White mold ripped yields so badly for some growers last year that making a profit was mission impossible. Many others suffered just moderate losses, or had none at all.
Nevertheless, white mold (sclerotinia stem rot) has become a major concern and profit stealer in the northern soybean-growing belt. And scientists now feel it can strike almost anywhere in northern states, if conditions are right.
"In our most severely hit areas, it was just as bad as in 1994, which was a bad year," notes Craig Grau, University of Wisconsin extension plant pathologist. "In some areas, it came in later than normal but did not seem to have much effect on yield.
"Here in the Madison and Arlington areas, however, we saw symptoms in late July, which is the earliest I have ever seen them. And we had yields in the 20s, where we normally expect yields in the 60-bushel range."
Grau, who coordinates white mold research work among several Midwestern universities, outlined the '97 situation:
Northeastern Iowa, southeastern Minnesota, the northern three-fourths of Wisconsin, and parts of Michigan had high incidences, while reports from Ohio, Indiana, Illinois and the majority of Iowa revealed lower incidences than in recent years.
Ironically, white mold is a disease found in high-yield environments. Weather, however, is probably the biggest factor in white mold incidence, they say. Cooler- and wetter-than-normal weather starting at early bloom stage is ideal for the disease. If weather turns hotter and drier than normal then, the disease is almost a non-factor on yield.
"Some growers had severe problems, while others did not," explains Brian Diers, Michigan State University plant breeder. "It was the luck of the draw where the rain fell during the critical early flowering period and where it didn't fall. But I think the inoculum is spread throughout the state in our soybean growing areas. When conditions are right, the disease pops up anywhere."
Yield losses of 5-10 bu/acre are fairly common under those cooler and wetter conditions, note scientists. But it's not uncommon to get up to a 30% rate of infected plants in a field, with 15-bu losses, or more.
Scientists in the North Central states are researching this production problem intensely, but are in a "holding action where we are buying time," says Grau, until more tolerant or resistant varieties are available.
Here are factors that favor white mold: drilled beans, which form dense canopies (good for weed control but ideal for white mold development because they retain moisture); high plant populations; and highly susceptible varieties.
Some growers hit with white mold have switched back to 30" rows rather than drilling. That could help some, but Grau and Diers think it's not necessary and probably is counterproductive, except in severe situations.
"I still think that, if you plant a variety that is rated as moderately resistant or tolerant, that should be the foundation of your management program to combat white mold," insists Grau. "And I think there are some varieties that you can plant in a drilled-bean situation successfully."
Adds Michigan's Diers: "We routinely see some varieties in our variety tests having less disease than others. So we believe variety selection is a very important factor - more important than anything else you can do."
Grau does suggest reducing plant population in drilled or narrow-row beans to around 200,000 plants per acre, if you're up in, say, the 240,000 range.
Grau and Diers qualify their drilled-bean support for extreme situations, such as a field where you've lost 25 or more bushels per acre. In those situations, they advise going back to 30" rows and planting a tolerant variety.
What about the effects of no-till and crop rotation?
Early research brought some varying results. But now, after several years of research in both Wisconsin and the cooperating states, Grau feels patterns are firming up.
"In these big on-farm studies, where we have different tillage systems, we are still measuring less white mold in the no-till areas. The number of sclerotia is lower in the no-till area than in the other systems."
Grau credits the less-dense canopy generally produced in no-till as helping reduce white mold, probably because more air circulates and because of less lodging in the less-rank plants.
"I've been rather lukewarm on crop rotation for combating white mold," says Grau. "But after three years of studying this, I think one year, and preferably two or more years, out of soybeans is making a difference in our test plots - if you plant a moderately resistant variety."
So how do you find the "right" variety to combat white mold?
Ask your seed supplier or extension personnel. They can tell you which ones might fit your situation.