Q&A: Grape Troubles
Q: My grapes are rotting on the vine and not even ripe yet. The grapes are getting brown spots on them. Help!
O.H., New Jersey
A: Although we don't have an image of your grapes, your location helps determine the cause of the problem — most likely a fungal disease. Black rot is a common disease affecting grapes on the East Coast, but it doesn't occur in grapes on the West Coast. (There, the problem may more likely be caused by downy mildew, powdery mildew or botrytis.) Generally warm and humid or rainy weather aggravates the onset of disease, and the moisture helps in spreading spores to other places within the plant or other plants.
With black rot, the fungus Guignardia bidwellii attacks virtually all top parts of the plant — the fruit, leaves, young shoots, tendrils and stems. When exposed to warm, wet weather for much of the growing season and left untreated, the fruit will completely rot on the vine.
There's a six-week window from right after bloom up to the color change in fruit shortly before harvest when symptoms of black rot show. The first signs of infection generally show up on leaves as irregular spots, yellowish-orange in color with dark rings on the outside. Brown spotting occurs on half-grown fruit and continues to develop until the fruit is completely infected. Ultimately they become bluish-black and mummified and look like raisins.
Although they have somewhat similar symptoms in the fruit, downy mildew and powdery mildew have different leaf symptoms than black rot.
Caused by the fungus Uncinula necator, powdery mildew appears mostly on the foliage and stems. Powdery white fungal growths appear on the undersides of the leaves and can be overlooked if you're not on the lookout for symptoms. These patches eventually spread over the entire leaf, giving off a dull-white, powdery glow. The fruit will also darken, and white patches form on the fruit's skin.
Sometimes confused with powdery mildew, downy mildew is caused by a fungus, Plasmopora viticola, that attacks when the fruit, leaves, shoots and tendrils are in the early stages of development. Again, symptoms first appear on the foliage. Pale yellow spots form on the upper side of the leaves, and white-to-gray fungal growths develop on the undersides at the point where the spotting occurs. The leaf tissue at these spots eventually die and fade to a brown color. Shoots and tendrils are dwarfed and contorted, while the young fruit starts to turn soft and shrivel. It may develop a downy white fungal growth on the skin.
Although Immunox is a fungicide recommended for treating black rot, it's best to make sure the disease is properly identified first. Take a good sample of the plant, including leaves, stems, flowers and fruit, to your local extension service office. They can identify the problem and make recommendations on a treatment regimen. As always, when using chemicals, read the product label before making any application and follow safety guidelines. Generally you can apply the treatment starting right after bloom until about late July or early August. If there has been a lot of rain, apply the treatment more often. If the weather has been on the dry side, apply less often.
However, if it's too late in the growing season to treat the disease, it's wise to prepare for next year's crop. Good cultural and sanitation practices are essential to controlling or preventing disease. Remove the mummified clusters of grapes and fallen leaf litter in the fall so the spores won't overwinter to reinfect next year's crop. Destroy this diseased material; don't throw it on the compost pile. Open the canopy to sunlight and air circulation through proper pruning. This keeps moisture from resting in the plant's interior, festering and encouraging disease. Get a jump on spray treatments early in the season next year to prevent disease from showing up later.