Wet Weather During Soybean Flowering, Optimum White Mold Conditions

Pioneer experts say in-plant tolerance offers best protection

DES MOINES, Iowa - The complexity of white mold continuously challenges soybean growers. Experts at Pioneer Hi-Bred, a DuPont business, say it's important for soybean growers to scout for this yield-robbing fungal disease that continues to spread in northern areas of the Corn Belt. If growers are in a risk area, they should plant tolerant varieties.

"White mold is a very difficult disease to manage, partly because growers who encounter white mold during the growing season have few control options," says Zach Fore, Pioneer area agronomist.

Though less problematic in 2010, long-term survival structures of this organism ensure inoculum always is available to attack the soybean crop should conditions allow. For that reason, growers in risk areas must treat white mold as a perennial threat to top yields and profits.

In 2009, weather conditions created the ideal combination for a white mold epidemic. Growers in risk areas need to be on full alert every year and intensify scouting efforts if conditions during flowering are moist for seven to 14 days with moderate temperatures at or below 85 degrees.

The disease manifests in the soil and can live up to 10 years in a field, even with crop rotation. White mold starts in the soil, moving up the soybean stem and into the canopy. Eventually disease pathogens use the flowering tissue to enter the plant.

According to the University of Wisconsin, symptoms of white mold can be observed at canopy level about six weeks after flowering. A closer inspection can reveal symptomatic plants three to four weeks after flowering.

The white mold disease cycle begins with the formation of tan, mushroom-like structures called apothecia. Spores from the apothecia infect maturing soybean flowers, and the fungus eventually grows to the soybean stem. Growers should examine stems for signs of white mold fungus, which begin as gray to white lesions at the nodes. Lesions vary and can be 3 to 18 inches long, rapidly progress above and below the nodes and eventually can encircle the whole stem.

To date, there is no genetic resistance to white mold. All varieties are capable of developing white mold symptoms under severe infestations.

"One of the management options for growers who've experienced the disease in the past is to select varieties that have a higher tolerance to the disease," Fore says. "There are varietal differences in the degree of tolerance. Pioneer provides scores for all its varieties adapted to locations where white mold occurs. These scores reflect varietal differences in the rate at which infection develops and the extent of the damage it causes."

Pioneer uses lab techniques for selecting varieties with stem tolerance to the pathogen. Pioneer also tackles white mold by improving the phenotype of the plant, identifying varieties with better standability as well as more narrow and shorter canopies so they dry out better.

Pioneer tests those experimental varieties across multiple field screening sites to further characterize for white mold tolerance across years and locations. Varieties presenting strong tolerance are characterized for white mold. Pioneer assigns each variety a score for white mold tolerance on a one to nine scale, with higher scores reflecting more tolerance.

White mold typically is found in northern states from Minnesota and Iowa to the Atlantic coast, where conditions are cooler and wetter. Due to yield-enhancing production practices, resulting in early, dense canopies, the incidence of the disease has increased.

Fore says fungicide products currently available on the market lack effectiveness. "White mold infection occurs during flowering, which lasts several weeks. Fungicide products currently on the market are not able to protect the soybean plant throughout this entire period," he says. "It can be hit and miss, and often it's better not to incur that cost if the chances of it working are minimal."

DuPontTMAproachTM, a new fungicide under development by DuPont Crop Protection, has demonstrated excellent soybean white mold control in university trials for the past several years and should be available for growers in 2012. AproachTM, along with the development of resistant varieties, will provide soybean growers with new tools for controlling this devastating disease.

"Infection can be reduced by keeping the lower canopy dry, so wider row spacing and reduced plant populations can lessen the impact of white mold. However, these practices should be used judiciously because they also can reduce yield potential," Fore says. "White mold has more than 400 host plants, which makes broadleaf weed control important. Growers also can use tillage and crop rotation as another tool, making sure the secondary crop isn't white mold susceptible."

White mold impacts a number of other crops including sunflowers, canola, dry beans and potatoes.

For more information on white mold, visit Pioneer's agronomy library on www.pioneer.com. To learn more about Pioneer®brand soybean varieties, visit www.pioneer.com/soybeans or talk to your local sales professional.

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