Canopy density modifies leaf predisposition to Plasmopara viticola but does not affect downy mildew epidemics in grapevine

Certain agricultural practices may influence the susceptibility of the plant tissue to pathogens, including practices that influence the canopy density, which in turn may favor disease development through changes in the canopy microclimate. This relationship has previously been demonstrated for downy mildew (DM), caused by Plasmopara viticola , through comparisons of different trellising systems. In this work, we investigated the extent to which grapevine canopy density affects the susceptibility of leaves to P. viticola and the development of DM epidemics during the season in a single VSP system subjected to different canopy densities through modulating agronomic interventions, including variations in bud load, fertilization, and irrigation. Leaf susceptibility was investigated through artificial inoculation of P. viticola sporangia on leaf discs excised from the leaves of plants with different canopies. DM progress in the vineyard was periodically assessed as disease severity and then expressed as AUDPC. Infection on leaves from plants with denser canopies was more severe than that in leaves from plants with sparser canopies; however, neither changes in leaf predisposition to infection nor differences in microclimatic conditions (with higher moisture levels at nighttime in denser canopies) significantly affected DM progress in field. These findings confirm that DM epidemics are driven by complex interactions among host plant components, pathogen infection cycle, and microclimate factors. Although denser grapevine canopies increase the predisposition of leaves to P. viticola and enhance the moistness of the within-canopy microclimate, canopy density alone does not necessarily favor more severe epidemic development within uniform VSP systems.