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New Zealand Plant Protection 60 (2007): 108-113

Reduced sensitivity to carbendazim in isolates of Botrytis allii

S.L.H. Viljanen-Rolinson, M.V. Maroni and R.C. Butler

ABSTRACT

Fifty isolates of Botrytis allii, causing onion botrytis neck rot, were obtained from onion seed and bulbs throughout New Zealand and tested for their sensitivity to carbendazim (benzimidazole) fungicide in a mycelial growth study. Agar plates amended with carbendazim at 6 or 8 different concentrations were inoculated with an 8 mm disc of mycelium of each of the isolates and incubated in the dark at 20C. Radial growth of the colonies was measured after 4 days. EC50 (effective concentration required to give half of maximum growth) values were estimated by fitting logistic curves, with growth as a proportion of the maximum growth. Of the 50 isolates, 16 were identified as resistant, 32 as sensitive and two had a variable response. The estimated EC50 values for the isolates that were sensitive ranged from 0.006 to 0.053 g/ml active ingredient. It is recommended that New Zealand onion growers closely follow the current benzimidazole resistance management strategy.

Keywords: botrytis neck rot, Botrytis allii, carbendazim, benzimidazole, fungicide resistance, fungicide sensitivity.

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