Diseases of grape (
Powdery mildew
Powdery mildew of grape is caused by the fungus Uncinula necator. Like other powdery mildew fungi, it grows on the surface of leaves (photo 1). In grape it also infects the fruit (photo 2) and other young tissue including flowers, shoots and petioles. The powdery growth may appear white but turn light gray to tan in older infections. Leaves and berries in shaded areas are more susceptible, and disease usually shows up there first. Powdery mildew appears in the spring on new growth and is found until warm temperatures retard growth of the fungus.
All varieties of grapes planted in Arizona are susceptible, but disease can be controlled with timely applications of sulfur, either dusting or wettable; sterol inhibitors Myclobutanil (Rally), Tebuconazole (Elite), Triflumizole (Procure), Fenarimol (Rubigan) and Triadimefon (Bayleton); strobilurins such as Azoxystrobin (Abound) and Trifloxystrobin (Flint); contact materials such as potassium bicarbonate (Kaligreen) or oil (JMS Stylet Oil); and biologicals AQ10 Biofungicide, Trilogy (neem extract) and Serenade (a bacterium). All these fungicides should be applied preventively, before infections are seen. Fungicides should always be rotated to avoid development of resistance by the fungus. If infections are easily observed, the disease is almost impossible to control. Once established in an area, disease occurs every year since the fungus survives in bud tissue.
Other photos and useful information may be found at:
http://www.ipm.ucdavis.edu/PMG/r302100311.html
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July 14, 2003