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. Müller-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.

Background and Aims: Downy mildew, which occurs worldwide, is one of the most destructive of all grapevine diseases. Several authors have examined the host–pathogen interaction in different Vitis species, crosses and hybrids, but only a few studies comparing this in true V. vinifera varieties have been undertaken. The aim of the present study was to examine the in-field susceptibility to infection by Plasmopara viticola, the causal agent of downy mildew, of 44 grapevine varieties, all belonging to the collection of the Misión Biológica de Galicia. Methods and Results: Over a period of 2 years, the incidence and severity of downy mildew on the leaves and clusters of all 44 varieties was determined using a visual scale. Some of those examined showed high susceptibility (Chenin Blanc, Albariño and Prieto Picudo), others showed low susceptibility (Silveiriña, Caiño Bravo, Follajeiro and Brancellao Blanco), and still others showed intermediate susceptibility. Some varieties showed high disease incidence but low disease severity and vice versa. No significant correlation was detected between disease incidence and severity in either leaves or clusters, nor indeed between leaves and clusters. Neither berry colour, cluster compactness nor any other studied variable bore any clear relationship with susceptibility. Conclusion: The different grapevine varieties examined showed great variation in terms of their susceptibility to downy mildew. The susceptibility of most varieties was the same in both study years. Significance of the Study: The results of this work could help provide new material that might be of use in grapevine genetic improvement programs, and in the study of resistance to downy mildew. The selection of old varieties from the Iberian northwest may contribute towards the growth of organic viticulture in this area and even other winemaking regions around the world.