Why Do Grapes Rot Before Ripening?

Improper Pruning

• The most likely scenario when you are faced with grapes rotting before ripening is that your vine is too overgrown and has produced too many grape clusters to ripen. A grapevine must allocate resources and energy to growing new leaves and vine, as well as storing water and carbohydrates in the form of sugar in the fruit. A vine with too many grape clusters and too much foliage will not have enough resources to ripen its fruit, and you will find a large amount of clusters rotting before ripening. Prune your vine back in the dormant season, typically December or January, to two main trunks with five arms each.

Black Grape Rot

• If you find your grapevines are losing fruit, with single grapes in a cluster turning black and shriveling up, or entire clusters drying up and falling off after having turned black, you may be facing black grape rot. Black grape rot is caused by a fungal infection in the vine and must be treated with fungicides. Myclobutanil is the most common ingredient in fungicide used for black rot. Follow the instructions on the bottle and continue treatments as a preventive measure after rot has disappeared.

Overwatering or Underwatering

• If your grapevine is not getting enough water, you may be seeing your grapes remaining small and eventually rotting on the vine. If your vine is overwatered, they may swell and burst, finally rotting away on the vine after the inner sugars are exposed to the air. Grapevines need water but do not like to have "wet feet," which means they do not do well in poorly drained soil.

Disease

• Grapevines are subject to a number of diseases, disorders and pests, such as grapevine skeletonizer bugs, mildew and dieback. Identifying the diseases and disorders of grapevines is an entire field unto itself, and your county extension service can likely help you identify and treat the issue. Remove a few affected grape clusters and leaves, take them to the extension service and ask for assistance in treating your issue.