Efficacy of fungicides and a systemic acquired resistance activator (acibenzolar-S-methyl) against tobacco blue mould

Tobacco blue mould, caused by Peronospora tabacina Adam (Peronospora hyoscyami f. sp. tabacina Skalicky 1964) can be an economically devastating leaf spot disease in shade and broadleaf cigar wrapper tobacco (Nicotiana tabacum L.) types grown in Connecticut and Massachusetts. We investigated the effects of dimethomorph plus mancozeb and azoxystrobin fungicides as well as acibenzolar-S-methyl, a systemic acquired resistance inducer, on disease severity over 2 years in both shade-grown and broadleaf tobaccos. All fungicide and fungicide plus acibenzolar-S-methyl treatments applied were effective in reducing the number of blue mould lesions per plant. Treatments containing acibenzolar-S-methyl were the most effective, resulting in almost complete control. Substituting two or three applications of acibenzolar-S-methyl at label rates for dimethomorph plus mancozeb treatments in a spray program increased blue mould control over the same number of dimethomorph plus mancozeb applications by 28–94 percent. The effects of acibenzolar-S-methyl application on cured leaf quality were determined in commercial shade tobacco fields in 2000 and 2001. Leaves were cured, processed and commercially evaluated for quality in a blind test. Standard fungicide applications of dimethomorph plus mancozeb applied on a 14-d interval were compared to three acibenzolar-S-methyl treatments. Economic value was not different between treatments in 2000, but acibenzolar-S-methyl applied at 10-d intervals was associated with reduced value in 2001 when plants were more subject to drought and heat stress.