Sclerotinia sclerotiorum infects a broad range of plant hosts, and is typically identified by the production of white mycelium and black sclerotia on infected plants and culture. Isolates of S. sclerotiorum with darkly-pigmented mycelium have been reported on Valencia peanut in New Mexico and Texas. This study was conducted to determine the relationship between dark pigmentation in mycelium, oxalic acid production, and pathogenicity of S. sclerotiorum on Valencia peanut. A darkly-pigmented (SD) and a mutant, non-pigmented (SW) isolate of S. sclerotiorum were compared for vegetative growth on various growth media under different environmental conditions, and for pathogenicity on peanut. The SD isolate became darker in mycelium pigmentation as temperature increased beyond 20 °C and at pH between 5.0 and 6.0. Pigmentation in the SW isolate remained unaffected. Oxalic acid production did not increase in either SD or SW isolates when grown on media amended with pure oxalic acid or its precursor arabinose. The SD isolate caused peanut plant death and necrotic lesions on leaflets, while the SW isolate did not cause any symptoms. The differential behavior of SD and SW isolates in pathogenicity does not appear to be associated with mycelium pigmentation.
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