AMES, Iowa – Grapes grow well in a wide range of soil conditions but do best on well-drained soils. Horticulturists with Iowa State University Extension and Outreach answer questions about growing and harvesting grapes. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or firstname.lastname@example.org.
Grapes should not be harvested until fully ripe. The best indicators of ripeness are color, size and flavor.
Depending on the variety, the berry color changes from green to blue, red or white as the grapes approach maturity. At the fully ripe stage, the natural bloom on the berries becomes more pronounced. However, color alone should not be the sole basis for harvesting grapes. Many varieties change color long before the grapes are fully ripe.
Size and firmness are other useful indicators of ripeness. The individual berries should be full-sized. They also become slightly less firm to the touch at maturity.
The final and most reliable test for ripeness is flavor. Taste a few grapes when size and color are good. If they are not sweet, leave the clusters on the vines. Grapes do not develop full flavor when harvested before completely mature.
Several factors could be responsible for the uneven ripening of the berries within a cluster. Possible causes are over-cropping (too many grape clusters on the vine), a potassium deficiency, moisture stress or 2,4-D damage.
Over-cropping is the most common cause for home gardeners. An average grapevine may have 200 to 300 buds which are capable of producing fruit. If grapevines are not pruned properly in late winter, the number of fruit clusters may be excessive. The vine is unable to ripen the large crop properly, resulting in uneven ripening of the berries within the clusters. In Iowa, 60 is the maximum number of buds that should remain on a grapevine after pruning.
Black rot is probably responsible for the damage to the grapes. Black rot is caused by the fungus Guignardia bidwellii. This fungal disease is common in home and commercial vineyards in Iowa, especially in warm, humid summers.
On grape foliage, black rot starts as tiny yellow spots that grow to about one-quarter inch in diameter. The centers of the spots turn rusty red. Tiny black dots usually form in the center of the spots. These dots contain thousands of spores that can produce new infections. Symptoms on fruit become apparent when the berries are about half grown. Infected berries can rot in a few days. They shrivel and become hard, black mummies, which contain the same tiny spore-containing structures as the leaf spots.
The black rot fungus survives the winter primarily in mummified fruit on the vine or on the ground. However, it also overwinters on diseased areas on the vine.
Sanitation is an important step in controlling black rot. Rake up all fallen mummies on the ground. Also remove any mummies that are still hanging onto the vines. The mummies should be removed from the area and destroyed. A preventive fungicide spray program, beginning shortly after bud break and continuing until the fruit begin to develop color, should help to suppress black rot in plantings where the disease has been a problem in previous years.