Geostatistical analysis of downy mildew (Plasmopara viticola) symptoms on grapevines

The current environmental issues require the agricultural sector to reduce plant protection products (PPP) used in cultivation. In winegrowing, the control of downy mildew (Plasmopara viticola) requires several PPP applications. This study aimed to analyze the spatial dependence of symptoms of downy mildew on the grapevine. The severity of symptoms was evaluated by visual quantification on 32 points during four surveys in a 1.1-hectare block. First, the McKinney Index measured the severity; then, the georeferenced values were used for a geostatistical analysis (experimental variogram and kriging interpolation). According to the results, the spatial dependence of the number of leaves was strong (Nugget effect = 7.5%) and increased during the growing season. Similarly, the spatial dependence of the number of bunches was strong (Nugget effect = 6.3%) without strong variation during the growing season. On the other hand, symptoms' spatial dependence was weak at the beginning of the growing season (Nugget effect= 77%), while it was strong in the middle of the growing season (Nugget effect = 7.8%). The analysis of covariates proved that the effect of elevation and vines' vigor on downy mildew was not significant. These results suggest that primary infection was slightly spatial dependent, while secondary infection of downy mildew was strongly spatial dependent.