Double resistance by citrus green mould Penicillium digitatum to the fungicides guazatine and benomyl

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Abstract
SUMMARY: In vitro dosage response data with different isolates of Penicillium digitatum and the fungicide guazatine indicated an approximate 10‐fold shift in tolerance when compared with wild type strains. ED50 values for resistant strains were approximately 0.5 μg/ml compared to approximately 0.05, μg/ml for the wild type strains. Colony growth of guazatine resistant isolates on selective media containing carbendazim showed that they were also resistant to the benzimidazole group of fungicides. In vivo tests in inoculated oranges with strains previously characterised by in vitro tests confirmed resistance to guazatine and benomyl. A combined treatment of these fungicides at 400 /μ/ml and 500 μg/ml respectively, which normally gives protection against decay, also failed to provide adequate mould control. Growth and pathogenicity of the resistant strains in these tests in oranges were indistinguishable from that of wild type strains.