Evidence for Differences in the Temporal Progress of Plasmopara viticola Clades riparia and aestivalis Airborne Inoculum Monitored in Vineyards in Eastern Canada Using a Specific Multiplex Quantitative PCR Assay

Four clades of isolated from wild and cultivated species were described in 2013. Only clades and have been detected in eastern Canada. To increase our understanding of the epidemiology of these clades of , airborne sporangia were monitored with spore samplers at two experimental vineyards from 2015 to 2018 and at 11, 14, and 15 commercial vineyards in 2016, 2017, and 2018, respectively. At each vineyard and in each year, airborne sporangia were assessed three times weekly from grapevine budbreak to harvest. To accurately monitor airborne inoculum, a specific and sensitive quantitative PCR assay for simultaneous quantification of clades and was developed. At the experimental site, in the vineyard planted with the hybrid grape variety Chancellor, mostly clade was detected. In vineyards planted with multiple grape varieties, airborne sporangia of clade were prevalent at the beginning of the season, whereas clade was mostly detected from midseason to harvest. At the commercial sites, airborne sporangia of clade were more prevalent in 2016, whereas clade was more prevalent in 2017 and 2018. The only significant difference between the inoculum progress curves was the time at which 50% of the seasonal inoculum was reached, with an average for the 3 years of 100.8 and 117.9 days since 1 May for clade and clade , respectively. When airborne sporangium concentrations were expressed as the proportion of the two clades, in general, the proportion of clade to that of clade was low at the beginning of the season and increased to reach approximately 0.9 to 1.0 at the end of the season. These results suggest that both clades of coexist, but that clade is predominant and that downy mildew epidemics caused by clade occur 2 to 3 weeks before those caused by clade