SIGNIFICANCE OF INSENSITIVITY OF SCLEROTINIA MINOR TO IPRODIONE IN CONTROL OF SCLEROTINIA BLIGHT OF PEANUT

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 extr act agar containing iprodione at 2 mu g/ml, only 6% of the isolates gr ew. These isolates produced more mycelial growth than corresponding is olates not previously exposed to the fungicide in agar, indicating tha t insensitivity developed in vitro. In field microplots artificially i nfested with sclerotia of the insensitive isolates and planted to pean ut for 7 yr, iprodione provided significant disease suppression even t hough insensitive isolates could still be recovered. In a separate 3-y r microplot study, the pathogenicity of an isolate (B-83-T2) of S. min or suspected of possessing field resistance to dicarboximide fungicide s was compared to a sensitive isolate (S-2) after treatment of peanuts with dicarboximide fungicides (iprodione or vinclozolin), aromatic hy drocarbon fungicides (dicloran or pentachloronitrobenzene [PCNB]), and an experimental fungicide (fluazinam). Without fungicide treatment, d isease incidence (stem lesions per microplot) at harvest averaged 19.9 in plots infested with isolate S-2 and 18.3 in plots infested with is olate B-83-T2 (no statistical difference), but yields were significant ly 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, respecti vely. Insensitivity in S. minor to the dicarboximide fungicides appear s low but persistent and does not pose a threat to the continued use o f dicarboximides or related fungicides in peanut production.