Black rot (Guignardia bidwellii) is a fungal disease that commonly attacks grape vines (Vitis spp.), which grow in home gardens throughout U.S. Department of Agriculture plant hardiness zones 3 through 10, depending on the variety. The fungal spores overwinter in diseased fruits and foliage, reviving in spring to start infecting new vines during wet, warm weather. The disease causes leaf spots and makes the grapes shrivel, rot and blacken. Once the infection invades your vines, it can destroy an entire crop within a few days. You can control and prevent black rot with chemical treatment and proper cultural care.
Spraying infected grape vines every 10 to 14 days with an appropriate fungicide helps cure black rot disease. Fungicides capable of controlling black rot include captan, copper and mancozeb. Carefully follow the mixing and application instructions on the product's label because instructions vary. One captan-based fungicide recommends treating black rot by mixing 1 to 1 1/2 tablespoons of product for every gallon of water. Using a handheld garden sprayer, evenly apply the solution until all of the foliage and grape clusters glisten. Effective treatment requires complete coverage. Time spraying for when dry weather is predicted for at least 24 hours. Captan-based fungicides can be used up to the time of harvest, but wash your grapes well under running water before eating. You can help prevent black rot by spraying your vines at 10- to 14-day intervals from just before bloom until about four weeks after.
Pruning out all infected canes, leaves and clusters as soon as you notice them helps get rid of black rot. Disinfect your hand pruners or loppers between each cut by dipping the blades into a solution of 1 part chlorine bleach and 9 parts water. Place infected prunings into a covered trash can. Because the black rot spores thrive in damp areas, reducing the moisture levels around your grapevines can help prevent infection. Prune vines in late winter during dormancy, selecting just two to four strong, healthy canes about as thick as your pinky finger with buds growing close together. Cut each of those canes back until each one bears only about 15 buds. Prune out all other canes. Opening up the plant this way helps increase airflow throughout the vine, which slows the spread of black rot disease.
Control black rot from the start by planting grape vines in an open area that offers full sun and ample air circulation. Keep the grapevines off the ground by tying them to a trellis, fence or stake. This reduces the time they remain damp from rain and dew, which lowers the risk of black rot infection. Keeping the soil around your vines free of weeds and grass also reduces relative humidity levels and allows your grapevines to dry faster during wet weather.
A Word of Warning
Fungicides contain chemicals that can irritate skin and eyes. Reduce your risk of chemical exposure by wearing waterproof gloves, long pants, long sleeves, shoes with socks, goggles and a face mask. Don't allow pets or people into the treatment area until the spray thoroughly dries. Some chemicals can also harm fish, so don't spray grape vines growing next to a water source.