Significance of Insensitivity of Sclerotinia minor to Iprodione in Control of Sclerotinia Blight of Peanut. F. D.Tad Smith, Former Graduate Student; Tidewater Agricultural Experiment Station, Suffolk, VA 23437-0099. P. M. Phipps, Professor, Tidewater Agricultural Experiment Station, Suffolk, VA 23437-0099; R. J. Stipes, Professor, Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg 24061-0331; and T. B. Brenneman, Associate Professor of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793-0748. Plant Dis. 79:517-523. Accepted for publication 18 January 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0517.

Four hundred isolates of Sclerotinia minor were obtained from diseased peanut plants (Arachis hypogaea) in field plots untreated or treated three times with iprodione (1.12 kg/ha) in 1987. On glucose-yeast extract agar containing iprodione at 2 µg/ml, only 6% of the isolates grew. These isolates produced more mycelial growth than corresponding isolates not previously exposed to the fungicide in agar, indicating that insensitivity developed in vitro. In field microplots artificially infested with sclerotia of the insensitive isolates and planted to peanut for 7 yr, iprodione provided significant disease suppression even though insensitive isolates could still be recovered. In a separate 3-yr microplot study, the pathogenicity of an isolate (B-83-T2) of S. minor suspected of possessing field resistance to dicarboximide fungicides was compared to a sensitive isolate (S-2) after treatment of peanuts with dicarboximide fungicides (iprodione or vinclozolin), aromatic hydrocarbon fungicides (dicloran or pentachloronitrobenzene [PCNB]), and an experimental fungicide (fluazinam). Without fungicide treatment, disease incidence (stem lesions per microplot) at harvest averaged 19.9 in plots infested with isolate S-2 and 18.3 in plots infested with isolate B-83-T2 (no statistical difference), but yields were significantly lower (10%) in microplots infested with isolate S-2. When averaged across isolates, disease incidence was suppressed 96, 63, 42,41, and 20% by fluazinam, vinclozolin, iprodione, PCNB, and dicloran, respectively. Insensitivity in S. minor to the dicarboximide fungicides appears low but persistent and does not pose a threat to the continued use of dicarboximides or related fungicides in peanut production.