Downy mildew of grape

Downy mildew, caused by the oomycete Plasmopara viticola, is considered the most destructive fungal disease affecting grapevines globally. This obligate parasite thrives in humid climates and can rapidly devastate entire vineyards if not managed with precise agricultural timing.

The infection manifests primarily on green tissues. Initial symptoms include translucent «oil spots» on the upper surface of leaves, which later turn yellow or brown. During periods of high humidity, a distinct white, fuzzy mycelial growth appears on the underside of these spots, marking active sporulation.

The disease development is heavily dependent on weather conditions. Primary infections are triggered by rainfall and warm temperatures. Water droplets are essential for the dispersal of zoospores, which travel from the soil to the lower leaves via rain splashes, initiating the infection cycle that repeats throughout the growing season.

The economic impact of downy mildew is severe. It leads to premature defoliation, which severely reduces the vine's vigor, prevents proper cane maturation, and makes the plant highly susceptible to winter frost damage. In addition, clusters and berries can be infected directly, leading to fruit rot and significant yield loss.

Effective management requires an integrated approach. Essential practices include ensuring proper canopy management through trellising and pruning to reduce humidity. Additionally, preventative fungicide treatments are crucial, especially before and after forecasted rain events, to keep the pathogen population below the economic threshold.