Why Do Grape Clusters Dry Out on the Vine?

Growing grapes (Vitis spp.) remains an interesting challenge to many gardeners. Some grapevines' fruits can be eaten straight from the vines, but most are used to make jams, jellies and wine. Although usually simple to grow, the fruits can be ruined after they have set, even when gardeners start with a healthy vine and optimal growing conditions. Grapevines are hardy in U.S. Department of Agriculture plant hardiness zones 2 through 10, depending on the cultivar or variety.

A grapevine may flower prolifically with clusters of fruits following. If the flowers are not pollinated properly by bees and other insects, however, the fruits can harden and wilt on the vine. Many grape varieties available to home gardeners are self-pollinating, eliminating the need for wild pollinators to aid in fruit production. It is important to know what variety of grapevine you grow and whether or not it requires assistance for pollination.

Grape is generally a hardy plant, but exposing its fruit to lingering, excessive heat and a lack of water -- especially when its fruits are setting -- will cause the fruits to wither on the vine. It must be watched closely during periods of lingering drought or high heat. Provide irrigation for your grapevine to make up for a lack of rain, especially as the vine blooms and fruits set. Water thoroughly, saturating the ground, but avoid leaving the roots standing in water or root rot may occur.

A grapevine may experience black rot (Guignardia bidwellii) fungus -- a serious disease in the eastern United States -- if its climate remains wet or humid. The condition results in rust-colored spots that appear first on leaves, and then the center of each spot darkens before black bumps emerge. The fruits darken, burst, dry out and wrinkle. Because the disease overwinters, it is imperative to remove damaged parts of an affected vine and remove all its dried fruit clusters to avoid a recurrence of the fungus the following season.

Botrytis bunch rot (Botrytis Cinerea) is caused by spores carried on wind, and its symptoms are similar to black rot's symptoms. It affects the entire plant, causing brown patches on the leaves and spreading to the fruits. After some fruits are infected, the disease spreads quickly through the fruits, turning them dark and then causing them to burst and wither. A grapevine already stressed from heat, drought or another fungus -- such as powdery mildew -- is more susceptible to this blight than a non-stressed grapevine. Air movement through a grapevine is important to avoid a repeat appearance of the blight. Keep the vine pruned, and avoid planting it close to another plant or object. Remove affected leaves immediately and dispose of them. Fungicides can be applied to save the fruits and kill the blight's spores.