Grapevine Trees With Fluffy Tops

Powdery mildew is one of the most common plant diseases, inflicting extensive damage on a wide range of host plants. To control the disease before it destroys your grape yield, you should recognize the visible symptoms on the infected grape vines and design a suitable treatment program. Among the most conspicuous symptoms of powdery mildew infection is the titular white powdery growth that breaks out on all parts of the grapevine.

Powdery Mildew

Powdery mildew is caused by a wide range of fungal species; on grapes, Erysiphe necator is the pathogenic species. Powdery mildew infection is more common after wet weather. Spores overwintering on the ground need to land on a moist leaf surface to germinate and infect the grapevine's foliage. Once infected, however, the disease can spread throughout the grapevine regardless of weather conditions.

Other Symptoms

Most gardeners are not aware that their grapevines are infected with powdery mildew until the causal fungus causes a white, powdery, fuzzy growth to break out over the grapevine's foliage. But in addition to this symptom, new growth on the grapevine will be noticeably diminished and distorted. Grapes also develop scars, crack and split and sometimes fall to the ground.

Natural Treatments

Most varieties of grapevines are susceptible to powdery mildew, but you may be able to find a few resistant varieties in your area. Cultivating a resistant grape variety is one of the most effective control methods for powdery mildew. Since shade favors powdery mildew infection, plant grapevines in sunny areas whenever possible. Prune the grapevines regularly to ensure good air circulation and avoid over-fertilization since these two factors greatly increase the likelihood of powdery mildew infection.

Fungicides

A number of approved fungicides can be used to control powdery mildew as well. Horticultural oil, fungicides containing sulfur, biological fungicides such as Bacillus subtilis and fungicides containing tebuconazole + trifloxystrobin, difenoconazole + cyprodinil, tebuconazole + sulfur and metrafenone are all effective at controlling powdery mildew. Always purchase a fungicide that is safe for use on grape vines and always follow the manufacturer's instructions exactly.

References

• University of California Statewide Integrated Pest Management Program: Powdery Mildew on Fruits and Berries
• Cornell University Cooperative Extension; Grapevine Powdery Mildew; Wayne F. Wilcox
• Washington State University Extension; New Fungicides for Grapevine Powdery Mildew Management (2011); Gary G. Grove, et al.