Yearly Fluctuation in the Occurrence of Thiophanatemethyl-resistant Strains of the Pear Scab Fungus in Pear Orchards at Karasu Town, Mie Prefecture

Abstract

Distribution of thiophanate-methyl-resistant strains of Venturia nashicola Tanaka et Yamamoto was surveyed in 1976-1980 at the Kawahara area, Karasu town, Mie prefecture, where Japanese pear had been severely damaged by the occurrence of tiophanate-methyl-resistant strains in 1975. The detection rate of resistant isolates was 93-96% in 1976 through 1980, suggesting the slow recovery of population of senitive strains at the Kawahara area. On the other hand, the detection rate of resistant isolates in 1977 through 1980 was 1.01% at the former Mie Univ, farm and other places where any preparation of thiophanate-methyl or benomyl had never been used. This result suggests that a few number of thiophanate-methyl-resistant strains of V. nashicola have occurred in nature. The present study supports our previous conclusion that the occurrence of the resistant strains at the Kawahara area in 1975 might be due to the selective elimination of the sensitive strains by the extensive use of thiophanate-methyl, resulting in the predominance of the resistant strains.