In September, 2013, symptoms similar to Sclerotinia blight caused by Sclerotinia minor were observed on Runner peanut (cv. FloRun 107) in a commercial field near Pocahontas, Arkansas, in Randolph County (2). Blighted plants with wilted leaves were observed in several small (30 × 30 cm) clustered foci located near the end of a 20-ha, furrow-irrigated field. Peanut stems within the lower canopy of symptomatic plants had straw-colored lesions, with white fluffy mycelium and small (<2.0 mm diam.), black, irregularly shaped sclerotia. Stems on plants with severe symptoms were shredded in appearance, with small black sclerotia inside the stem tissue (2). Final disease incidence near harvest in mid-October was less than 1% of the field. Sclerotinia blight symptoms were also observed in 2013 on Runner (cvs. FloRun 107, Georgia 09B, and Florida 07) and Spanish peanut (cvs. OLin and OL06) research plots near Newport, AR, in Jackson County. Disease incidence among cultivars in these research plots was <1% for all cultivars except FloRun 107, which had a disease incidence of 2.6% for a 849.8 m2 plot. Isolations from surface-disinfected leaves on potato dextrose agar (PDA) consistently yielded white, fluffy mycelia with small (0.5 to 2.0 mm diam.), black, irregularly shaped sclerotia typical of S. minor (2). Six-week-old peanut plants (cv. FloRun 107) growing in pots were used to test pathogenicity. Each plant was inoculated by placing an agar plug (5 mm diam.), collected from the edge of an actively growing S. minor culture, on the main peanut stem. Plants (n = 5) were incubated for 8 days in a humidity chamber where temperatures ranged from 24 to 30°C and relative humidity remained >95%. Characteristic symptoms of Sclerotia blight were observed on all inoculated peanut plants whereas none of the plants (n = 3) inoculated with sterile PDA agar plugs expressed symptoms. Pathogenicity tests were repeated on peanut cvs. Flavor Runner 458 and Georgia 09B with similar results. S. minor was consistently isolated from symptomatic tissue on PDA, fulfilling Koch's postulates. To our knowledge, this is the first report of S. minor on peanut or any host in Arkansas or the Mid-South region. The two peanut fields with Sclerotinia blight had a history of soybean production, and S. minor may have gone undetected on soybean or one of many host weed species (1). Since S. minor is a major economic pathogen of peanut, commonly causing yield losses of 10% (2), it will likely be a significant factor in Arkansas and Mid-South peanut production.
References: (1) M. S. Melzer et al. Can. J. Plant Pathol. 19:272, 1997. (2) D. M. Porter and H. A. Melouk. Sclerotinia blight. Page 34 in: Compendium of Peanut Diseases, 2nd ed. N. Kokalis-Burelle et al., eds. The American Phytopathological Society, St. Paul, MN, 1997.
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