Resistance of Venturia nashicola to Thiophanate-methyl and Benomyl: Existence of Weakly Resistant Isolates and its Practical Significance

Abstract

High correlation was observed between conidial germ tube length and number of septa formed in the germ tube of pear scab fungus (Venturia nashicola). Germ tube elongation of sensitive conidia, obtained from leaf lesions, was remarkably suppressed by thiophanatemethyl or benomyl. It was considered that these fungicides act as antimitotic agents to the fungus, and consequently inhibit the septal formation and germ tube growth. As a result of testing for the degree of fungicide resistance by several methods, it was revealed that the “weakly resistant” isolates as well as the “highly resistant” isolates exist in the field. In the artificial inoculation, the “weakly resistant” isolate could also produce sporulating lesions on the Japanese pear leaves treated with thiophanate-methyl at recommended concentration. These results show that the “weakly resistant” isolates play an important role in poor disease control in the field as well as the “highly resistant” isolates.