Soybean Growers: Keep an Eye Out for White Mold

June 22, 2010

WOOSTER, Ohio – Ohio soybean growers may not have to worry about soybean rust, but they should be keeping their eyes out for potential white mold developments.

"The weather conditions over the past few weeks are very similar to last year. If it stays cool and wet, then white mold will be the next issue to monitor," said Anne Dorrance, an Ohio State University Extension plant pathologist with the Ohio Agricultural Research and Development Center.

Last year, rainy weather and cooler-than-normal summer temperatures resulted in the first major white mold outbreak in Ohio in nearly a decade.

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," said 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 are those that favor germination of the sclerotia," said Dorrance.

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. 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," said Dorrance. "Another issue is fungal resistance to the chemicals. We don't want the fungus building up tolerance to that fungicide."

Dorrance said that fungicide applications should only be made prior to infection if conditions are favorable for fungal build-up. And then, few fungicide products show any consistent control.

"The only fungicide we have a lot of data on that consistently shows reduction is Topsin M (a thiophanate-methyl product). Our recommendations are targeted mainly to seed producers to make applications during stages R2 and R3 only if the canopy is closed at flowering," said Dorrance. "For standard bean producers, we don't recommend fungicide applications unless that producer is growing a high-value bean."

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," said Dorrance.

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

• 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 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," said Dorrance.

For more information on white mold and how to manage it, 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.

Author(s): 
Candace Pollock
Source(s): 
Anne Dorrance