Farmers who only see $$$ when they look at a dense, green soybean stand in July may not know that white mold can ruin such a field in August. The healthiest looking fields are often at greatest risk.
“White mold frequently penalizes those growers who manage their soybean crop most intensively, pushing it to its highest potential,” says University of Wisconsin-Madison agronomist Ed Oplinger.
“It”s the most complex disease of field crops I”ve been involved with,” adds UW-Madison plant pathologist Craig Grau, who has studied white mold since the 1970s.
“Virtually everything we typically urge a grower to do to improve plant health and yield ends up increasing the risk of white-mold problems,” Grau says. “Practices such as planting soybeans early and planting them close together have helped many farmers increase soybean yields. But those same practices favor white mold development.”
With Wisconsin”s soybean acreage doubling since 1990, more and more Wisconsin farmers must now learn how to manage the disease, according to Oplinger. Even the most skillful management program can”t eliminate losses to the fungus, he warns.
“Farmers who are satisfied with yields of 35 to 40 bushels per acre can plant soybeans in 30-inch rows and don”t have to be too fussy about the variety they select,” Grau says. “They”ll have some losses but probably will be OK. However, farmers need to be careful if they are aiming for yields of 50 bushels per acre or more and plant soybeans in 7-inch or 15-inch rows.”
Grau, Oplinger and plant pathologist James Kurle have spent the last several years studying how different management practices affect white mold development on soybeans. The three are with the College of Agricultural and Life Sciences and the UW-Extension. As participants in an 18-member research team studying white mold in seven Midwestern states, they examined how crop rotation, variety selection, tillage, row width, plant density and herbicides affected the disease.
The Wisconsin scientists found that the best management plan is to combine careful variety selection with practices that reduce the presence of white mold in a field and that minimize damage if it is present. Those practices include canopy management, tillage and crop rotation.
How farmers combine those elements depends on their past problems with white mold, according to Grau. Farmers who are planting a field to soybeans for the first time often have few problems with the fungus. Farmers who had only minor problems with white mold the last time they planted soybeans should avoid susceptible varieties and not plant soybeans back to the same fields for at least a year. If the disease attacked many or most plants the last time soybeans were planted, growers need to be very defensive. That means planting one of the most resistant varieties, planting in wide rows, using no tillage if possible and only planting a field back to soybeans after two or three years.
“The long-term solution to the white-mold problem is developing soybean varieties with high resistance to the fungus,” Grau says. “Many people are working on that, but it”s not going to happen this year or next.
“Planting a variety with the appropriate amount of resistance is the foundation of any plan to control the disease. We don”t have any varieties that are highly resistant to the disease, but we do have about 40 that are moderately resistant. Although white mold infection kills 10 to 20 percent of our most resistant ones, it decimates our most susceptible varieties, killing most plants and reducing yields by about 50 percent.”
On the Internet, farmers can quickly find resistant and susceptible varieties at Grau”s soybean health homepage, http://www.wisc.edu/plantpath/soyhealth The homepage contains updated information on ways to control white mold and other soybean diseases. Growers also can get this information from their county UW-Extension offices.
The Wisconsin researchers found that the most effective practice to control white mold damage was managing the plant canopy. Warm, humid weather favors the fungus, while dry, hot weather limits losses to white mold, according to Grau. The goal of canopy management is to avoid the warm, humid conditions that can develop inside a dense soybean canopy.
“White mold losses escalate when high plant populations or excess fertility produces a dense canopy,” Grau says. “Adjusting the planting date, fertility, tillage system, row width and plant spacing all affect the soybean canopy. Growers can limit losses to the disease by implementing practices that produce a less dense canopy.”
Managing the canopy can be tricky because practices that reduce the risk of white mold losses also reduce yields. “Planting soybeans in narrow rows may not always increase yields, but it generally leads to greater problems with white mold,” says Grau. “Growers must do some tinkering with row width and plant density to get optimal yields.”
When it comes to white mold and tillage, Grau says the less soil disturbance the better. The disease develops from fungal resting stages that remain in fields and decay slowly over several years. Because the resting stage is more likely to decay when close to the surface, no-till fields tend to have the fewest problems with white mold. When fields are rotated to other crops, farmers should use the least amount of tillage necessary, such as cultivation, for weed control. Minimum tillage also tends to keep the disease in patches rather than spreading it throughout fields.
Rotating crops has not been as effective in controlling white mold as variety selection, canopy management and even tillage, according to Grau. “Because the fungus can grow on several other crops and hundreds of wild plants, and because the white mold resting stage can survive for several years in fields, the fungus is often still around after a rotation.”
However, Grau notes, a rotation with a small grain does seem to be beneficial for soybeans. He suggests a rotation of corn, soybeans, and a small grain – in that order – with oats, wheat or barley as the small grain crop.
“After one or two years away from soybeans, most growers can replant them,” he says. “Even if they had severe losses to white mold in a field previously, they can get reasonable yields by planting moderately resistant varieties in wide rows.”