Powdery Mildew Management Tips for Vineyards

Last year, many growers in California reported heavier disease pressure than usual. After a winter of heavy rain and some late spring rains, this year growers may see even greater rates of powdery mildew infection. However, there are actions you can take to reduce powdery mildew in your vineyard.

Powdery mildew shown on grapes. (Photo: L. J. Bettiga)

Disease Management
There are two important strategies for control of powdery mildew: reducing initial inoculum and preventing infection.

In most areas, powdery mildew overwinters as fruiting bodies called chasmothecia, which contain the spores (called ascospores) of powdery mildew. In warm climates, it can also overwinter as mycelium in dormant buds. An early season application of lime sulfur or horticultural oil (such as JMS Stylet Oil) should be applied at budbreak if temperatures are optimal for ascospore release: between 70° F to 80° F. This application significantly reduces spores released from chasmothecia and delays initial infections.

After the initial period of ascospore release, preventing further infection entails a season-long control strategy which includes fungicides (conventional or organic) and cultural controls. Further fungicide types and application intervals may be timed using the powdery mildew risk index.

Powdery Mildew Risk Index
Grape growers can use the powdery mildew risk index model to micro-adjust their spray interval (http://ipm.ucanr.edu/calludt.cgi/GRAPEPMVIEW1). The model correlates the pathogen biology with the canopy temperature and leaf wetness to assess the disease pressure during the growing season.

The model contains two stages: an ascospore and a conidial stage. The ascospore model is used to determine the risk of ascospore release and primary infection. Daily average temperature and duration of leaf wetness are put into the calculation.

Once ascospore infection has occurred, growers can switch to the risk index (RI) to determine the potential of secondary infection by conidia. At this stage, pathogen population increase is mainly based on the canopy temperature. There are two stages for calculating the risk index: initiating the risk index and adjusting the spray timing.

Initiating the risk index: Once there are three consecutive days with six or more continuous hours of canopy temperatures from 70°F to 86°F, it means the index reaches 60 and an epidemic is under way. Growers should begin using the spray-timing phase of index.

Adjusting the spray timing: After the index reaches 60, the calculation of index is simply based on the daily canopy temperature. Basically, 20 points are added to the index when six or more continuous hours of canopy temperatures occur from 70° F to 86° F. Similarly, 10 points are subtracted from the index when fewer than six continuous hours of canopy temperatures occur from 70° F to 86° F or temperatures reach 95° F for more than 15 minutes. According to the RI, local conditions and field scouting, growers can tighten or loosen the spray interval to confront the different levels of disease pressure in their vineyards (Table 1).

Resistance Management
Growers can use the RI to micro-adjust spray intervals and reduce unnecessary sprays when the disease pressure is low. However, alternating fungicides with different modes of action is critical to reduce the risk of developing resistance. Growers should avoid applying two sequential sprays of any fungicide without alternating with a fungicide of a different mode of action.

Fungicide formulations that mix more than one active ingredient may be useful to delay the development of resistance and improve disease control by broadening the spectrum of activity. Whenever possible, tank mix conventional or biological fungicides with sulfur. Because the risk of resistance to sulfur is minimal, tank mixing with sulfur reduces the chance that resistant isolates of powdery mildew will successfully infect plants and spread.

The Importance of Coverage
Like any spray program, coverage is one of the most important factors that determine the success of the powdery mildew fungicide spray. Vine vigor, trellis types, irrigation/fertilization, canopy management and sprayer/nozzle types can all be managed in order to achieve better coverage. Water-sensitive paper and proper sprayer calibration should be used as tools to validate and improve spray coverage. Canopy management, such as shoot thinning, leaf removal, and hedging, are commonly applied, when appropriate, to increase the fruit quality, and these practices can also open the canopy for better spray coverage and improved disease control.

Reducing Risk
Powdery mildew is one of the most common disease issues grape growers face. However, with a comprehensive and well-timed management program, your risk of crop loss can be significantly reduced.
As always, follow label guidelines for application rates, minimum spray intervals, and pre-harvest intervals. Rotate fungicides between different modes of action to prevent resistance. If you have any questions, consult your local farm advisor or Extension agent.

Topics:

Leave a Reply

One comment on “Powdery Mildew Management Tips for Vineyards

Grapes Stories
Grapes
September 26, 2017
New England Researchers Study Viability of Seedless Table Grapes Varieties
University of New Hampshire project looks at growing systems, varieties to determine what type of production would be suited for cooler climates. Read More
Grapes
September 12, 2017
Heat Wave Leads to Early Winegrape Harvest in California’s Sonoma County
Ask any winegrape grower to describe the grape harvest in Sonoma County and you typically hear “harvest time is like Read More
Grapes
September 8, 2017
Spotted Lanternfly Population Large This Year
Thanks to mild winter, numbers for the pest are growing, especially in southeast Pennsylvania. Read More
glass of red wine
Grapes
August 30, 2017
ASEV Confirms Enologist as President
Washington State University’s James Harbertson to lead American Society for Enology and Viticulture (ASEV) as the 2017-2018 president. Read More
Grapes
August 26, 2017
Ode to Being a Part-time Farmer [Opinion]
I work with many grape growers in Virginia. Some of them work full time at a single vineyard operation. Many Read More
GenNext Growers
August 8, 2017
Winegrape Society Awards $100,000 in Scholarships
American Society for Enology and Viticulture bestows awards at its 68th national conference. Read More
Grapes
July 7, 2017
California Winegrowers Applaud $5 Million to Battle Disease
Governor approves Pierce’s Disease Control Program money for the first time since 2011. Read More
Grapes
July 3, 2017
Climate Doesn’t Weigh Heavy On Ste. Michelle Wine Estates
While warming trend has been a part of past few seasons, the vineyard is incorporating new technologies to grape growing versus altering production techniques. Read More
Grapes
July 3, 2017
How to Handle Warming Pacific Northwestern Vineyards
The warming temperatures in the Pacific Northwest are well-suited for red winegrape production, but other changes in viticulture practices must be made. Read More
GenNext Growers
June 29, 2017
Cornell Graduate Student Earns Grape Disease Research
Doctoral candidate recognized by American Society of Enology and Viticulture for work studying sour rot pathogens. Read More
Grapes
June 29, 2017
Cornell University Wants You to Name That Grape
Contest to name cold-tolerant variety a seedless ‘Concord’-type berry runs through July 31. Read More
Grapes
June 26, 2017
Washington Grape Grower Battles New Threat
Apples, cherries, and pears are also threatened by Tobacco Ringspot Virus, a pathogen never before seen in Washington state. Read More
Food Safety
June 21, 2017
Organization Seeks Winegrape Exemption Produce Safety Rule
California Association of Winegrape Growers says because winegrapes are grown for processing, they should be exempt from produce safety rule. Read More
Grapes
June 5, 2017
California Winegrowers Award Scholarships to Employees’ Kids
California Association of Winegrape Growers Foundation awards $30,000 in student scholarships. Read More
Grapes
June 5, 2017
Getting to the Bottom of Berry Scarring
It’s a costly defect, but learning what causes berry scarring can help you prevent it. Read More
The Latest
Grapes
October 13, 2017
Fairgrounds Host California Fire Evacuee…
Gallo will contribute $1 million to fire recovery effort and will match employee donations two-for-one. Read More
Grapes
October 10, 2017
Wildfires Hit California Wine Country
California Gov. Jerry Brown declares state of emergency for northern counties impacted by flames. Read More
Grapes
October 7, 2017
Tips for Effective Vine Mealybug Managem…
Scouting, trapping, and mating disruption can help prevent populations of this pest increasing in your vineyard. Read More
Grapes
September 26, 2017
New England Researchers Study Viability …
University of New Hampshire project looks at growing systems, varieties to determine what type of production would be suited for cooler climates. Read More
Grapes
September 12, 2017
Heat Wave Leads to Early Winegrape Harve…
Ask any winegrape grower to describe the grape harvest in Sonoma County and you typically hear “harvest time is like Read More
Grapes
September 8, 2017
Spotted Lanternfly Population Large This…
Thanks to mild winter, numbers for the pest are growing, especially in southeast Pennsylvania. Read More
Grapes
August 30, 2017
ASEV Confirms Enologist as President
Washington State University’s James Harbertson to lead American Society for Enology and Viticulture (ASEV) as the 2017-2018 president. Read More
Grapes
August 26, 2017
Ode to Being a Part-time Farmer [Opinion…
I work with many grape growers in Virginia. Some of them work full time at a single vineyard operation. Many Read More
GenNext Growers
August 8, 2017
Winegrape Society Awards $100,000 in Sch…
American Society for Enology and Viticulture bestows awards at its 68th national conference. Read More
Grapes
July 7, 2017
California Winegrowers Applaud $5 Millio…
Governor approves Pierce’s Disease Control Program money for the first time since 2011. Read More
Grapes
July 3, 2017
Climate Doesn’t Weigh Heavy On Ste. Mich…
While warming trend has been a part of past few seasons, the vineyard is incorporating new technologies to grape growing versus altering production techniques. Read More
Grapes
July 3, 2017
How to Handle Warming Pacific Northweste…
The warming temperatures in the Pacific Northwest are well-suited for red winegrape production, but other changes in viticulture practices must be made. Read More
GenNext Growers
June 29, 2017
Cornell Graduate Student Earns Grape Dis…
Doctoral candidate recognized by American Society of Enology and Viticulture for work studying sour rot pathogens. Read More
Grapes
June 29, 2017
Cornell University Wants You to Name Tha…
Contest to name cold-tolerant variety a seedless ‘Concord’-type berry runs through July 31. Read More
Grapes
June 26, 2017
Washington Grape Grower Battles New Thre…
Apples, cherries, and pears are also threatened by Tobacco Ringspot Virus, a pathogen never before seen in Washington state. Read More
Food Safety
June 21, 2017
Organization Seeks Winegrape Exemption P…
California Association of Winegrape Growers says because winegrapes are grown for processing, they should be exempt from produce safety rule. Read More
Grapes
June 5, 2017
California Winegrowers Award Scholarship…
California Association of Winegrape Growers Foundation awards $30,000 in student scholarships. Read More
Grapes
June 5, 2017
Getting to the Bottom of Berry Scarring
It’s a costly defect, but learning what causes berry scarring can help you prevent it. Read More