Competition Between Plasmopara viticola Clade riparia and Clade aestivalis: A Race to Lead Grape Downy Mildew Epidemics

Abstract

There is evidence of five clades of Plasmopara viticola in the world. Only two clades, riparia and aestivalis, have been identified as responsible for the downy mildew epidemics in Quebec, Canada. It was reported in 2021 that epidemics caused by clade riparia start 2 or 3 weeks before those caused by clade aestivalis and that clade aestivalis was more aggressive than clade riparia. The objective of this work was to study the competition between P. viticola clade riparia (A) and clade aestivalis (B) and to compare the aggressiveness of both clades in mono- and co-infection situations. Suspensions of sporangia from both clades with six percentage combinations (AB 100–0; AB 89–11; AB 74–26; AB 46–54; AB 23–77 and AB 0–100) were inoculated on leaf discs (cultivar Vidal), and three other combinations (AB 88–12; AB 68–32 and AB 47–53) were inoculated on living leaves of grape plants (cultivar Vidal). Subsequently, sporangium production expressed as percentage of sporangia produced by each clade was estimated on leaf discs after eight cycles of infection-sporulation and then validated on living grape leaves after five cycles. The aggressiveness of clades in mono-infection situations on leaf discs was compared to that in co-infection situations. The results show that the percentage of sporangia produced by clade aestivalis increases with the infection-sporulation cycle while that produced by clade riparia decreases. The area under the sporangium production progress curve (AUSPPC) of clade aestivalis was significantly higher than that of clade riparia. The aggressiveness of P. viticola clades riparia and aestivalis in co-infection situations was different from those in mono-infection situations and was strongly influenced by percentage of each clade in competition. These results suggest that on the grapevine cultivar Vidal, P. viticola clade aestivalis is more competitive than clade riparia and that the percentage of each clade present in the vineyard should be considered for management of downy mildew.