DES MOINES -- Last season, white mold ravaged many soybean fields, and this growing season, weather conditions in late July and early August may determine if the fungus will threaten fields again in 2010, say experts at Pioneer Hi-Bred, a DuPont business.

"White mold prefers cool, damp conditions," says Martin Fabrizius, Pioneer research scientist, Redwood Falls, Minn. "Temperatures were below average in July 2009, and the worst infections occurred in areas with significant moisture, especially in northeast Iowa, southeastern Minnesota, Wisconsin and northern Illinois."

Last year's cool conditions allowed fungal mushrooms to launch spores under lush soybean canopies during flowering, says Don Kyle, Pioneer research scientist, Princeton, Ill. Due to temperatures of 80 degrees or less, soybeans were very susceptible to white mold. He says last year was the worst broad outbreak of white mold since the mid-90s.

"Once the canopy closes, it will not dry out as quickly and this will increase the potential for white mold infection," says Paul Stephens, Pioneer senior research director of soybean product development. "Often it appears in lower ground where high humidity stays around longer in the morning or areas where there are lush soils, but last year it affected more acres because it was cooler than normal."

In terms of economic impact, white mold can be one of the most devastating challenges to soybean growers after it establishes.

"For example, growers in northern Illinois who typically see 50 to 60 bushels per acre may have harvested only 30 bushels per acre last year," Stephens says.

Fabrizius says they do not expect temperatures to be as cool as last year.

"The long-range forecasts from NOAA (National Oceanic and Atmospheric Administration) and others suggest July 2010 temperatures could be warmer than July 2009," he says. "This could help control white mold."

Many growers rotate their soybean crops, generally interrupting disease cycles. However, white mold pathogens can stay in the soil for 10 years, so growers need to monitor fields and keep diligent records.

"The No. 1 thing growers can do to defeat the fungus is with variety selection," Kyle says. "In fields with a strong history of white mold or when planting varieties with less tolerance, additional agronomic practices may be warranted, such as lowering plant populations, increasing row widths and utilizing chemical control."

Scouting fields to judge risk is difficult because the disease doesn't show until symptoms manifest. Although no rescue treatment exists after symptoms appear, there are a couple management options to help reduce the incidence of white mold.

"A product called Cobra® can be sprayed from the V5 to V6 stage through the beginning of the flower stage," Fabrizius says. "The other option is a biological control product called Contans® WG that, according to the manufacturer, attacks the overwintering structure of white mold. It's applied in the fall through early vegetative stages. It uses parasites to attack sclerotia, or the overwintering structures."

Experts say treating with fungicides may be challenging because it's difficult to penetrate the soybean canopy and onto the flower where the product needs to go.

To combat white mold, Pioneer researchers and breeders use a two-prong approach.

"Using a lab technique, we select for varieties with stem tolerance to the pathogen itself," Stephens says. "We also tackle white mold by improving the phenotype of the plant. We identify varieties with better standability as well as narrow and shorter canopies so they dry out better. Then, we test those experimental varieties across multiple field screening sites to further characterize them for white mold tolerance across years and locations."

Pioneer researchers rely on maps to study where the disease occurs and to predict where it's going.
"Maps help us position the scope and size of our resources," Stephens says. "We want to ensure we have the most elite products on the market."

Pioneer assigns each variety scores for white mold tolerance on a one to nine scale, with higher scores reflecting more tolerance.

"Not every product has above-average tolerance," Kyle says. "Our goal is to continually improve white mold tolerance in our new varieties. Pioneer has always been about trait packages. It's not about just one trait, it's about the entire trait package -- whether it's brown stem rot (BSR), sudden death syndrome (SDS), soybean cyst nematode (SCN), herbicide resistance or another."

Growers can use trait characterization ratings to choose the best variety for their field.

"Pioneer is committed to the correct characterization of the tolerance scores," Fabrizius says. "They are very reliable. Yet, we want growers to know white mold still can occur even with above-average tolerance because there's no complete genetic resistance available at this time."

Pioneer uses sophisticated screening techniques to rule out highly susceptible varieties.

"We only advance products with some level of tolerance," he says. "We screen products at multiple testing stages, and we've set up additional white mold research screening sites that include misting systems to set up the right environmental conditions to test different seeds. These nursery sites are specifically set up to score white mold tolerance because fighting white mold is a priority for Pioneer."

Pioneer researchers look for better tolerance by continuing to screen novel soybean germplasm and even exotic, alternative germplasm sources that hold native tolerance to white mold. Researchers continue to explore the use of genes outside of soybeans for a transgenic approach in the future.

SOURCE: Pioneer Hi-Bred.