Abstract
Brown stem rot (BSR) of adzuki bean [Vigna angularis (Willd.) Ohwi & Ohashi], caused by Phialophora gregata (Allington et Chamberlain) W. Gams f. sp. adzukicola, is one of the most serious diseases of adzuki bean in Hokkaido. It is difficult to control BSR through chemical or cultural methods because BSR is soil-borne. The most effective means of controlling this disease is through the breeding of resistant cultivars. In 1996, a new race (race 2) of BSR was identified, which was virulent to all the commercial adzuki bean cultivars in Hokkaido. Therefore, greenhouse and field experiments were conducted to seek sources of resistance to BSR race 2. Field experiments consisted of two fields; one field mainly infested with BSR race 1 and the other with BSR race 2. Most adzuki bean cultivars previously selected for their resistance to BSR race 1 were susceptible to BSR race 2. Of 236 adzuki bean varieties and 36 wild adzuki bean accessions evaluated, one adzuki bean variety and one wild adzuki bean accession [Vigna angularis var. nipponensis (Ohwi) Ohwi & Ohashi] were found to be more tolerant to BSR race 2 than other adzuki bean germplasm accessions, based on both greenhouse and field evaluation. These two accessions, that were resistant to BSR race 1 and race 2, were selected as BSR resistance breeding gene sources. F1 and F2 hybrids between these resistant sources and susceptible sources were evaluated for the reaction to BSR race 2 in the greenhouse. The segregation ratio fitted to a 3 : 1 (resistant : susceptible) ratio, indicating that the newly selected gene sources resistant to race 2 were mainly controlled by a single dominant gene.
