It lives best when the soil temperature is below 82 degrees and the humidity is high. The bigger the plants are the larger the threat. It causes peanuts the most problem when the plants have lapped the middles. Left alone, this soil-inhabiting fungus can literally take the peanuts right out of the pods.
With a little scouting and the use of weather-based advisory systems, however, the damaging effects of Sclerotinia blight can be moderated. In addition, peanut growers in the V-C region this summer will have access to Omega 500 in their annual battle against Sclerotinia blight.
A problem in the region since the 1970s, Sclerotinia blight attacks the stems of peanut vines that are at or near ground level. The fungus that causes Sclerotinia blight leaves a tell-tale cotton-looking web in the vines of affected plants. The fungus kills the stems and pegs, and colonizes and rots the peanuts pods, says Pat Phipps, Virginia Tech Extension plant pathologist, who helped to develop the weather-based index.
The Sclerotinia advisory system, developed at Virginia Tech, considers a number of parameters in figuring when conditions are right for an outbreak of the disease and the subsequent control application.
The growth and stage of the plant figures heavily in the Sclerotinia advisory. “The bigger the canopy the more the soil surface is shaded, which means higher humidity and more soil moisture in the canopy when it rains,” Phipps says.
Combined with soil temperatures below 82 degrees, at least 95 percent humidity and canopy conspire to make a bed for Sclerotinia.
“If the total rainfall in the last 10 days was an inch or more, then that's enough for the fungus to grow or if it totals more than a half an inch for the previous five days,” Phipps says. “Less than half an inch of rain in five days, the moisture is so short lived that the fungus doesn't continue to grow.”
The beauty of this advisory system is that it automatically figures each of the parameters and tips the scale in favor of the peanut farmer when Sclerotinia is a concern.
In 16 years of research, the average date of the onset of Sclerotinia blight has been July 27. The earliest was in June; the latest in September.
Since it's a predictive model, the Sclerotinia blight advisory system usually recommends spraying about five days before the peanut farmer sees the disease.
The spray traditionally provides protection for 21 days. Subsequent sprays are determined by weather conditions at 11 locations in Virginia.
This season, producers have an additional weapon in the arsenal against Sclerotinia. Omega 500, fluzainam, received Section 18 approval in North Carolina, Virginia, Oklahoma and Texas. Manufactured by ISK Biosciences and marketed and sold by Syngenta, Omega 500 is recommended at a pint per acre. It can be used up to four pints in a season.
On the Internet, the Sclerotinia advisory system is located at www.ipm.vt.edu\infonet\. It's also available at 1-800-795-0700 in Virginia.
“Once our system indicates a need for peanuts to be sprayed in an area, we want growers to go out and look at their fields for traces of the disease,” Phipps says.
“Just those fields that have a history of the problem or are beginning to show evidences of Sclerotinia are what we're looking at,” he says.
To avoid Sclerotinia, a three- to four-year crop rotation is essential, Phipps says. “The use of the leafspot advisory system is highly recommended because it reduces the number of trips across the field. The more times you drive across peanuts with tractor tires and bruise the vines, the more likely you'll have a spread of Sclerotinia.”