Effects of oat host genotype, leaf age and position and incubation humidity on germination and germling development by Erysiphe graminis f. sp. avenae

Abstract

The early stages of germling development by Erysiphe graminis were compared on young and old seedling and adult leaves from a susceptible and an adult plant resistant genotype, incubated under either humid or dry conditions. Preparative procedures for optical microscopy were employed which did not displace ungerminated or loosely attached germlings, but allowed the resolution of primary germ tubes (PGTs), localized autofluorescent host cell responses (LAHR) associated with PGTs, and appressorial germ tubes. Under dry conditions fewer conidia germinated to form a PGT, and fewer of these formed an appressorial germ tube, compared with the humid environment. Host genotype had no effect on germination, and although significantly fewer conidia germinated on old than on young, and on seedling than on adult, leaves, these effects were small in absolute terms. A higher percentage of PGTs was associated with LAHR on the susceptible than the resistant host, on young than old leaves and on adult than seedling leaves. There appeared to be a partial correlation between the proportion of germling PGTs associated with LAHR, and the proportion of germlings that formed an appressorial germ tube, as more did so on the susceptible than the resistant host and on young than old leaves. Although a slightly higher percentage of PGTs was associated with LAHR under the humid than the dry environment it was clear that where PGTs were associated with LAHR a very high proportion of germlings formed an appressorial germ tube in both environments. This contrasted with the situation where PGTs were not associated with LAHR where, under the dry environment, the majority failed to form an appressorial germ tube.