PHENOTYPIC CHARACTERIZATION OF THE INTERACTION BETWEEN PLASMOPARA VITICOLA AND VITIS VINIFERA.

Plasmopara viticola (Berk. et Curt.) Berl. and De Toni is an obligate biotrophic oomycete which causes grapevine downy mildew, one of the most important grapevine disease, affecting particularly the European grapevine Vitis vinifera L. The main objective of the study is to find possible sources of resistance to P. viticola in the less common V. vinifera germplasm (Caucasian origin) through the screening of an ample collection of cultivated and wild Caucasian accessions, grown in greenhouse and in open field. This could really simplify the genetic improvement programs. The level of susceptibility towards P. viticola has been assessed by experimental inoculations on 148 cultivated Caucasian (mainly Georgian) and Iranian accessions (V. vinifera var. vinifera) and 35 wild Georgian accessions (V. vinifera var. sylvestris) grown in pots in greenhouse. Moreover, natural downy mildew occurrence has been evaluated on 94 Georgian accessions cultivated in vineyard. These activities have been carried out for three consecutive years. Several accessions were characterized by an interesting resistance level, and in particular, the Georgian cv Mgaloblishvili N. was characterized by the most stable behavior. For this reason, further investigations, aiming at more deeply investigating the plant-pathogen interaction at the genetic and histological levels, were carried out on this cultivar. In order to get insights on the resistance mechanism at the genetic level, 272 Mgaloblishvili progenies were obtained by self pollination, open pollination and pollinated with ‘Pinot noir’ N and experimental inoculations have been carried out over a three-year period. The characterization of the interaction between P. viticola and Mgaloblishvili by histological analysis at different time points was assessed, in order to evaluate the type and the timing of the plant response and the development of the pathogen. To thoroughly analyze the effective level of resistance in Mgaloblishvili, experimental inoculation with different P. viticola strains characterized by different aggressiveness levels were carried out. Finally, the genetic variability of P. viticola in Italy was investigated to get insights on the possible durability of resistance in open field. The genetic diversity of P. viticola populations, collected from 13 different geographic Italian regions, has been investigated during one-month period at INRA, Institute de la vigne et du vin, Villeneuve d’Ornon (Bordeaux) in France to evaluate if the genetic differences could modulate the aggressiveness level of the pathogen.