Competition Between Clade and Clade : A Race to Lead Grape Downy Mildew Epidemics.

There is evidence of five clades of in the world. Only two clades, and , have been identified as responsible for downy mildew epidemics in Quebec, Canada. It was reported in 2021 that epidemics caused by clade start 2 or 3 weeks before those caused by clade and that clade was more aggressive than clade . The objective of this work was to study the competition between clade (A) and clade (B) and to compare the aggressiveness of both clades in mono- and coinfection 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). Then, sporangium production, expressed as the 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 monoinfection situations on leaf discs was compared with that in coinfection situations. The results show that the percentage of sporangia produced by clade increases with the infection-sporulation cycle while that produced by clade decreases. The area under the sporangium production progress curve (AUSPPC) of clade was significantly higher than that of clade . The aggressiveness of clades and in coinfection situations was different from that in monoinfection situations and was strongly influenced by the percentage of each clade in competition. These results suggest that, on the grapevine cultivar Vidal, clade is more competitive than clade and that the percentage of each clade present in the vineyard should be considered for management of downy mildew.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.