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Corn+Soybean Digest

Weather Thwarted White Mold In '98

White mold, sometimes called the "fuzzy fungus from the North," took a hit in 1998. Generally, it was less severe.

The reason? Most probably, it was higher air temperatures during the critical infection period, during and after bloom.

"Overall in Wisconsin, we had record soybean yields, and overall the white mold situation was not very severe. So I think we really dodged a bullet with the higher air temperatures, primarily in July," notes Craig Grau, University of Wisconsin extension plant pathologist.

The situation was similar in other Northern states with a history of white mold, the common name for sclerotinia stem rot.

Nevertheless, white mold has rapidly become one of the most important diseases of soybeans in the North Central Region. That includes Illinois, Iowa, Minnesota, Indiana, Ohio, Michigan, Wisconsin, South Dakota, North Dakota and Nebraska, notes Wayne Pedersen, University of Illinois extension plant pathologist.

White mold was reported in Minnesota, Wisconsin, Michigan and Ontario back in the 1970s. But it didn't start playing real havoc with many soybean growers in the Upper Midwest until the 1990s.

"Until then, in other parts of the Midwest, the disease was mostly a curiosity, with only scattered outbreaks," explains Brian Diers, a plant breeder at the University of Illinois.

"Today, white mold is the most widely recognized soybean disease in Minnesota," declared Ward Stienstra, University of Minnesota plant pathologist, in a 1997 bulletin on the disease.

In fact, Stienstra said that at 1997 winter meetings he heard of some farmers who put their grain drills up for sale because of white mold.

Weather has a big impact on severity of the disease. In 1994 and 1997, white mold ripped yields in infected fields in Wisconsin, Minnesota and Michigan by 15-30 bu/acre - even more in very severe cases. In 1994 through 1996, white mold became a severe problem in Illinois, Iowa, Ohio and Indiana. Scientists now believe the inoculum is widespread in the Upper Midwest, and the disease can pop up most anywhere when conditions are right.

Ironically, white mold is a disease found in high-yield environments that include dense soybean canopies created by planting in narrow row widths, high seeding densities, early planting, high soil fertility and other factors that promote good plant health.

"This situation is very unfortunate," Wisconsin's Grau declares, "because white mold penalizes the really progressive soybean grower."

Unfortunately, there are no sure-fire cures for combating white mold - yet. Some varieties definitely show varying degrees of tolerance or resistance, but none has been found with total resistance. There is hope, say Grau and Diers, that biotech-produced or transformed soybeans, with more-resistant genes from other plants, will be significantly more resistant. And they are coming.

Scientists are hard at work on this project right now and have tested one gene from barley that shows promise.

"Variety selection, I believe, is still the key," declares Grau. "And if you choose a variety that doesn't have a good white mold track record, then you better start modifying that canopy. But there are varieties that I think you can forge ahead with."

What about giving up on drilled or narrow-row (narrower than 20") beans? Some scientists lean that way, but Grau and Diers think that's a mistake, except in cases with a history of severe yield losses due to white mold.

"Our research data keeps saying that if you plant a variety that truly has a degree of moderate resistance or tolerance, you will still have more bushels in a drilled situation than in planting that same variety in 30" rows," Grau says. "You might actually have a little less white mold in the wider rows, but you will not have as many bushels of beans with the wider rows."

There are exceptions to that statement, Grau admits. But, say, over a five-year average in their research, drilled beans with a moderately resistant or tolerant variety produced more yield.

Studies show that, because of the ability of the soybean plant to compensate some for stand losses, yield loss becomes significant only when percentage of diseased plants reaches 20% or more.

Undeniably, any practice that produces a denser canopy that holds moisture and humidity longer favors white mold development. So if you have a high- yield environment situation where you have been clobbered by white mold, you might want to consider widening rows, reducing planting rate, planting a little later and using a variety with some tolerance.

Rotation likely helps some, too. But the white mold pathogen can survive in the soil for up to five to seven years. It also has a large number of hosts, including weeds like lambsquarters, pigweed, velvetleaf, ragweed, nightshade, Canada thistle and mustard. So rotation isn't the strong answer it is with many other disease problems.

Recent research shows the pathogen can also be seedborne. So, besides today's earlier planting in higher-residue situations, that's another reason to use seed treatment.

In 1997, University of Illinois scientists, supported by Illinois soybean checkoff funds, screened nearly 250 commercial varieties and experimental soybean lines for white mold resistance. A list of the 30 most resistant varieties is available from the University of Illinois at 217-333-3847 or 217-244-3258, or on the Illinois Soybean Checkoff Board's home page ( html).

Additional information on variety reactions to white mold can be found on Grau's Soybean Plant Health Web site (www. The site gets support from the Wisconsin Soybean Marketing Board and the North Central Soybean Research Program.

If you have had white mold problems, check with your seed company reps to obtain the most resistant or tolerant varieties with high-yield potential that they recommend.

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