Downy Mildew

The fungus attacks all green parts of the vines, particularly the leaves. Depending on the incubation period and leaf age, lesions are yellowish and oily or angular, yellow to reddish and brown and limited by the veins. Sporulation of the fungus appears as a delicate, dense, white, cottony growth in the lesions. Infected shoot tips thicken, curl ("Shepherd's Crook") and become white with sporulation. They eventually turn brown and die. Similar symptoms are seen on petioles, tendrils and young inflorescences, which, if attacked early enough, ultimately turn brown, dry up and drop. The young berries are highly susceptible. They appear grayish when infected (gray rot) and become covered with a downy felt of fungus sporulation. Berries become less susceptible as they mature, but rachis infections can spread into older berries (brown rot, no sporulation). Infected berries of white cultivars may turn dull gray‑green, while those of black cultivars turn pinkish red. Infected berries remain firm, compared to ripening healthy berries, and drop easily. Portions of the rachis or the entire cluster also may drop.

Grape downy mildew occurs mainly in regions where it is warm and wet during the vegetative growth of the vine. Limited rainfall in spring and summer generally limits the spread of the disease in California. Surviving inoculum may be present in California at low levels and initially may have been introduced on plant material from outside of California. In most regions the fungus survives the winter mainly as oospores in fallen leaves. However, in California's generally mild winters, survival of the fungus in buds, shoot tips, and persistent leaves may be more important than in other grape-growing regions.

The pathogen is dispersed by splashing rain and wind. The infection process can take less than 90 minutes. Infection generally occurs in the morning and the incubation period is about 4 days. Downy mildew is favored by all factors that increase the moisture content of soil, air and host plant. Rain and irrigation practices are principal factors in promoting epidemics. The optimum temperature for development of the disease is 68° to 77°F (20° to 25°C) with extremes ranging from 50° to 86°F (10° to 29°C). In California the greatest potential for disease development exists when a wet winter is followed by late spring rains. The potential is high as well in the event of early fall rains.

Preventive management consists of effective soil drainage and reduction of sources of overwintering inoculum. In a vineyard that depends on sprinkler irrigation, extend the interval between irrigations as long as possible.

Fungicides for use against downy mildew can be categorized as either preventive or curative. The preventive fungicides must be applied before an infection period begins. New growth following application will not be protected. Include a spreader/sticker agent to prevent the material from washing off with rain. In vineyards with a history of downy mildew, apply early season copper sprays as part of a preventive program, especially during wet springs.

For more information on choosing a fungicide for grapes, refer to General Properties of Fungicides used in Grapes, Fungicide Efficacy—Conventional Products, Fungicide Efficacy—Biologicals and Natural Controls, Treatment Timing for Key Diseases, and Fungicide Resistance Management.

	Common name	Amount per acre**	REI‡	PHI‡
	(Example trade name)		(hours)	(days)
Pesticide precautions Protect water Calculate VOCs Protect bees
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
A.	AZOXYSTROBIN
	(Abound)	11–15.4 fl oz	4	14
	MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Begin applications at budbreak. Alternate with chemicals that have a different mode of action. Do not apply more than 2 sequential applications of this material or more than 6 applications per year.
B.	PYRACLOSTROBIN + BOSCALID
	(Pristine)	8–10.5 oz	24	14
	MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
	COMMENTS: Do not apply on Concord, Worden, Fredonia, Niagara, and related varieties. Do not make more than 2 sequential applications; rotate to a fungicide with a different mode of action. The R.E.I. is 5 days when conducting cane tying, turning, or girdling.
C.	KRESOXIM-METHYL
	(Sovran)	3.2–4.8 oz	12	14
	MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Begin application at budbreak.
D.	MEFENOXAM + COPPER HYDROXIDE
	(Ridomil Gold Copper)	1–2 lb	48	42
	MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) and Multi-site contact (M1)
	COMMENTS: Apply up to four times beginning before bloom. Do not apply more than a total of 0.4 lb active ingredient mefenoxam per crop per season. Do not apply after bloom. Do not use on copper-sensitive varieties.
E.	COPPER HYDROXIDE
		Label rates	48	See label
	MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
	COMMENTS: Use 1 to 3 lb hydrated lime/acre in combination with cupric hydroxide. May be applied either as a dilute or concentrate spray. Use for the last 1 or 2 late-season applications following early-season application of another fungicide. Slight to severe foliar injury may occur on copper sensitive varieties.

**	Apply with enough water to provide complete coverage.
‡	Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI The longer of two intervals is the minimum time that must elapse before harvest.
#	Acceptable for use on organically grown produce.
1	Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different Group number are suitable to alternate in a resistance management program.