Rainy weather and cooler-than-normal summer temperatures have resulted in a white mold outbreak in soybeans throughout parts of the Midwest. For Ohio growers, this is the first major outbreak of white mold in the state in nearly a decade.

Anne Dorrance, an Ohio State University Extension plant pathologist, says that the disease is affecting soybean fields mainly throughout southern and northeast Ohio -– areas of the state that have been receiving the most consistent rainfall.

The good news is that it takes a lot of disease to affect yields, and this outbreak will have limited yield impacts. Right now, Ohio appears on pace for record soybean yields of 47 bushels per acre.

"Studies out of Iowa State have shown that it takes about 10% infection, heavily scattered throughout a field, before you begin seeing a yield reduction," Dorrance says. "Right now, the infection is just in pockets throughout fields. If we start seeing infection in 50 out of 100 plants, then we may be looking at upwards of 20% to 25% yield loss."

In Indiana, several soybean fields in western and southwestern Indiana have been observed with white mold present. Several of these fields are south of I-74 in an area that does not have frequent white mold outbreaks in Indiana, say Kiersten Wise, a Purdue University pathologist. Producers in areas that typically battle white mold, such as the northeast corner of Indiana, are not observing white mold in their fields, Wise adds.

"The majority of fields with confirmed white mold are past growth stage R3, and although growers are interested in managing the disease, foliar fungicide applications at this point may not be beneficial in reducing disease severity," Wise says.

Wise says Topsin-M and Domark are currently labeled for use against white mold; however, timing is critical with a foliar fungicide application for white mold control because the fungicide must be able to penetrate the canopy and protect the soybean flowers.

She says previous research from the University of Illinois indicates that the optimum timing for a foliar fungicide application of Topsin-M to manage white mold is at R1. Wise adds that fungicides containing a strobilurin mode of action, such as Headline, Quadris, Quilt and Stratego, are not labeled for use against white mold.

Paul Esker, University of Wisconsin, plant pathologist, says conditions are favorable for a white mold outbreak in much of the state. In spite of the dry weather during the early summer period in many parts of the state, the amount of dew has been high, thus increasing the risk for white mold.

In Iowa, white mold has become evident in soybeans during the last 2 weeks, especially in eastern Iowa. Although infection occurred shortly after the beginning of flowering in late June and early July, the characteristic white myecial growth on infected plants has only become apparent the past 2 weeks, says Darren Mueller, Iowa State university plant pathologist.

"Really, the only good news about this disease is that it does not have too much of a secondary disease cycle. In other words the disease itself is no longer spreading or is spreading one plant at a time," Mueller says.

Illinois has experienced some white outbreaks for the past several weeks, but with soybeans developed beyond R1 and R2 stages in the majority of cases, there isn't much that can be done at this point to control it, says University of Illinois plant pathologist Carl Bradley.

"The warmer weather that has moved into the state will slow down or stop white mold progression within infected plants," Bradley says. "When temperatures move into the upper 80s and 90s, the white mold fungus shuts down."

White mold, also known as Sclerotinia stem rot, is a common fungal disease that spreads by infecting old, decaying soybean stem tissue or blossoms prior to flowering (R1 stage) and during flowering (R2 stage). The fungus invades the plant by producing a compound called oxalic acid, which kills plant tissue and allows the fungus to take hold.

"It's not a great pathogen. It just needs some decaying material to get started. The base of dying plants will have large, fluffy white legions and the stem will be bleached white with large, copious amounts of mycelia growth," Dorrance says. "Fields most prone are those in high-yielding sites, where the canopy formed early and the fields received timely moisture, and in areas where humidity has built up and there is little airflow."

For producers facing fields with visible symptoms, little can be done to stop the infection. Specialists do not recommend applying fungicides.

"We are worried about efficacy. Fungicides are generally used as a protectant, and when you have thick, white mycelium already infecting the plant, fungicides won't impact that fungus at all," Dorrance says. "Another issue is fungal resistance to the chemicals. We don't want the fungus building up tolerance to that fungicide. With the damage done so close to flowering, growers will just have to deal with it."

Alternate management practices for controlling white mold include:

• Crop rotation to prevent Sclerotinia from building up in fields year after year. "Just a reminder that whenever we've got a problem in the field, rotate out of that crop the following year," Dorrance says.

• Plant resistant varieties. "The challenge seed companies face is that major white mold outbreaks are so infrequent that it's challenging to develop soybean variety resistance to this disease," Dorrance says.

• Avoid introduction of the fungus into the field by cleaning seed. The fungus is present in soybean stems and debris, which can be carried by the combine at harvest. Seed should be well cleaned to remove sclerotia to avoid introduction of the fungus into the field.

• Practice good weed management. Sclerotinia has a very wide host range, attacking common weeds like lambsquarters and pigweed.

• Till to bury infected residue deep in the soil. Deep plowing can prevent fungal germination. However, practice no-till or other conventional tillage practices thereafter to prevent Sclerotinia from rising to the soil surface and germinating.

For more information on white mold and how to manage it, refer to Sclerotinia Stem Rot (White Mold) of Soybean.