WOOSTER, Ohio – Rainy weather and cooler-than-normal summer temperatures have resulted in a white mold outbreak in soybeans throughout parts of Ohio. It's the first major outbreak of the disease in the state in nearly a decade.
Anne Dorrance, an Ohio State University Extension plant pathologist with the Ohio Agricultural Research and Development Center, said 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.
"Studies out of Iowa State have shown that it takes about 10 percent infection, heavily scattered throughout a field, before you begin seeing a yield reduction," said Dorrance. "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 percent to 25 percent yield loss."
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," said Dorrance. "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," said Dorrance. "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."
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.
• 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 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 Web site at http://www.planthealth.info/whitemold_basics.htm.
Despite the white mold outbreak, Ohio soybean growers are on tap to harvest near-record yields. According to the Ohio Agricultural Statistics Service, Ohio's average soybean yield is forecast at 47 bushels per acre.