HOST SETTLEMENT OF PLASMOPARA-VITICOLA ON DIFFERENT SUSCEPTIBLE HOSTS

Leaf discs of grapevine varieties and species of various resistance to downy mildew were infected by Plasmopara viticola, fixed after 6, 24 and 48 h of incubation and coloured with optical brighteners or fuchsi n acid solution. The different stages of infection were investigated q ualitatively and quantitatively.Zoospores of P. viticola settled at th e stomata of leaves of resistant and susceptible cultivars. The first phenomenon of resistance occurs when the germination tube tries to ent er the substomatal hole. In clones of Vitis riparia and V. rupestris g ermination tubes of the pathogen were frequently hindered to enter the substomatal hole. In V. riparia and the resistant variety Fr 946-60 P . viticolawas often already stopped before forming a primary haustoriu m. In the resistant varieties Fr 946-60, Fr 993-60, Castor and SV 5-27 6 the pathogen was stopped in many cases after the formation of the fi rst haustorium. At this time degeneration of the parasitized host-cell s could often be observed. In the resistant variety Pollux cell degene rations mostly occur after the formation of the secondary haustorium.