Host-parasite relationships in bean cultivars of varying susceptibility tobean rust

Components of disease development were measured in three cultivars of Phase olus vulgaris L. (common bean) infected with Uromyces appendiculatus (Pers. :Pers.) Unger (bean rust fungus) race O. Disease measurements and light and electron microscope data of host-parasite relationships were obtained and analyzed. Uredinial size, infection efficiency, latent period, and fungal c olony radius were measured from infected bean leaves that were grown under controlled conditions. Phaseolus vulgaris cultivar Pinto 111, a highly susc eptible check, displayed the largest uredinia, the highest infection effici ency, large colony radii, and a short latent period. Cultivars Early Gallat in and Kentucky Wonder (K.W.) 814 displayed moderate and low susceptibility , respectively. Cultivar Early Gallatin had smaller uredinia, reduced infec tion efficiency, and longer latent period when compared with cv. Pinto 111. Cultivar K.W. 814 was characterized by minute pustules, restricted colony expansion, and the longest latent period. Ultrastructural data of host-para site relationships were collected from infected leaf tissues and prepared f or transmission electron microscopy by high-pressure cryofixation and freez e substitution. In 'Pinto 111' the collars around haustorial necks were com posed of a fibrillar network embedded in an electron transparent matrix. Ul trastructural observations indicated that Cultivars K.W. 814 and Early Gall atin deposited more collar material than 'Pinto 111.' Networks of tubular e ndomembranes developed near the host-parasite interface in the host cytopla sm of cultivars K.W. 814 and Early Gallatin. The tubules showed continuity with the extrahaustorial membrane and contained an amorphous, electron-dens e material in the lumen. Tubular endomembranes were not seen in the highly susceptible cultivar Pinto 111.