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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 qPCR assay.

Four clades of Plasmopara viticola isolated from wild and cultivated Vitis species have recently been described. Currently, only P. viticola clades riparia and aestivalis had been detected in eastern Canada. In order to increase our understanding of the epidemiology of these clades of P. viticola, 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 qPCR assay for simultaneous quantification of P. viticola clades riparia and aestivalis was developed. At the experimental site, in the vineyard planted with the hybrid grape variety Chancellor, mostly P. viticola clade riparia was detected. In vineyards planted with multiple grape varieties, airborne sporangia of P. viticola clade riparia were prevalent at the beginning of the season, while P. viticola clade aestivalis was mostly detected from mid-season to harvest. At the commercial sites, airborne sporangia of P. viticola clade riparia were more prevalent in 2016, while P. viticola clade aestivalis 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 three years of 100.8 and 117.9 days since May 1 for P. viticola clade riparia and clade aestivalis, respectively. When airborne sporangium concentrations were expressed as the proportion of the two clades, in general, the proportion of clade aestivalis to that of riparia was low at the beginning of the season and increased to reach around 0.9 to 1.0 at the end of the season. These results suggest that both clades of P. viticola coexist, but that clade aestivalis is predominant and that downy mildew epidemics caused by P. viticola clade riparia occur 2-3 weeks before those caused by clade aestivalis.