Strobilurin plus acibenzolar-S-methyl controls white rust without inducing leaf chlorosis in spinach
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White rust (Albugo occidentalis) is a major foliar fungal disease of spinach (Spinacea oleracea) in Texas and the Southeastern USA. It causes leaf yellowing, decreasing product quality and marketability. There is a need for evaluating broad spectrum fungicides and new plant defence activators that have the potential to enhance leaf quality in spinach. Two of those fungicides are acibenzolar-S-methyl (ACIB) and strobilurin derivatives (STRO). ACIB appears to mimic natural systemic acquired resistance response in plants, while STRO inhibits mitochondrial respiration in fungi. We conducted three consecutive studies to a) determine the effects of ACIB rates and combinations with STRO and mefenoxam + copper hydroxide (MEFE-Cu) for the control of white rust, and b) evaluate potential phytotoxicity expressed as leaf chlorosis. After two weekly applications in 1998, ACIB, STRO and ACIB + STRO significantly reduced the % of leaf area infected with white rust in the spinach cv. Cascade. In 1999, ACIB + STRO and ACIB + MEFE-Cu had the best white rust efficacy, with ACIB equally effective when applied at reduced rates. After three applications in 2000, STRO, ACIB + STRO, and ACIB + MEFE-Cu had significantly lower white rust values than the control, ACIB or MEFE-Cu. The combination of ACIB + STRO did not cause leaf chlorosis in any year. Use of preventive applications of ACIB + STRO at 7-10 day intervals appear to be an effective strategy to improve leaf quality in spinach.
