September 29, 2004
From: ProMED-mail<promed@promedmail.org>
Source: Southwest Farm Press, 29 Sep 2004 [edited]
<http://southwestfarmpress.com/news/9-28-04-peanut-disease>

Peanut disease baffles scientists, frustrates West Texas farmers

West Texas peanut farmers and agricultural scientists have been frustrated and baffled this summer by a peanut disease that defied identification. "It is Sclerotinia blight [Sb]," says Terry Wheeler, research plant pathologist at the Texas A&M Research and Extension Center in Lubbock. Wheeler says that it looks and acts like Sb in the field and when it is isolated in the lab it is Sb.

Some farmers, mostly in Gaines County, expressed frustration at a recent field day at the Western Peanut Growers Research Farm near Seminole. The crux of their beef is a delay in identification of the organism. Peanut specialists on hand at the field day explained that the pathogen that was observed this year resembled another peanut disease organism, Botrytis blight [Bb], caused by _Botrytis cinerea_.

The fungicide of choice for Bb, Topsin-M, costs considerably less than Omega, the recommended treatment for Sb. Moreover, Topsin-M, which is not labeled for Sb, "stopped the infection," Wheeler says. "It's not supposed to, but it did."
 
According to Extension plant pathologist Chip Lee, samples from Gaines County and those from Lee were the same organism. He says, "The samples do not look like Sb, but in every sample, isolation showed it to be Sb. Every one."

According to Wheeler, peanut specialists looked at the disease in the field and agreed that it was not Sb. Using standard isolation techniques, she could not identify the disease as Bb, which is easy to identify in the lab, but not in the field.

Wheeler says the Gaines County farmers apparently have a new strain of the fungus. "This is a clear change in a well-studied disease," she said. "And it is a very aggressive form of the disease. It has been very severe in Gaines County this year."

The outbreak runs from around Seminole north to Seagraves, near the Terry County line. A sample from Yoakum County is also being analyzed.

According to Wheeler, the bad thing about Sb is that once it gets started in a field, it remains for the life of that field.

[Byline: Ron Smith ]

[The most logical explanation for the current disease is that a mutation has occurred or a new strain has evolved. Sclerotinia blight [Sb], caused by the fungus _Sclerotinia minor_ [Sm], is a destructive mid-to late-season disease of peanuts. 1st discovered on peanut in Virginia in 1971, it subsequently spread to North Carolina, Oklahoma, and Texas. Sm survives in soil as sclerotia (resistant seed-like structures) and attacks plants near the soil line. Sm occurs worldwide, primarily in cool, moist regions. Its host range includes canola, sunflower, bean, sweet potato, cole crops, and soybean. In Oklahoma, yield losses in experimental plots infested with Sb ranged from 7 percent to 80 percent.

Spread of the disease is limited because Sm does not Sm produce airborne spores during the cropping season. Sclerotia are the major means of long-distance movement. Sm is seedborne and is found as a contaminant in soils cropped by peanut. Infection by Sb can be very damaging and is difficult to control. Disease management basically depends upon preventing movement of contaminated soil to clean areas. Farm implements coming off contaminated soils should be washed thoroughly to remove soil and straw. A combination of cultural and chemical management practices will usually provide adequate control. New resistant cultivars have been developed recently by USDA.

Links: <http://pearl.agcomm.okstate.edu/plantdiseases/f-7186.html>
<http://www.ars.usda.gov/is/AR/archive/aug03/peanut0803.htm>
<http://www.ars.usda.gov/is/pr/2003/030813.htm>
<http://www.ars.usda.gov/is/AR/archive/aug03/peanut0803.pdf> - Mod.DH]