The influence of structural leaf epidermis morphology of summer barley upon spore deposition and infection by Erysiphe graminis f. sp. hordei

The landing and adhesion of conidia from Erysiphe graminis f. sp. hordei on barley leaves were investigated under the influence of simulated rain. The leaves to be inoculated were placed at diverse angles to the stems under wind-free conditions. The exposition of the inoculated leaves to rain surrogate (deion, water) took place in a specially constructed closed system. In the case of horizontally positioned barley leaves, the trichome density scarcely interfered with the deposit of conidia under wind-free conditions. Vertically positioned leaves with less trichome density were more vulnerable to conidial deposition than those more horizontally positioned and/or with greater trichome density. Increasing trichome density of the host leaf epidermis lessens the tendency of mildew conidia to be washed away by precipitation. The ablution rate of the conidia is in positive correlation to the duration of precipitation and, conversely, to the number of trichomes/square centrimetre of the host leaf. The combination of densely occurring trichomes as well as stomata creates an especially favourable microclimate for E. graminis. These factors provide, in spite of the higher ablution rate caused by increased precipitation, a constant infection potential. The presence of trichomes is not fundamentally a passive resistance mechanism against E. graminis as it effects two opposing phenomena: - reduction of conidial deposit when the host leaves are vertically exposed, whereby the conidia rebound from the trichomes. -protection of deposited conidia from exposure to rain and wind as well as securing a favourable microclimate for germination on the leaf surface. Epicuticular wax deposits appear to contain stimulatory factors for spore germination, so that there is a correlation between susceptibility of the various cultivars to infection and the density of their epicuticular waxes