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

Components of disease development were measured in three cultivars of Phaseolus 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 colony radius were measured from infected bean leaves that were grown under controlled conditions. Phaseolus vulgaris cultivar Pinto 111, a highly susceptible check, displayed the largest uredinia, the highest infection efficiency, large colony radii, and a short latent period. Cultivars Early Gallatin and Kentucky Wonder (K.W.) 814 displayed moderate and low susceptibility, respectively. Cultivar Early Gallatin had smaller uredinia, reduced infection 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-parasite relationships were collected from infected leaf tissues and prepared for transmission electron microscopy by high-pressure cryofixation and freeze substitution. In 'Pinto 111' the collars around haustorial necks were composed of a fibrillar network embedded in an electron transparent matrix. Ultrastructural observations indicated that Cultivars K.W. 814 and Early Gallatin deposited more collar material than 'Pinto 111.' Networks of tubular endomembranes developed near the host-parasite interface in the host cytoplasm of cultivars K.W. 814 and Early Gallatin. The tubules showed continuity with the extrahaustorial membrane and contained an amorphous, electron-dense material in the lumen. Tubular endomembranes were not seen in the highly susceptible cultivar Pinto 111.