White mold could be a problem again in 2010

By Matt Reese
Chances are looking all too good for another bout with white mold this year in Ohio soybeans.
Anne Dorrance, Ohio State University Extension plant pathologist, said if the white mold producing material (Sclerotinia) is in a field, conditions may be right for it to be there again this year.
“Sclerotinia white mold, also known as Sclerotinia stem rot, has a very interesting disease cycle. The inoculum comes from very small fruiting bodies called apothecia that form from the sclerotia. They puff their spores up onto the stems and infect the old blossoms and they can kill the plants in the bottom third of the stem,” Dorrance said. “We have historic fields that have had white mold since the early 90s and late 80s. Every once in a while we get a blow up. Last year conditions were perfect and this year conditions are good again.”
The moisture this year has been favorable for the development of the disease.
“This was a bit of a surprise as the 2 weeks prior to this were dry, but rains did fall 3 to 4 days prior, the night time temperatures hit below 70 a couple of nights and more importantly, there was still heavy dew on the plants at noon,” Dorrance said of an infected field in Clark County.
Because of the potential for problems this year, it will be important to carefully scout fields with a history of white mold. Fields that have formed a dense canopy prior to flowering and experience consistent moisture and a few cool nights are at the highest risk for this disease, Dorrance said.
“These fields are those that always have some white mold in them — this disease does not occur in every field across the state,” Dorrance said. “For those historically positive fields, this is a year to watch.”
To treat white mold, the fungicide Topsin M needs to be applied at the R1-R2 growth stage, Dorrance said. And, with Ohio soybeans at just about every growth stage from V1 to R4, Dorrance points out that this could be a challenging undertaking.
“All fungicide applications work best when applied prior to arrival of fungal spores, as protectants on the leaves or stems depending on the disease in question,” she said. “Whenever you apply a fungicide, always leave a control strip or two — not at the edge of the field — to evaluate if the treatment worked.”
Thus far, white mold has proven to be tricky to control.
“The good news is that we have new chemistries coming down the pike. The OK news is that the treatments we have right now can knock it back so we have less disease in the field at the end of the season. The bad new is that we still have all of this sclerotia and we have this inoculum buildup over time,” she said. “The short term fix for managing this is resistance, but the companies have had a tough time because the breeding is very different for this disease. We really do not have good screening systems to use in the lab and greenhouse to expedite this process. There are some products right now that we’re evaluating and there are GMO varieties as well. There will be some things in the pipeline in the next two to three years.”

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