Infection behavior of Venturia nashicola, the cause of scab on Asian pears

The infection of Japanese pear by Venturia nashicola, the cause of scab on Asian pears (Japanese pear, Pyrus pylifolia var. culta; Chinese pear, P. us suriensis), was examined using light and electron microscopy to determine t he mechanism of resistance in pears. Early stages of infection were similar on the susceptible cv. Kosui, the resistant cv. Kinchaku, and the nonhost European pear (P. communis) cv. Flemish Beauty. V. nashicola penetrated onl y the cuticle layer on pear leaves and formed subcuticular hyphae on all th ree cultivars. Hyphae were localized in the pectin layer of pear leaves and never penetrated into the cytoplasm of epidermal cells. This restriction o f fungal growth suggested that pectinases released by infection hyphae or s ubcuticular hyphae may be important in infection. Subcuticular hyphae were modified ultrastructurally in the pectin layer of resistant pear cultivars accompanied by fungal cell death. In contrast, fungal cells appeared intact in susceptible pear cultivars, suggesting the existence of resistance mech anisms.