May 2005
Fungal Mutation Explains Resistance
Early blight, caused by Alternaria solani, is an important foliar disease of potato and other crops. Alternating wet and dry periods provided by consistent dew formation are particularly favorable for the development of the disease. As a result, yield losses of 30 percent caused by early blight have been reported. Few potato cultivars are resistant to the disease; therefore, early blight is managed primarily through the use of foliar fungicides. Broad-spectrum fungicides such as chlorothalonil and mancozeb control early blight disease under moderate disease pressure but not always under irrigated potato production. As a result, in the late 1990s, potato producers took advantage of the high level of disease control (>90%) provided by strobilurin fungicide chemistry (Q(o)I fungicides) such as azoxystrobin (Quadris®) and pyraclostrobin (Headline®). In 2000, one year after the registration of the first strobilurin fungicide, isolated potato fields were identified with significant levels of early blight despite several applications of Quadris® made on the crop.
Isolates of A. solani recovered from these fields, and from other fields throughout the Midwest in 2001, confirmed that the early blight fungus had reduced-sensitivity to azoxystrobin due to the presence of the F129L mutation. Previous studies confirmed that azoxystrobin reduced-sensitive isolates are also reduced in sensitivity to pyraclostrobin (Headline®) but less so to trifloxystrobin (Gem®). In recent studies, it has been demonstrated that this mutation does not appreciably affect disease control provided by the non-strobilurin Q(o)I fungicides famoxadone (Tanos®) and fenamidone (Reason®). Early blight disease control provided by famoxadone is significantly better than the level of control provided by fenamidone, primarily due to higher intrinsic activity on the early blight fungus. Interestingly, A. solani isolates possessing the F129L mutation are significantly more sensitive to boscalid (Endura®) in vitro than the sensitive wild-type. Fungicide rotations of famoxadone and boscalid may be effective in slowing the development of resistance in A. solani to both chemistries. (Plant Disease, March 2005).
