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New Zealand Plant Protection 66 (2013): 293-302

A synthetic agar assay for determining sensitivity of Venturia inaequalis to anilinopyrimidine fungicides in New Zealand apple orchards

N.J. Larsen, R.M. Beresford, P.N. Wood, P.J. Wright and B.M. Fisher

ABSTRACT

A synthetic agar mycelial growth assay, adapted from a Botrytis cinerea method, was used to determine the sensitivity of Venturia inaequalis isolates (the cause of apple black spot) to anilinopyrimidine (AP) fungicides. Nineteen single-conidium isolates were classified as sensitive (S) or resistant (R) to cyprodinil or pyrimethanil after 21 days growth, according to concentrations that inhibited colony diameter by 50% (EC50). Isolate classification on agar was compared with leaf disease control by AP fungicides on potted apple trees inoculated with S or R isolates. Three S isolates were completely inhibited by AP fungicides on plants. Three R isolates were partially inhibited on fungicide-treated leaves. One isolate, classified as R for both fungicides on agar, was inhibited by cyprodinil, but not pyrimethanil, on plants. The agar method will be used for screening V. inaequalis populations for AP resistance in orchard surveys, with plant tests on some isolates to determine implications for orchard disease control.

Keywords: fungicide resistance management, mode of action, apple scab, disease management.

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