Why Do Grapes Rot Before Ripening?

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Growing grapes on an arbor helps keep them dry.
Growing grapes on an arbor helps keep them dry. (Image: janaph/iStock/Getty Images)

Whether you're harvesting fresh, juicy grapes or simply looking for a vigorous leafy vine, grapes (Vitis spp.) pull their weight in the yard. If the bunches of grapes are rotting before they have a chance to ripen, the plant is likely suffering from a common and widespread fungal disease known as black rot. Though black rot is destructive, you can combat it with fungicides and proper grapevine care.

Watch for Wet Weather

Black rot attacks both cultivated and wild grape varieties, causing symptoms on fruits when the grapes are still green and not yet full size. First, whitish tan spots, often surrounded by a brown halo, appear on fruits. Within a few days, the grapes blacken and shrivel up, appearing mummified. The disease emerges in the spring and summer during long, warm rainy periods. It requires two to three days of wet weather to spread, and is slowed by cool weather. Young leaves will show blotches and lesions before the symptoms appear on the grapes.

Follow Sanitation Guidelines

Black rot spreads easily, and may overwinter on plant debris and re-infect healthy plants the following spring. Remove and destroy all rotten fruits and tendrils. Keep the area around the vines weed-free, as weeds inhibit air circulation. Prune any infected plant parts, including canes and any leaves with lesions. Disinfect your pruning equipment by soaking it for five minutes in a solution of either 1 part rubbing alcohol and 1 part water or 1 part bleach to 3 parts water, rinsing the equipment afterwards with water and allowing it to air dry.

Apply a Fungicide

In spring, when new growth is 4 to 6 inches long, apply fungicide to the vine to prevent black rot. Thoroughly mix 1 1/4 ounces of a fungicide containing myclobutanil in 1 gallon of water and, using a handheld sprayer, uniformly apply the solution to all parts of the vine until it runs off the leaves and vines. Reapply the solution every seven days during wet weather and every 10 days during dry weather, applying up to five times during the season. The grapes stop being susceptible to the disease when they have a sugar content of 6 to 8 percent, which usually occurs when they begin changing color. When applying the fungicide, wear long sleeves, long pants and rubber gloves and follow all safety instructions on the product's label. Store unused solution in a secure location away from children and pets.

Care for Your Grapevines

Planting your grape plant in the right conditions and giving it good care will help to prevent problems. Grapes vary in hardiness: common grape (Vitis vinifera) grows in U.S. Department of Agriculture plant hardiness zones 5 to 9, although this varies by cultivar. Other grape species may grow in colder climates. Plant the grapevine in a bright, sunny spot with well-draining, deep soil. Water the plant regularly to keep the soil moist.

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