Leaf Hairs as a Basic Protective Barrier against Downy Mildew of Grape

The influence of hairs during the interaction of Plasmopara viticola with leaves of four Vitis species was studied both under natural conditions and upon treatment with the detergent Tween 20. The hydrophobic felt consisting of hairs on the adaxial leaf sides repels water from the leaf surface preventing a successful penetration of the host via fungal germ tubes. Treatment with a detergent solution led to an enhanced attachment of water droplets. As a consequence of the detergent treatment, infection structures of P. viticola were formed on the field-resistant hosts Vitis doaniana and Vitis davidii as in the susceptible Vitis vinifera hosts. It can be concluded that covering the leaf surface with hydrophobic hairs represents a major defence strategy and that the infection process is dependent on wetness. Although in Vitis cinerea and Vitis labrusca primary infection structures can be monitored after detergent treatment, the growth of the fungus was found to be restricted in these species indicating the action of additional or different defence strategies.