Black Rot
Xanthomonas campestris pv. campestris
Overview
Black rot is a destructive fungal disease of grapes caused by Guignardia bidwellii (also placed in Phyllosticta). It also affects related vines and can ruin clusters in humid growing regions. The fungus overwinters in mummified berries and infected canes. Spring rains release spores that infect young shoots, leaves, and fruit. Warm, wet weather during bloom and early fruit development drives epidemics. Once berries begin to color, they become less susceptible, so early-season protection is critical.
Symptoms
Leaf lesions are roughly circular tan to reddish-brown spots with dark borders. On shoots, elongated black cankers appear. Young berries develop light spots that enlarge, turn black, and shrivel into hard mummies that often remain attached. Entire clusters can be lost in wet seasons. Fruit infections frequently start at the blossom end or where spores land during early berry growth. Mummies hanging in the canopy are a hallmark of black rot.
Environmental Conditions
Humid
Prevention
Plant grapes in sunny sites with good air movement. Choose cultivars with better black-rot tolerance for humid climates when quality goals allow. Train vines for an open canopy from the start. Avoid overhead irrigation on fruiting vines. Scout weekly in spring so you catch the first leaf spots before fruit infection takes off. Keep wild grapes near the vineyard managed if they contribute inoculum.
Management
Sanitation is essential: remove and destroy mummified fruit and infected canes during dormant pruning. Open the canopy with thoughtful training and leaf pulling so fruit dries quickly after rain. Organic copper and other labeled protectant fungicides, applied from early shoot growth through early fruit set according to local schedules, help where humidity is high. Remove severely infected clusters during the season. After harvest, clean up fallen mummies from the trellis and ground.