Midwest no-tillers should scout their soybeans to see if white mold disease is developing, says Anne Dorrance, an Ohio State University plant pathologist.

“The weather conditions over the past few weeks in Ohio are very similar to last year," Dorrance says. "If it stays cool and wet, then white mold will be the next issue to monitor.”

In 2009, rain and cooler-than-normal summer temperatures resulted in the first major white mold outbreak in Ohio in nearly 10 years.

White mold — which is 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.

The pathogen needs some decaying material to get started, says Dorrance, adding that it takes a lot of disease to affect yields.

Visible symptoms include fluffy white lesions on the base of dying plants and the stem will be bleached with white fluffy growth of the mycelium from the fungus. 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.

Fields where the soil never dried out favor germination of the sclerotia, Dorrance says.

Little can be done to stop the infection once symptoms become visible. Specialists do not recommend applying fungicides after symptoms have developed.

“We are worried about efficacy,” Dorrance says. “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. Another issue is fungal resistance to the chemicals. We don’t want the fungus building up tolerance to that fungicide.”

Fungicide applications should only be made prior to infection if conditions are favorable for fungal build-up, she says. Few fungicide products show consistent control.

“The only fungicide we have a lot of data on that consistently shows reduction is Topsin M (a thiophanate-methyl product),” Dorrance says. “Our recommendations are targeted mainly to seed producers to make applications during stages R2 and R3 only if the canopy is closed at flowering.

“For standard bean producers, we don’t recommend fungicide applications unless that producer is growing a high-value bean.”

Other management practices for controlling white mold include:

• Rotating crops 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,” said Dorrance.

• Planting 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,” said Dorrance. But if you have options, select the resistant variety

• Avoiding the 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.

• Managing weeds well. Sclerotinia has a very wide host range, attacking common weeds like lambs quarters and pigweed.

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

Like all diseases, development of white mold is highly weather dependent.

“One hot dry week close to flowering, where the top 2 inches of soil dries out, will knock this one out of contention as a problem,” Dorrance says.

For more information managing white mold, refer to OSU Extension’s fact sheet, “Sclerotinia Stem Rot (White Mold) of Soybean” at http://ohioline.osu.edu/ac-fact/pdf/AC_45_08.pdf, or log on to the Plant Health Initiative website at http://www.planthealth.info/whitemold_basics.htm.