Influence of fungicide spray schedules on the sensitivity of Cercospora beticola to the sterol demethylation-inhibiting fungicide flutriafol

A field experiment was conducted over a three-year period (1997–1999) to study the efficacy of several fungicide spray-schedules and their effects on the sensitivity of Cercospora beticola populations to the DMI fungicide flutriafol. Spray applications of flutriafol, either alone at the recommended dose, or at a reduced dose in mixtures with maneb, or in alternation with tank mixed fentin-acetate and maneb, were included in the spray programs. Applications of flutriafol at the recommended dose showed a significantly greater control efficacy in comparison with the other treatments, while applications of flutriafol alternated with tank mixed fentin-acetate and maneb showed lower efficacy in comparison with the remaining flutriafol treatments. Fungal populations from plots continuously treated with flutriafol, either alone at the full dose or at reduced dose with maneb, had lower sensitivity to flutriafol in comparison with populations from plots treated alternatively with flutriafol and tank mixed fentin-acetate and maneb. Repeated applications of flutriafol, at full or reduced doses, favoured the selection of highly resistant strains. Since applications of flutriafol in alternation with tank mixed fentin-acetate and maneb do not maintain a high level of disease control, the only available anti-resistance strategy would be the restriction of the number of flutriafol treatments, by applying them only when environmental conditions are favourable for disease development and by using alternative fungicides during the rest of the growing season.