Limitations in the exploitation of N ‐phenylcarbamates and N ‐phenylformamidoximes to control benzimidazole‐resistant Venturia nashicola on Japanese pear

The efficacy of N ‐phenylcarbamates and N ‐phenylformamidoximes against benzimidazole‐resistant strains of Venturia nashicola was tested experimentally in Japanese pear orchards. Both compounds gave effective control when applied in an orchard where highly benzimidazole‐resistant strains were predominant. However, unsatisfactory control was observed in an orchard where the fungal population was dominated by fungal strains with intermediate or weak benzimidazole resistance. Strains resistant to N ‐phenylcarbamates and N ‐phenylformamidoximes were widely distributed in Japanese pear orchards, suggesting that effective use of these compounds to control benzimidazole‐resistant strains of V. nashicola would not be practicable. Increased sensitivity to N ‐phenylformamidoximes in highly benzimidazole‐resistant isolates was shown to be controlled by a single chromosomal gene, but progenies that were highly resistant to both a benzimidazole fungicide and a N ‐phenylformamidoximes appeared in crosses between parents resistant to each fungicide alone. Similar strains were also found in pear orchards, and this‘double resistance’was shown to be heritable.