The course of colonization of two different vitis genotypes by Plasmopara viticola indicates compatible and incompatible host-pathogen interactions

The course of colonization of leaf mesophyll by the causal agent of grapevine downy mildew, Plasmopara viticola, in a susceptible and a resistant grapevine genotype was examined in order to characterize the development of the pathogen in compatible and incompatible host pathogen interactions. Within a few hours after inoculation, the pathogen was established in the susceptible Vitis vinifera cv. Muller-Thurgau and formed primary hyphae with a first haustorium. No further development occurred in the following 10 to 18 h. The next step, in which the hyphae grew and branched to colonize the intercellular space of the host tissue, was observed 1.5 days after inoculation. After 3 days, the intercostal fields were entirely filled with mycelium and sporulation was abundant under favorable environmental conditions. The first infection steps were essentially the same in the resistant V rupestris. However, the invasive growth of P. viticola was delayed, and further development ceased before the intercostal fields were filled with mycelium.