How to Manage Pests

UC Pest Management Guidelines

Grape

Powdery Mildew

Pathogen: Erisiphe necator

(Reviewed 6/06, updated 10/08)

SYMPTOMS AND SIGNS

Symptoms of powdery mildew include red blotchy areas on dormant canes. On leaves, initial symptoms appear as chlorotic spots on the upper leaf surface. Signs of the pathogen appear a short time later as white, webby mycelium. As spores are produced, the infected areas take on a white, powdery or dusty appearance. On fruit and rachises the pathogen appears as white, powdery masses that may colonize the entire berry surface.

COMMENTS ON THE DISEASE

The fungus survives the winter as dormant mycelium in buds or as chasmothecia (spore structures). Chasmothecia are the most important sources of overwintering inoculum. They mature in late summer and fall on infected green tissue and are washed onto the cordons and spurs with fall and winter rainfall. On warm winter and spring days when moisture is abundant, cleistothecia burst and release ascospores. Conidial spore production occurs 7 to 10 days after primary infection by ascospores and will continue throughout the season as long as moderate temperatures (70° to 85°F) exist.

MANAGEMENT

Season-long control is dependent upon reducing early-season inoculum and subsequent infection. Thus treatment must begin promptly and be repeated at appropriate intervals. Timing of the first treatment depends on fungicide used and growth stage. Frequency of treatment thereafter depends on fungicide choice and weather conditions. Monitor and use the powdery mildew index (PMI) model to determine necessary spray intervals. Treatment may be discontinued for wine and raisin grapes when fruit reaches 12 Brix but should be continued up to harvest for table grapes.

All powdery mildew fungicides, with the exception of oil, are best used as protectants. Discontinue the use of soft chemistry products (sulfurs, biologicals, systemic acquired resistance products, and contact materials) when disease pressure is high because by themselves they will not provide adequate control. If eradication is necessary, a light summer oil may be used anytime in the season if there is no sulfur residue present (i.e. at least 2 weeks after a sulfur treatment). Basal leaf removal can improve coverage and efficacy of powdery mildew fungicides on clusters.

Organically Acceptable Methods

Sulfur, Serenade, Sonata, and Organic JMS Stylet Oil are acceptable on most organically certified grapes; check with your certifier for details.

Monitoring and Treatment Decisions

In spring, the overwintering chasmothecia produce ascospores, which are the primary source of infection. Ascospores are released when 0.1 inch of rain or irrigation is followed by 13 hours of leaf wetness when temperatures are between 50° and 80°F. Seven to 10 days after this initial infection, monitor vineyards for the presence of powdery mildew by collecting 10 to 15 basal leaves from 20 or so vines at random and examining the undersurface for powdery mildew spores. If spores are found, then monitor disease development by using the powdery mildew risk assessment index.

Powdery Mildew Index (PMI)

Once initial infection occurs, ideal temperatures for growth of the fungus are between 70° and 85°F. Temperatures above 95°F for 12 continuous hours or longer cause the fungus to stop growing. The powdery mildew index assesses the risk of disease development by relating it to air temperature and tells you how often you need to spray to protect the vines. When using the powdery mildew index, always monitor the vineyard for signs of the disease. If evidence of the disease is not recent, don't treat. You may monitor temperatures in your own vineyard and calculate the PMI using the rules below, or you may use weather equipment that has the UC Davis PMI built into its software.

Initiating the index

After you find powdery mildew, an epidemic will begin when there are 3 consecutive days with 6 or more continuous hours of temperatures between 70° and 85°F as measured in the vine canopy.

1. Starting with the index at 0 on the first day, add 20 points for each day with 6 or more continuous hours of temperatures between 70° and 85°F.
2. Until the index reaches 60, if a day has fewer than 6 continuous hours of temperatures between 70° and 85°F, reset the index to 0 and continue.
3. If the index reaches 60, an epidemic is under way. Begin using the spray-timing phase of the index.

Spray timing

Each day, starting on the day after the index reached 60 points during the start phase, evaluate the temperatures and adjust the previous day's index according to the rules below. Keep a running tabulation throughout the season. In assigning points, note the following:

• If the index is already at 100, you can't add points.
• If the index is already at 0, you can't subtract points.
• You can't add more than 20 points a day.
• You can't subtract more than 10 points a day.

1. If fewer than 6 continuous hours of temperatures between 70° and 85°F occurred, subtract 10 points.
2. If 6 or more continuous hours of temperatures between 70° and 85°F occurred, add 20 points.
3. If temperatures reached 95°F for more than 15 minutes, subtract 10 points.
4. If there are 6 or more continuous hours with temperatures between 70° and 85°F AND the temperature rises to or above 95°F for at least 15 minutes, add 10 points. (This is the equivalent of combining points 2 and 3 above.)

Use the index to determine disease pressure and how often you need to spray to protect the vines. Spray intervals can be shortened or lengthened depending on disease pressure, as indicated in the table below.

RESISTANCE MANAGEMENT

Alternating fungicides with different modes of action is essential to prevent pathogen populations from developing resistance to fungicides. This resistance management strategy should not include alternating or tank mixing with products to which resistance has already developed. Do not apply more than two sequential sprays of a fungicide before alternating with a fungicide that has a different mode of action.

Common name		Amount/Acre**	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
Treatments can be made in conjunction with plant growth regulators and other applications.
STEROL INHIBITORS
A.	TEBUCONAZOLE
	(Elite) 45DF	4 oz	12	14
	MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
	COMMENTS: Begin treatment when shoots are 8 to 10 inches long. Can be applied earlier but research shows that a wettable sulfur application (5 lb/100 gal water/acre) at budbreak should be used first. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14 to 21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 8 to 10 inches and treatments with sterol inhibitors or strobilurins begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry.
B.	TRIFLUMIZOLE
	(Procure) 50WS	Label rates	12	7
	MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
	COMMENTS: Begin treatment when shoots are 8 to 10 inches long. Can be applied earlier but research shows that a wettable sulfur application (5 lb/100 gal water/acre) at budbreak should be used first. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14 to 21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 8 to 10 inches and treatments with sterol inhibitors or strobilurins begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry. Do not apply more than 32 oz of product/acre/season.
C.	MYCLOBUTANIL
	(Rally) 40WP	4 oz in 50 or	24	14
		more gal water/acre
	MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
	COMMENTS: Begin treatment when shoots are 8 to 10 inches long. Can be applied earlier but research shows that a wettable sulfur application (5 lb/100 gal water/acre) at budbreak should be used first. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14 to 21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 8 to 10 inches and treatments with sterol inhibitors or strobilurins begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry. Apply no more than 1.5 lb maximum/season. Do not apply by air.
D.	FENARIMOL
	(Rubigan) EC	3–6 oz	12	30
	MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
	COMMENTS: Begin treatment when shoots are 18 inches long. Precede with a wettable sulfur application (5 lb/100 gal water/acre) that is applied at budbreak. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14–21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 18 inches and treatments with this fungicide begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry. Do not apply more than 19 oz/acre/season.
STROBILURINS
A.	AZOXYSTROBIN
	(Abound)	11–15.4 fl oz	4	14
	MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Begin treatment when shoots are 8 to 10 inches long. Can be applied earlier but research shows that a wettable sulfur application (5 lb/100 gal water/acre) at budbreak should be used first. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14 to 21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 8 to 10 inches and treatments with sterol inhibitors or strobilurins begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry.
B.	TRIFLOXYSTROBIN
	(Flint)	1.5–2 oz	12	14
	MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Do not apply to Concord grapes or crop injury may result. Begin treatment when shoots are 8 to 10 inches long. Can be applied earlier but research shows that a wettable sulfur application (5 lb/100 gal water/acre) at budbreak should be used first. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14 to 21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 8 to 10 inches and treatments with sterol inhibitors or strobilurins begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry. Do not apply more than 8 oz/acre/season.
C.	KRESOXIM-METHYL
	(Sovran)	3.2–6.4 oz	12	14
	MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Begin treatment when shoots are 8 to 10 inches long. Can be applied earlier but research shows that a wettable sulfur application (5 lb/100 gal water/acre) at budbreak should be used first. During cool springs when growth is slow, an additional wettable sulfur treatment is advisable 14 to 21 days later. Apply subsequent sulfur treatments at 14- to 21-day intervals until shoots reach 8 to 10 inches and treatments with sterol inhibitors or strobilurins begin. Because shoot growth rate is weather dependent, shoot length should not be used as a spray date indicator after the first treatment. Alternate use with fungicide of different chemistry. Do not apply more than a total of 1.6 lb/acre/year.
D.	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 use on Concord, Worden, Fredonia, Niagara, or related grape varieties. Do not make more than 2 sequential applications before rotating to a fungicide with a different mode of action.
SULFUR COMPOUNDS
A.	SULFUR#	Label rates	see comments	see label
	(dust, wettable, flowable, or micronized)
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
	COMMENTS: In some counties there is a 3-day restricted entry period when using sulfur; consult your county agricultural commissioner. To help prevent off-site drift, use wettable sulfur instead of dusting sulfur when canopies are minimal (less than 12 inches). Begin treatment at budbreak to 2-inch shoot growth. Reapply at 7-day intervals if treating every other middle or at 10-day intervals if treating every middle. Reapply if sulfur is washed off by rain or irrigation. Sulfur can cause injury to foliage and fruit when applied just before or on days when the temperature exceeds 100°F. The amount/acre may be reduced during periods of high temperature to prevent burning. Do not apply within 3 weeks of an oil application.
BIOLOGICALS
A.	BACILLUS PUMILIS#
	(Sonata)	2–4 qt	4	0
	MODE OF ACTION: A biological fungicide.
	COMMENTS: Begin making applications before disease onset or when disease pressure is low. Repeat at 7- to 10-day intervals until disease pressure is intermediate, then switch to a strobilurin, sterol inhibitor, oil, or sulfur. Apply in sufficient water to obtain thorough coverage.
B.	BACILLUS SUBTILIS#
	(Serenade Max)	1–3 lb	4	0
	MODE OF ACTION: A biological fungicide.
	COMMENTS: Begin making applications before disease onset or when disease pressure is low. Repeat at 7- to 10-day intervals until disease pressure is intermediate, then switch to a strobilurin, sterol inhibitor, oil, or sulfur. Apply in sufficient water to obtain thorough coverage.
SYSTEMIC ACQUIRED RESISTANCE PRODUCTS
A.	HARPIN PROTEIN
	(Messenger)	4.5–9 oz	4	0
	MODE OF ACTION: Unknown.
	COMMENTS: Begin applications when new shoot growth is present. Apply as a foliar spray on 7- to 14-day intervals before the onset of the disease when disease pressure is light. Discontinue use under moderate to heavy disease pressure.
B.	GABA/L-GLUTAMIC ACID
	(AuxiGro)	4 oz or 2-4 oz as a tank mix	4	0
	MODE OF ACTION: Unknown.
	COMMENTS: Must be applied before the onset of powdery mildew infections. May be applied alone, in alternating applications with other powdery mildew products, or in tank mix combinations with other powdery mildew products. Discontinue use when disease pressure is moderate to heavy. Do not exceed 24 oz/acre/crop.
CELL SIGNALING INHIBITOR
A.	QUINOXYFEN
	(Quintec)	3–4 fl oz	12	14
	MODE OF ACTION GROUP NAME (NUMBER1): Quinoline (13)
	COMMENTS: Spray on a 14-day interval, otherwise use 5-6.6 fl oz to spray on a 21-day interval.
CONTACT MATERIALS
A.	NARROW RANGE OIL#
	(Organic JMS Stylet Oil, Saf-T-Side, etc.)	2%	4	0
	MODE OF ACTION GROUP NAME (NUMBER1): A contact fungicide with smothering and barrier effects.
	COMMENTS: Never mix oil and sulfur or apply one within 2 weeks of the other. Can be used as a protectant or eradicant. As a protectant, alternate it prebloom with the sterol inhibitors. At the 2% rate, this oil is an excellent eradicant and can be used as a stand-alone program at anytime during the season (except within 2 weeks of a sulfur treatment); good coverage is essential. Apply at 14- to 18-day interval. Do not use on table grapes after berry set.
B.	POTASSIUM BICARBONATE#
	(Kaligreen)	2.5–5 lb	4	1
	(MilStop)	2.5–5 lb	1	0
	MODE OF ACTION: An inorganic salt.
	COMMENTS: Conditionally acceptable for use on organically grown produce; check with your certifier. Apply by ground only in sufficient water (25 gal/acre minimum) to ensure complete and thorough coverage of foliage and crop. Most effective when alternated with a sterol inhibitor and used as a protectant. Field reports suggest this material has eradicant activity; but this has not been demonstrated in University research. If used as an eradicant, contact of the disease organism is essential. Use of non-acidifying spreader/sticker or nonphytotoxic crop oil is recommended.
C.	INSECTICIDAL SOAP#
	(M-Pede)	1.5–2%	12	0
	MODE OF ACTION GROUP NAME (NUMBER1): A contact fungicide with smothering and barrier effects.
	COMMENTS: Alternate use with one of the sterol inhibitors. Apply in 100–150 gal water/acre. Complete coverage of upper and lower leaf surfaces, as well as grape clusters, is essential for control. Apply every 7–10 days. Do not combine with sulfur or apply within 3 days of a sulfur application. Do not apply to Calmeria or Italia varieties of grapes.
**	Apply with enough water to provide complete coverage.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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. For more information, see http://www.frac.info/.

PUBLICATION

UC IPM Pest Management Guidelines: Grape
UC ANR Publication 3448

Diseases

W. D. Gubler, Plant Pathology, UC Davis
R. J. Smith, UC Cooperative Extension, Sonoma County
L. G. Varela, UC IPM Program, Sonoma County
S. Vasquez, UC Cooperative Extension, Fresno County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Research Center, Parlier
A. H. Purcell, Environmental Science, Policy and Management, UC Berkeley
Acknowledgment for contributions to Diseases:
G. M. Leavitt, UC Cooperative Extension, Madera County

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