What Causes Grape Vines to Become White?

• Healthy grape vines. Buena Vista Images/Photodisc/Getty Images

  The grape vines you are cultivating in your home landscape are under threat by a variety of pest and disease problems. When grape vines become white, this is a signal that an underlying problem is occurring that may lead to fruit loss and plant damage. A springboard for protecting your grapes is maintaining vigorous vines through consistent maintenance. Grow vines in locations that offer full sunlight and well-drained soil.

Western Grapeleaf Skeletonizer

• Western grapeleaf skeletonizer, Harrisina brillians, are pests of grape vines. These pests are skeletonizing bugs that feed in their larval stage, chewing a large portion of leaf tissue excluding veins. Leaves resemble skeletons, with their vein structures remaining more prominently displayed. Larvae display bodies that change during each stage of life from cream to brown to a multi-striped yellow, purple and black. In maturity, moths are a green/black or blue hue with a metallic cast. As larvae feed, they eat only the undersides of leaves. The remaining tissue is thin and resembles white paper, according to the University of California IPM Online. To control this problem and keep grape vines from becoming white, home gardeners should consider the release of insect parasites like Amedoria misella that hunt and kill skeletonizers. Parasites are available in garden supply stores, catalogs and online. For extreme infestations, pesticides with the active ingredient spinosad or methoxyfenozide are reliable controls.

Powdery Mildew

• Powdery mildew is a fungal infection responsible for grape vines taking on a white appearance. Caused by the pathogen Erisiphe necator, powdery mildew is a disease that typically begins its infection of grape vines on moist, warmer days of spring or winter. Within approximately one week of infection, powdery mildew exhibits symptoms of fading spots on leaves. As fungi increase and release spores, spores gather and cover leaf surfaces, resulting in the appearance of white grape vines. Gardeners should apply fungicides like sulfur-based options or myclobutanil, according to the University of California IPM. As powdery mildew exists in a range of severity, the best means of management is contacting a local county extension agent or licensed professional for a program tailored to your particular problem.

Downy Mildew

• Downy mildew is a fungal infection that attacks grape vines. Caused by the fungal pathogen Plasmopara viticola, downy mildew is most prevalent in warm, moist climates. Symptoms occur as lesions, or infected areas, of plant tissue that take on a yellow, brown or red color in geometric shapes as with an oil-soaked appearance. As the disease progresses, spores gather in the lesions in masses of white, cotton-like formation. In addition to the white growths, shoots infected with downy mildew take on a white color, turn brown and die. For control of this problem, maintain extremely well-drained soil and utilize a fungicide. Apply a preventive fungicide with the active ingredient mancozeb or copper to avoid infection, according to the University of California IPM Online.