Viticulture Notes

Vineyard and Winery Information Series:
Vol. 21 No. 3, May-June 2006

Dr. Tony K. Wolf, Viticulture Extension Specialist

Table of Contents

I. Current situation

Grape Disease Control, 2006: The attached "Grape Disease Control, 2006" document was prepared by Dr. Wayne Wilcox, at Cornell University's Geneva Experiment Station. Dr. Wilcox has provided an annual Grape Disease Control summary for many years, and we are grateful that he generously shares the information. As in previous years, the summary is broken down into sub-components:

- fungicide changes and new registrations
- review of the biology of the principal grape fungal diseases that we can exercise some control over
- a concluding section "Putting it all together" that outlines comprehensive disease management options

Dr. Wilcox's Grape Disease Control recommendations are written with a focus on New York conditions, but he broadens the discussion so that the information is generally applicable to Virginia growers as well as growers in other eastern US states and Canada. While pesticides will have a federal (EPA) label registration, the labeling varies from state to state so be certain to check within local extension offices or other sources to ensure that a product has been registered within that state.

Closer to home: I encourage readers to pay particular attention to the following article by Ashley Myers and Anton Baudoin concerning powdery and downy mildew resistance to fungicides. This is an issue that we have written and spoken extensively about in previous years, and which bears repeating on an annual basis. With the registration of the strobilurin fungicide, Abound, on grapes in 1997, the concern for resistance development with both powdery and downy mildew was heightened. It was not so much a matter of "if" but "when". As recently as 2005, the thinking was that a total of 15 to 20 applications of any of the stand-alone strobilurin fungicides (e.g., Abound, Sovran, or Flint) to a vineyard, over time, could result in resistance development by powdery mildew. I regret to say that our research vineyard at the Southern Piedmont AREC in Blackstone is a casualty of this scenario. Since 2001 (5 growing seasons), we applied 17 applications of either Abound or Flint to the vineyard. We now have a demonstrated (A. Baudoin) powdery mildew resistance to Abound at that site. In hindsight, we might have gotten several more years of efficacy from the strobilurins had we limited our strobilurin sprays at Blackstone to no more than 2 applications per year, the current seasonal prescription. There are various reasons for doing what we did, but the stark reality is that we appear to have reached a critical usage point beyond which resistance development occurred. The "15 to 20 application" range was an accurate prediction of this threshold in our case. If you haven't done so, go back over your spray records of the last 5 to 8 years and count up the total number of strobilurin applications that you've made to your vineyard. Be vigilant for mildew (either powdery or downy mildew) outbreaks if your vineyard has an extensive history of strobilurin usage. Resistance to one of the strobilurin fungicides conveys resistance to all the strobilurin fungicides, so switching to another strobilurin fungicide is not an option. Our concerns about powdery mildew resistance extend to the older, sterol-inhibiting fungicides as well and we also have concerns about downy mildew resistance to the strobilurin fungicides. Read on…

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II. Pièce de Résistance (The Mildews)

What do culinary excellence and grape mildews have in common? Little to nothing, but it's an interesting title for an article about resistance. Conveniently, the other definition of "pièce de résistance" is outstanding achievement. Hmm…an outstanding achievement, pathologically speaking? Resistance development! When a pathogen develops the ability to resist a chemical control it is evolutionarily outstanding and makes life more difficult for those trying to suppress the pathogen population. Just as human pathogens become resistant to antibiotics after continuous use of certain drugs, several of our common grape pathogens have the ability to evolve resistance mechanisms.

For years when only protectant fungicides, such as mancozeb and captan, were used, no resistance was observed, presumably because these fungicides affect several biochemical processes of the pathogen and many genetic changes would need to occur for resistance to develop. With the introduction and widespread use of the systemic fungicides, especially benomyl, then later metalaxyl, the sterol-inhibiting, and strobilurin or QoI fungicides, strains of fungi resistant to one or more of these fungicides began to appear. Resistant strains most commonly develop when single-site or site-specific chemicals are used extensively for their control. Single-site fungicides (often referred to as systemic fungicides because most single-site fungicides are locally systemic) are specific in their site of action, only affecting one or two steps of genetically controlled events in fungal metabolism. As a result, resistant populations can quickly arise, requiring only a single mutation followed by selection of resistant individuals in a population.

In 1997, the first strobilurin was registered for grapes in the United States as azoxystrobin (trade name: Abound). Azoxystrobin was highly effective against downy mildew, powdery mildew, and black rot, with the added benefit of a short PHI (pre-harvest interval). The strobilurin class of fungicides (QoI fungicides) inhibit mitochondrial respiration by blocking electron flow through the electron transport chain by binding to the Qo-site (hence the name, QoI or Qo-site Inhibitor) of the cytochrome bc1 complex. To simplify, strobilurins block one vital metabolic component of the pathogen's cell, and if that one component changes, even slightly, the fungicide no longer works. As a result, the potential of resistance development to the strobilurins is high and methods to minimize resistance appeared on product labels. These include limiting the number of applications per season and guidelines for rotation to non-related chemistries. One group of rotational partners is the SI (sterol-inhibiting) fungicides.

The SI fungicides are site-specific inhibitors of cell membrane formation. Used often since the early 1980s for control of grapevine diseases such as powdery mildew and black rot, shifts towards insensitivity and practical resistance to the SIs have occurred in some vineyards. Higher application rates of the SIs will often still provide powdery mildew control, and there appear to be no well-documented cases of SI resistance in black rot. Bayleton, however, is reputed to have lost much of its efficacy against powdery mildew in some vineyards in the eastern United States. Other SI fungicides are myclobutanil (Nova), tebuconazole (Elite), fenarimol (Rubigan) and triflumizole (Procure).

In 2000, grape downy mildew strains resistant to strobilurins became a problem in several viticultural areas of Europe; currently resistance is well-documented in Italy, Switzerland, Austria, and France. Additional fungi exhibiting resistance to the strobilurins are powdery mildew of wheat and barley, cucumber powdery and downy mildew, gummy stem blight, and leaf spot in turf (USA).

In July 2005, Virginia Tech's Dr. Anton Baudoin began a small resistance survey of downy mildew isolates after a vineyard on the Eastern Shore had an unexplained increase in downy mildew, and poor control with pyraclostrobin (trade name: Pristine) and azoxystrobin at labeled rates. The 2005 survey, funded by the Virginia Wine Board, revealed that in addition to the Eastern Shore site, two vineyards in central Virginia and two in western North Carolina, had populations of downy mildew that were resistant to pyraclostrobin and azoxystrobin. Strobilurin resistance of grape downy mildew had not been previously reported in North America but had been present for several years in Europe. Because resistance was detected in three widely separated locations, we should assume that downy mildew resistance to the strobilurins may be more widespread. This would include the strobilurin products Abound, Sovran and Flint, as well as Pristine. Pristine is a binary product that contains the strobilurin, pyraclostrobin, and the unrelated fungicide boscalid (trade name: Endura). For all intents and purposes, only the pyraclostrobin component of Pristine is active against downy mildew. The boscalid is formulated with pyraclostrobin to delay powdery mildew resistance development, but would not be expected to significantly affect downy mildew resistance development.

What is the situation with powdery mildew? Because powdery mildew resistance to the strobilurins has been observed in New York and Pennsylvania since 2002, powdery mildew resistance in Virginia would come as no surprise. Indeed, powdery mildew isolates collected in the fall of 2005 from one northern Virginia, one Southside Virginia, and two central Virginia vineyards tested resistant to azoxystrobin and pyraclostrobin. One resistant isolate from each of these vineyards was sent to Switzerland for testing by Syngenta (Abound's manufacturer) and the G143A mutation was found in three of the four isolates tested. G143A was also found in several of the downy mildew isolates, and is a mutation that confers high levels of resistance to the strobilurins. Dr. Baudoin is currently conducting tests for sterol-inhibitor (SI) fungicide resistance on the same group of isolates, with some isolates showing willingness to grow on leaves treated with 3 ppm myclobutanil (trade name: Nova) and comparable concentrations of triadimefon (trade name: Bayleton). Although more tests are needed, these preliminary assays indicate that some erosion of powdery mildew's sensitivity to sterol-inhibiting fungicides is very probable in Virginia.

Let's review the situation, as it can be confusing: The limited resistance testing conducted in Virginia in 2005 provided evidence that both downy mildew and powdery mildew isolates have evolved resistance to strobilurin fungicides, and that at least some powdery mildew isolates have increased resistance to sterol-inhibiting fungicides.

To prepare for the upcoming growing season, you should have a plan of action for a powdery or downy mildew resistance scenario. For those who have carefully followed resistance management recommendations, continuing to do so may extend the life of strobilurins as well as the sterol-inhibitor (SI) fungicides. We will call this scenario one. If in the past you have relied heavily on strobilurins or SIs (15 or more applications of a strobilurin fungicide), you should anticipate resistance. We will call this scenario two. It's a good idea to examine both scenarios as the points made in each are not exclusive of one another.

In scenario one, resistance management will be continued. The most critical component of the spray program (this applies to all spray programs) is canopy management. Reducing humidity and improving air circulation and direct sunlight exposure is very important for powdery mildew control, as powdery mildew requires warm, humid conditions for spore germination and development, and is favored by the reduced irradiance within a dense canopy. Good canopy management will also improve spray coverage. Thorough coverage is critical, and poor canopy architecture is often the major cause of disease control failure. Use at least 75 gallons of water per acre and place spray cards in the canopy to monitor coverage. Use full rates of product. This is very important with the SI and strobilurins; using lower rates is analogous to only using half of a prescribed antibiotic. Keep the spray interval narrow (no more than 14 days). Include sulfur at 2 lb/acre where permissible as a tank mix. Use a higher rate (6 - 8 lb/acre) of sulfur in sprays where sulfur is the only powdery mildew product. Alternate chemistries; do not rely on an SI or strobilurin for season-long management.

Where resistance occurs or is suspected (scenario two), it is important to maintain sulfur in your program, using it as a rotational partner or including it with synthetics at the lower rate (2 lb/acre). Quinoxyfen (trade name: Quintec) and Pristine have shown good control of powdery mildew, and may be used as rotational partners with sulfur (or with sulfur-sensitive varieties). Quintec is a fungicide for powdery mildew control only. It has no post-infection activity and therefore should only be used as a protectant. As described above, Pristine is composed of two chemical components, a strobilurin (pyraclostrobin) component and an anilide (boscalid) component. However, if resistance has developed the pyraclostrobin is providing zero control and the boscalid is doing all the work. Both quinoxyfen and boscalid are also at some risk for powdery mildew resistance development; therefore, to keep additional resistance from developing, add sulfur to your Pristine application at the tank mix rate of 2 lb/acre. You may be asking why use Pristine at all? It also has activity against black rot, phomopsis, botrytis, and downy mildew. But wait, isn't there a downy mildew resistance issue with pyraclostrobin and the other strobilurins? Yes, and boscalid has no activity against downy mildew. Therefore, incorporate a downy mildew protectant fungicide as needed into your sprays, and be sure to scout* for the occurrence of downy mildew. Be prepared to use a non-strobilurin downy mildew eradicating fungicide if an infection period has occurred or young lesions are visible. Horticultural oils also provide good foliar protection against powdery mildew but not as much protection on waxy fruit, due to difficulty in achieving good coverage. Additionally, horticultural oils have no activity against downy mildew. If using horticultural oils for powdery mildew control, be sure to get good coverage (100 gallons/acre) and consider label warnings of 10 days between oil and sulfur applications.

There are a few members of the "rescue squad" should you find your vineyard in a resistance situation with a full-blown powdery mildew infection. They include horticultural oils, sulfur (5 lb/acre), Oxidate, and Armicarb or Kaligreen. These materials must be in direct contact with the pathogen and are especially useful for eradication of berry infection. For berry infection, eradication sprays should be made, via back-pack sprayer or nozzle adjustment, directly into the fruiting zone. It is important to use maximum rates, achieve good coverage, and follow all label instructions for application.

For our 2006 spray schedule at the Winchester research station, we will be following scenario two's guidelines. The backbone of our spray schedule will be mancozeb or captan and sulfur (6 - 7 lbs/acre), relying heavily on sulfur for powdery mildew control except when temperatures exceed 95°F. It is a good idea to test sulfur toxicity with your varieties and weather conditions, especially if you have not often used sulfur in the past. Quinoxyfen (trade name: Quintec, maximum of 3 applications per year) at 3 - 4 fl oz/acre will be rotated with sulfur for powdery mildew control. One or two applications of Pristine (maximum of 2 applications/year) tank mixed with 2 lb/acre of sulfur will be made around bloom. We will reserve horticultural oils or Oxidate or Kaligreen for powdery mildew outbreaks. We will also potentially use one or two sprays of Ridomil MZ (metalaxyl and mancozeb), weather-depending, and phosphorous acid for downy mildew control.

Powdery mildew is one of the most destructive diseases of grapevines. Essentially all hybrid and vinifera wine grape varieties grown in Virginia are either moderately or highly susceptible. Chardonnay is perhaps the most susceptible of the commonly grown varieties and should be regularly monitored* for control failure. Once powdery mildew is present in a vineyard, it is difficult to bring under control. Severe epidemics can seriously affect grape and wine quality, as well as yields, and reduce vine growth and winter hardiness. The most devastating consequence is complete loss of fruit due to heavy powdery mildew fruit infection. The resistance issues of this growing season promise to be challenging to grape growers, especially if weather conditions are favorable for powdery mildew development. Please be prepared - have a plan and stay informed of the information and chemicals available.

Throughout the season, please be sure to check my homepage (http://faculty.vaes.vt.edu/ashley08) for information on Virginia's grape diseases, as well as fungicide resources tailored specifically to Virginia growers. Information on fungicides and spray timing is available in the 2006 VT Pest Management Guide (http://www.ext.vt.edu/pubs/pmg/hf3.pdf). Additional options for treating existing powdery mildew infections can be found in a previous Viticulture Notes article at http://www.ext.vt.edu/news/periodicals/viticulture/05julyaugust/05july.html#II.

* Scouting or monitoring means looking for pest, pathogens, or symptoms in the vineyard at critical times in their development and recording their incidence. Scouting is not riding through the vineyard in your tractor, gator, or golf cart. To effectively scout, you must look inside the vine canopy at the interior leaves and fruit i.e. areas prone to prolonged moisture or wetness. For powdery mildew, closely check leaves and shoots near the cordon and trunk where the cleistothecia (overwintering spores) on the older wood would easily splash onto green shoots and cause infection. Additional tips are to scout with the sun behind you, check borders and interior areas of the vineyard separately, include areas adjacent to woods, monitor at least 100 vines per vineyard, look in hot spots with a history of problems, and inspect both sides of the vine.

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III. Upcoming meetings

Regional meetings
(Thanks to Penn State extension agent Mark Chien for compiling this listing of up-coming regional events)

MAY
10	Virginia vineyard meeting. Acorn Hill Vineyard, Madison, VA. 11 a.m - 2 p.m. Topics include new planting management (disease, insect, nutrition, etc.). See April 26.
11	Wine Blending Workshop with Lucien Guillemet from Chateau Boyd-Contenac, Jim Law at Linden Vineyards (northern Virginia), Carl Helrich - Allegro Vineyards (Southcentral PA). Blending wines fits the style and realities of Eastern wine growing. The idea here is to learn about the fine art of blending wines, mainly reds but some whites, too. The workshop will be held at Linden Vineyards from 9 a.m. to 1 p.m. Limited to 25. Cost is $80/person. For information, call Mark Chien. 717-394-6851.
15 - 17	Pennsylvania Wine Association Annual Meeting. Wyndham Hotel Harrisburg/Hershey. Harrisburg, PA. Invited speakers focus on current topics important to the PA wine industry. Enology, wine marketing and viticulture topics are all on the program. Pesticide credits available. Awards banquet and annual PWA business meeting. For information, please call Jennifer at 717-234-1844.
TBA	Twilight meeting. Chester County. Vineyard to be announced. Free. No pre-registration required. Pre-bloom disease and pest control. Canopy management and weed control. Vineyard tour by owner.
JUNE
17	Maryland Grape Growers' Association Field Day. Upper Marlboro University of MD Center. 2005 Largo Road, Upper Marlboro, MD 20774. Informational contact: Mr. Bill Kirby 410-822-4421 or Bob White bobwhite@erols.com, or Joe Fiola jfiola@umd.edu
21	Virginia vineyard meeting. Horton Vineyard and Winery (meet at the winery), Gordonsville, VA. Topics include seasonal insect control update and viticultural management strategies. See April 26.
25	Vineyard Management Seminar. Linden Vineyards. Linden, VA. The focus on this session is the finer points of day to day management of a producing vineyard. Canopy management to impact quality and flavors is the main emphasis. Horticultural decisions such as pruning, training, pest management and vine nutrition are also covered. Pre-registration required. Limited space. http://www.lindenvineyards.com/.
28 - 30	American Society for Enology and Viticulture Annual Convention. Sacramento, CA. ASEV is the professional association of the U.S. wine industry. The focus is on viticulture and enology research with a large trade show. For more information, go to http://www.asev.org.
JULY
9 - 12	American Society for Enology and Viticulture Eastern Section Annual Meeting. Rochester, NY. This is an important opportunity for non-western states growers to hear the latest research results from their regions include student papers and Viticulture Consortium projects. Pre-conference tour of Finger Lakes wineries is available. For more information, visit the ASEV-ES web site at http://www.nysaes.cornell.edu/fst/asev/
AUGUST
9	Virginia vineyard meeting. Chateau O'Brien, Markham, VA. Topics include cover crops, crop estimation, pruning, post harvest disease management, late season insect scouting and control, WPS review. See April 26.
13	Advanced Wine Making Workshop. Linden Vineyards. Linden, VA. The finer points of artisan winemaking are covered is this seminar with time spent in the vineyard, cellar, classroom and tasting. Style and quality issues are the focus. Participants should have some winemaking experience or have taken the Winemaking Basics Seminar. Pre-registration required. Limited space. http://www.lindenvineyards.com/
17	Pennsylvania Association of Winegrowers Annual Summer Vineyard Walk Around. At the Fruit Research and Extension in Biglerville, PA. 9 a.m. to 5 p.m. Tour of variety and pathology experiments run by Dr. Jim Travis and his team at FREC and tasting of research wines by extension enologist Stephen Menke. Registration and pre-registration required. Pesticide credits are available. For information, call Mark Chien or Stephen Menke.

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"Viticulture Notes" is a bi-monthly newsletter issued by Dr. Tony K. Wolf, Viticulture Extension Specialist with Virginia Tech's Alson H. Smith, Agricultural Research and Extension Center in Winchester, Virginia. If you would like to receive "Viticulture Notes" as well as Dr. Bruce Zoecklein's "Vinter's Corner" by mail, contact Dr. Wolf at:

Dr. Tony K. Wolf
AHS Agricultural Research and Extension Center
595 Laurel Grove Road
Winchester, VA 22602

or e-mail: vitis@vt.edu

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