What Is Ripe Rot on Grapes?

Causal Agent

• Ripe rot is a fungal disease caused by the Colletotrichum fungi. Fungal spores overwinter in fallen fruit and start to get active during moist spring weather, spreading mostly with splashing water. The disease is more common in grape growing regions that experience warm temperatures of about 77 to 86 degrees Fahrenheit at the time of fruit maturation.

Symptoms

• Early symptoms of ripe rot include the appearance of round, decaying spots on the fruit skins. With disease progression, the spots get sunken and the decay worsens to a point where affected fruit darkens and shrivels entirely. Fungal fruiting bodies or reproductive spores grow under fruit skin. Once mature, the pink-colored spore mass breaks through the fruit skin. These spores are responsible for spreading the infection to healthy fruit and plants.

Damage

• Ripe rot fungus infects grapes during all stages of growth. The unripe fruit does not show symptoms until it starts to ripen, however. Rotted fruit either falls off the vine or mummifies and stays attached. Spores spread rapidly through the vine. The disease can cause heavy loss in vineyards with susceptible cultivars. Rains during harvest accelerate progression of disease. Dark-skinned grape varieties are usually more resistant to ripe rot as compared to the white-skinned varieties, but does not meant that the darker varieties such as chardonnay or merlot are resistant to the fungus.

Management

• Control ripe rot by treating vines with preventative fungicides early in the season. Repeat applications again during bloom time and again during the early fruit development stages. Recommended fungicide includes the use of captan. Remove all affected fruit from and under the vine to reduce chances of infection in spring. Thin the vine and prune regularly to allow air and sunlight into the inner areas