected (Figure 1). Recognize that these symptoms can be confused with several other diseases including midseason Phytophthora, brown stem rot, and stem canker (see Table 2 for comparisons). However, plants that were killed or that are dying due to white mold typically have small amounts of cottony fungal growth on the surface of infected plant parts. Furthermore, at crop maturity, irregularly shaped sclerotia (hardened fungal survival structures) will be noticeable on and within white mold-infected stems and pods (Figure 2). White mold may go unnoticed until the sclerotia is found during or after harvest.
If conditions are right for white mold in some fields or parts of your fields, what can be done at this point? Due to the depressed crop price, it is very unlikely that fungicide applications are economical. If applied properly, Topsin M can be used to manage white mold. Two applications are necessary, the first applied between 25% and 50% of full bloom and the second applied 10 to 14 days later. Good canopy penetration is crucial because the fungus is active in the top few inches of the soil and ejects spores from there into the lower canopy. Even if you don't plan to spray for white mold, accurate diagnosis is important for long-term disease management planning. Recently, the North Central Soybean Research Program introduced a White Mold Coalition Web site (http://www.plantpath.wisc.edu/ncsrpwhitemold/summ99.htm), which provides a great deal of user-friendly information about short- and long-term management strategies, including varietal resistance information.--Bruce Paulsrud