Variability and mechanism of Saskatoon disease response to Entomosporium mespili
'Entomosporium' leaf and berry spot, is a serious disease of saskatoon (Amelanchier alnifolia Nutt.). The objectives of this study were to assess the variability of saskatoon disease response to Entomosporium mespili, and to investigate the structural mechanisms of host resistance. Cultivars of saskatoon were characterized for incidence and severity of 'Entomosporium' leaf and berry spot. The cultivars Parkhill, Regent and Success showed consistently lower values for the incidence and severity of fungal sporulation on leaves and fruit. Variation in leaf disease response to 'E. mespili' was observed among self- and cross-pollinated seedling populations derived from saskatoon cultivars. Population means for leaf acervuli number ranged from 63.4 for seedlings of Honeywood to 237.5 for seedlings of Smoky. Plants with resistance to leaf spot were identified among self-pollinated populations derived from saskatoon cultivars; as well as three other species of 'Amelanchier'. Population means for leaf acervuli number ranged from 2.0 for 'A. intermedia' to 134.8 for 'A. fernaldii'. Scanning electron microscopy was used to elucidate the infection process of 'E. mespili' on leaves of saskatoon. Conidiospore germination occurred rapidly on leaves of two cultivars, usually by the extrusion of a single major germ tube. Spore penetration of the leaf cuticle involved appressoria, formed at germ tube termini or directly below the germinating spore. Sub-spore appressoria were most common and resulted in the early formation of leaf acervuli. At 2 days post inoculation, 43% of germinating conidiospores had penetrated the leaf surface of Buffalo, compared with 12% on leaves of Success. Pre- and post-infectional structural defense mechanisms were observed in inoculated leaves of saskatoon cultivars. Cuticle thickness was uniform across resistant and susceptible cultivars, but differences in leaf thickness and anatomy appeared to be important. Papillae composed of suberin or callose were formed at sites along the adaxial epidermal layer. In leaves of resistant cultivars, the formation of suberin deposits in palisade mesophyll cells appeared to confine fungal activity to epidermal cells. Suberization of mesophyll cells at the periphery of the lesion sequestered hyphae and limited the number of sporulation sites on the leaf surface.