COMPONENTS OF PARTIAL RESISTANCE IN PHASEOLUS BEANS AGAINST AN ETHIOPIAN ISOLATE OF BEAN RUST

Phaseolus bean cultivars, obtained from the Ethiopian national breedin g programme, and cultivars widely grown in the country, 15 in total, w ere tested in a greenhouse for five components of partial resistance t o one isolate of bean rust. The single-pustule isolate came from Ambo, a site where bean lines are tested against rust because the climate i s conducive to bean rust. The components examined include latent perio d (LP(50)), infection efficiency (IE), sporulation capacity (SC), infe ctious period (IP) and pustule size (PS). Differences in cultivar resp onses were found for all PR components. Differences were largest, howe ver, for infection efficiency and sporulation capacity. Cultivars Exri co 23, A 176, Veracruz 10 and BAT 1198 had a high level of PR to the i solate tested. Mexican 142, a widely grown cultivar in Ethiopia was in termediate, whereas Red Wolaita, an important cultivar in the south, s howed a low level of PR. Linear correlations between LP(50) and IE, an d between SC and PS were high. Linear correlations between IE, SC, or PS with IP were not significant. Though differences in cultivar respon se were found for all components, any one parameter may not suffice to explain the PR potential of a particular cultivar. The study suggests that latent period, infection efficiency and pustule size should be u sed in selection for PR. For the evaluation of large numbers of bean c ultivars in the greenhouse, IE and PS are preferable to minimize labou r requirements.