Splash dispersal of Plasmopara viticola primary inoculum

Primary infections of Plasmopara viticola are caused by zoospores originating from oospores that overwinter in leaf litter or in soil. The inoculum is carried from the ground to the grape leaves by splashing rain, but little information exists about the relationships between rainfall, travelling distance and distribution of the inoculum within grapevine canopies. Experimentally, the soil of single curtain-trained plants was uniformly covered with a powder of two colours: red, under the projection of the canopy on the ground (row); blue, outside this projection (inter-row), in order to mark the splashes from raindrops that fell in these areas. Twelve traps for splashes were placed within the canopy at 3 different heights (40, 80, and 120 cm above the ground) to mimic leaves. Blotting papers were arranged in the abaxial part of the traps, substituted after each rainfall and observed for the number and dimension of the droplets. More than 23,000 droplets were collected (average of 3.9% of the trapping surface covered). The numbers of red and blue droplets were not significantly different, but the former were 1.6 times bigger than the latter. More than 99% of the total droplets were collected at 40 cm above the soil. Rain events lasted 2 to 19 hours, with 1.6 to 64.2 mm of water, 1.3 to 3.8 m/s of wind (max gusts of 30.6 m/s), but these differences did not influence droplets number and distribution significantly. Continuation of these studies will contribute to better understand the relationships between rainfall and primary inoculum of grapevine downy mildew.