Viticulture Notes

Vineyard and Winery Information Series:
Vol. 23 No. 3, November - December, 2008

Dr. Tony K. Wolf, Viticulture Extension Specialist, AHS Jr. Agricultural Research and Extension Center, Winchester, Virginia
vitis@vt.edu
http://faculty.vaes.vt.edu/vitis

Table of Contents

1. Looking back on 2008 season
2. Dr. Mizuho Nita joins AHS AREC faculty as grape pathologist
3. Grape leafroll virus
4. Question from the field: cane-pruning vs. spur-pruning
5. Other news
6. Upcoming vineyard meetings

I. Reflections

The 2008 season was a model growing season for many in Virginia. Generally speaking, we had ample, sometimes too frequent, rains in the early part of the season and, excepting some stray tropical depressions, we had a relatively dry late-summer and fall. There were some vineyards hit by hail storms and there were assorted other misadventures reported including a few vineyards that had steel line-posts fail during thunderstorms, a few cases of downy mildew and powdery mildew getting out of control, and a higher than usual pressure from wildlife in some situations. Some of these observations warrant some elaboration, others are normal fare for grape growing in the mid-Atlantic.

Meteorologically, the 2008 season was cooler than 2007, at least here in Winchester: by the end of September, our accumulated growing degree days since 1 April were about 3200 (about the long-term average for Winchester) versus about 3500 for the 2007 season. Rainfall for the same periods was 28 inches in 2008 and 17 inches in 2007. We had 3.9 inches of rain this September compared to 3.3 inches in September 2007. Harvest commenced in late-August and, excepting some very late-maturing varieties, was finished by late-October. Scattered frosts around the 9th of October forced some harvests. The 2008 crop will likely be one of Virginia’s largest, if not the largest. We had a number of reports of higher than expected crop levels and some of this appears to be related to both larger clusters and more clusters per shoot or vine than intended. Fruit quality was variable: while some growers commented on the cleanness and high quality of fruit, others commented on the slowness with which fruit ripened in September. The reasons for the latter are no doubt variable – the rains of September, leaf area compromised by downy mildew, over-cropping, leafroll virus, and cooler than average weather may have all contributed in some fashion in those vineyards where ripening was delayed. Where crop levels were managed, and where the crops dodged rains, the fruit quality was often very high. A common thread, however, was higher than usual juice acidity and somewhat lower than usual pH. Many growers mentioned that harvest was delayed a week to 10 days. Our own harvest here at Winchester was fairly close to the long-term average established during the nineties. For example, Chardonnay was harvested around the 21st of September (23 Brix, 3.43 pH and 0.71 TA). Given the advancing harvest date of the last 5 years, one could argue that 21 September was somewhat delayed. Lessons learned in 2008 to apply in the future?

• Accurate crop prediction and crop level management is required every year;
• grape contracts come and go – seek long-term commitments;
• irrigation can advance grape maturity relative to drought-stressed vines;
• wildlife damage comes in many forms (many reports of raccoons and hornets, not just deer);
• and leafroll virus is an emerging threat (see related article, below).

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II. New grape pathologist hired at Virginia Tech:

I’m happy to report that our search this summer/fall for a grape plant pathologist culminated in our hiring Dr. Mizuho Nita, who will formally start here at Winchester in early January 2009. Dr. Nita, who is originally from Japan, received both his MSc and PhD degrees from The Ohio State University. His PhD research involved management of Phomopsis cane and leaf spot of grape. He developed a disease forecasting system for phomopsis based primarily on temperature and leaf wetness, and tested the system over several years and different locations. The system was effective in reducing the number of fungicide applications in several years at two locations. Additional work showed that dormant-season fungicide applications could reduce inoculum and subsequent infections, although there was considerable variation in the response. Mizuho also used spatial analysis tools to map the severity of phomopsis in Ohio as a function of weather and disease management practices. Mizuho will provide an introductory talk at the upcoming Virginia Vineyards Association meeting in February and will include a short review on his PhD research with phomopsis. Since obtaining his PhD in 2005, Dr. Mizuho has been working as a research associate, conducting epidemiological studies of a fungal disease of wheat. His interest in grape disease research and extension, however, sparked his interest in applying for our position. The grape pathology position will be roughly 50% research and 50% extension. We anticipate that Dr. Nita will engage in a range of research, including fungicide testing, variety susceptibility evaluation, exploration of cultural and chemical means of disease management, and development of novel diagnostic tools. Extension programs will target growers as well as Cooperative Extension agents with training in disease identification and management.

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III. Leafroll virus issues:

Symptoms: The most telling symptoms of leafroll virus occur with red-fruited varieties that synthesize and express a high proportion of red pigments (anthocyanins) in the fruit and leaves. The symptoms are noticeably expressed in late-summer and fall and appear as a reddening of the interveinal regions of the leaf with the leaf veins remaining bright green (Figure 1). As the disease name suggests, the leaves may also exhibit a downward curling or rolling of the edges. The symptoms start on the more basal leaves of the shoot but rapidly advance to younger leaves, including laterals. Although not as obvious, leafroll also causes a fading of the green color in white-fruited varieties, again with the leaf veins and immediately adjacent tissue remaining deep green and the leaf margins curling downward as the symptoms worsen. The symptoms could be confused with nutrient deficiency (such as phosphorus deficiency), but nutrient deficiencies usually affect many vines in a contiguous area of the vineyard, whereas leafroll virus will normally appear on individual vines, at least in young vineyards prior to potential spread (more on that shortly).

Effects: Leafroll virus reproduces in the phloem or food-conducting vascular tissues. Similar to the effects of phytoplasmas (also phloem-inhabiting microbes) found in grapevine yellows-affected vines, the leafroll virus particles disrupt the flow of carbohydrates and other metabolites to fruit and other sinks. The effect of this disruption is a decline of vine vigor, a reduction in crop yield (as much as 30% decrease), and a significant decrease in the “ripeness” of fruit on affected vines. Fruit on affected vines can be 3 or more degrees Brix lower than that of adjacent, healthy vines. Worse, the affected fruit often has very little varietal aroma and flavor characteristics, poor coloration, and fruit may be tart, although this is likely due to the relatively low sugar levels attained in fruit of affected vines. Leafroll is not isolated to vinifera varieties. Hybrids and even American-type varieties can be affected, although the effects of the virus are apparently less severe or non-apparent in American-type varieties.

The virus(es): Leafroll is caused by up to 9 distinct but closely related viruses that vary somewhat in their effects on the host plant. The leafroll viruses appear to be restricted to Vitis hosts. Native grapevines may be asymptomatic hosts (as is the case with grapevine yellows phytoplasmas), although this has not been thoroughly evaluated. Of the 9 (or more) viruses that comprise the grape leafroll family, grape leafroll 3 (GLRaV-3) appears to produce the most intense symptoms and adverse effects on vine health.

Spread: The grape leafroll viruses are all spread through propagation of infected plant material and they are graft-transmissible. Before the nineteen-eighties, this was assumed to be the principal, if not exclusive means of transmission. In the eighties, researchers began to accumulate evidence that mealybugs (small, soft-bodied insects) were capable of transmitting the leafroll viruses from vine-to-vine. California researchers confirmed this mode of transmission in the early nineties and found that four mealybug species commonly found at low populations in California were capable of transmitting GLRaV-3, and one (longtailed mealybug) was capable of transmitting GLRaV-5. Attention in California is also focused on a fifth mealybug – vine mealybug – which has the potential to be an even more effective vector. Increased incidence of grape leafroll virus in California vineyards in the last 15 years has led to speculation on several possible reasons, including expansion of known (mealybug) vector populations, potential involvement of other non-identified vectors, a heavy shift away from rootstocks such as St. George and AXR #1, which might have masked leafroll symptoms, and the possibility that one or more new strains of leafroll virus may have been introduced in the last 20 years. Recent research conducted by researchers at Cornell in New York State have shown a widespread occurrence of leafroll virus in Finger Lakes region vineyards; approximately two-thirds of 95 surveyed vineyard blocks had leafroll-positive vines present (GLRaV-1, GLRaV-2, and GLRaV-3 were all detected). We have invited Dr. Marc Fuchs of Cornell University to share the results of their work with leafroll virus at the upcoming Virginia Vineyards Association annual technical conference in February.

Although we have not conducted formal surveys of leafroll in Virginia vineyards, we’ve recognized its presence here since the early eighties. Anecdotal observations suggest that within-vineyard spread may also be occurring in our vineyards. A Charlottesville area vineyard experienced a dramatic population surge of grape mealybugs in 2006, to the point that some fruit was culled out in the harvest because of the “honey dew” excretions on the fruit. Incidence of leafroll symptoms, including leaf discoloration, lower yields, and decreased fruit quality have essentially increased to 100% of the vines by 2008. The affected vineyard blocks are being removed this fall for eventual replanting. Closer to home, we established a small block of Cabernet Sauvignon and Chardonnay here at the AREC in Winchester in 1990. We noted a few apparently leafroll infected Cabernet vines by 1995 when we were in the midst of a crop level experiment. Since 2000 we have noticed more symptomatic vines and we’ve observed that fruit did not ripen to the degree of ripeness that we had achieved in the vineyard’s earlier years. This was also about the time we started observing mealybugs in the vineyard, at a time coinciding with our experiments with the use of under-trellis cover crops. The under-trellis vegetation and the presence of mealybugs may be purely coincidental; or not. There is much we don’t know about this aspect of vineyard ecology. What was obvious by 2008, and confirmed by lab analysis of a dozen samples sent to Dr. Fuchs at Cornell University, was that our Cabernet and apparently the adjacent rows of Chardonnay were uniformly infected by leafroll virus. A survey of the vineyard in late-September revealed that every vine showed pronounced leafroll symptoms. This story is muddied a bit by the fact that we planted the Cabernet (CS clone #08) in 1990, at a time when the source wood was known to harbor GLRaV-3 virus. So, did the vines just take 10 years to show advanced symptoms? Is vine-to-vine spread occurring? Good questions for a grape pathologist and entomologist to collaborate on....

Recommendations: The research with leafroll is evolving with groups in both California and New York State actively involved. For new plantings, there is no change in the recommendation to use only certified wood to reduce the likelihood of planting grapevines that are already infected with leafroll or other pathogens. Some popular clones are known to be infected with one or more leafroll viruses: Cabernet Sauvignon clones #337 and #191 (ENTAV), for example, are known to be infected with the GLRaV-2 virus. Some argue that this virus does not measurably impact fruit quality, although if true in specific rootstock/environment situations, that may not necessarily be a universal truth. The consensus of virologists on this issue is to avoid plant material that has ANY leafroll virus. The potential vector situation is also clouded by lack of information: while we know that one or more mealybug species occur in Virginia and can be found in vineyards, we are uncertain of their vector status. There is also a potential for other, yet to be identified phloem-feeding vectors, although this is not necessarily a strong possibility. Given (a) the potential for vine-to-vine spread within the vineyard and (b) the known, negative impact of leafroll virus on grape and wine quality, it makes abundant sense to rogue out clearly symptomatic vines and to replant those vines with clean stock. Remember, the infected vines cannot be cured and they may be serving as a source of inoculum for further vine infections. It may pay at this time to monitor for mealybugs in your vineyard. If warranted, the current Virginia Cooperative Extension Pest Management Guide for grapes (http://www.ext.vt.edu/pubs/pmg/) lists six different insecticides that are labeled for this target.

Testing services: Confirmation testing for leafroll viruses can be done by commercial labs. Although several labs may provide this service, STA Laboratories (http://www.stalabs.com/) in Gilroy, CA is one that has specific experience with grape pathogens, including viruses. The STA Laboratories website contains much helpful information but you will need to contact the lab directly to obtain their current fee structure and packaging/shipping requirements. Bear in mind that leafroll virus particles are not uniformly distributed throughout the plant and there is potential for false negative tests – negative results when indeed the vine is infected. Suspect vines can be sampled during dormancy or with vegetative tissues in late-summer.

The following two references offer more detailed descriptions of grape leafroll virus research findings in California and New York State, respectively:

Sweet, N. 2008. Leafroll virus threatens California vineyards. FPS Grape Program Newsletter, October, 2008. Available online at: http://fps.ucdavis.edu/WebSitePDFs/Newsletters&Publications/GrapeNewsletterOct2008.pdf

Martinson, T. et al. 2008. Grapevine leafroll – an increasing problem in the Finger lakes, the US and the world. Cornell University Cooperative Extension. Available online at: http://winegrape.ag.psu.edu/docs/winegrapeinfo/attach/Grapevine_Leafroll_Cornell_Report.pdf

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IV. Question from the field regarding pruning:

A. You’ve asked several questions here, one about cold hardiness, one about timing of pruning, and one about use of wound protectants; I’ll comment on each.

Cold hardiness: Until recent years, I did not recommend planting Merlot in most areas of Virginia due to its apparent cold tenderness, as judged by grower experience. With a trend towards warmer winters, the threat of winter cold injury has lessened, and Merlot acreage has rapidly expanded and now comprises over 10% of Virginia grape acreage. According to the 2007 Virginia Agricultural Statistics Service commercial grape report, Merlot was the 3^rd highest volume of Virginia grapes harvested in 2007, following only Chardonnay and Cabernet franc. Although Merlot can be considered relatively cold-tender, I would not expect its response to cold stress to differ from that of other varieties with respect to how it’s pruned. There are two issues – effects of pruning on cold hardiness of retained wood, and the effects of training on the ability to compensate for injury in the event cold injury occurs. Some research suggests that cordon-trained and spur-pruned vines might have a slight cold hardiness advantage over head-trained, cane-pruned vines. However, crop load, disease incidence, canopy characteristics and other factors will have a much greater impact on cold hardiness. I would consider reasons other than cold hardiness potential when deciding whether to use canes or spurs. The second issue with this answer concerns compensating for injury: the more perennial wood maintained (e.g., cordons and high trunks), the more time is required in removing cold-injured wood and retraining the vines to the desired training system in the event of substantial cold injury to the vine. This argues against using elaborate training systems such as open-lyre and Geneva Double Curtain with extensive cordons where cold-tender varieties are grown in locations subject to damaging winter temperatures. These are more “training” issues than “pruning” issues per se though.

Timing: I would suggest pruning as late in the winter as you can. If you can complete the task in a month, starting in early March should be acceptable in most years here in northern Virginia. The research data on time of pruning effects on cold hardiness of retained wood generally trends towards “no impact”. However, once the wood is removed, there’s little or no room for compensation if you have a subsequent low temperature episode that injures buds or wood. This ability to compensate for cold injury by leaving more buds or extra canes is the principal reason for delaying pruning as long as possible.

Wound protectants: This topic has been covered in two previous Viticulture Newsletters (http://www.ext.vt.edu/news/periodicals/viticulture/05february/05february.html#II), although not exhaustively. Dr. Doug Gubler spoke about their research in California with pruning wound protectants at the Virginia Vineyards Association’s annual meeting in February 2008. You asked if you should use a pruning sealant on the canes? No, there’s no point in protecting the cut end of canes that would be removed in the following year. On the other hand, there may be merit in protecting larger (nickel-sized or larger) cuts made in the head region of the vine if you have evidence of Eutypa dieback or Botryosphaeria canker rots in the vineyard (see the previous newsletter topics on this subject). On a more general note, both Eutypa and Botryosphaeria rot are common in older Virginia vineyards. While we don’t fully understand if topical wound protectants serve to reduce vine infection by these fungal pathogens in our environment, there is some evidence of their protective nature in studies done elsewhere. Topsin M WSB fungicide has a Special Local Needs label registration for this purpose in Virginia (EPA SLN No. VA-080003) and can be used at 3.2 oz per gallon as a painted-application or as a sprayable suspension for treating wounds. This application is specific for Virginia; however, several other states have similar special local needs labels for this purpose, so non-Virginia readers may wish to check with their own state sources. As Dr. Gubler pointed out this past February, the application of a fungicide would have to be repeated following rains to ensure wound protection. If you’re spur-pruning, you should probably also be treating the cut ends of the spurs – they are presumably no less susceptible to fungal infection than are the larger pruning cuts made into wood that’s two years old or older. Obviously, this will start to become a huge job with a spray bottle or a paintbrush!

You didn’t ask the question, but a sidebar to this discussion is whether one should cane-prune or spur-prune for vine management reasons other than cold hardiness issues. Cordon-training and spur-pruning is commonly used and has both advantages and some disadvantages to cane-pruning. Advantages of cordons and spurs over cane-pruning include:

• Elimination of labor involved with tying canes to trellis wire.
• Ability to achieve more uniform shoot growth along a longer vine space distance. A cane laid out 3 or 4 feet often exhibits non-uniform shoot growth – vigorous shoots near the head and towards the distal end of the cane, but relatively poor shoot growth in the mid-cane region. This non-uniformity is not as obvious with cordon-training.
• Ability to use double-pruning of spurs as a means of suppressing basal shoot development on the spurs and buying a few days of frost injury avoidance.
• Probably easier to “rough prune” vines using unskilled labor.

Advantages of head-training and cane-pruning, on the other hand, include:

• Fewer pruning cuts per vine.
• Better node fertility with varieties that have low fruitfulness of basal buds (e.g., Sauvignon blanc and Nebbiolo).
• Less shoot thinning required with varieties that have propensity to push a lot of non-count or base buds (e.g., Cabernet Sauvignon).
• Less pest pressure in some cases. For example, older wood of cordons and spurs can harbor phomopsis cane and leaf spot inoculum, mealybugs, and European red mites.
• In high-vigor situations, the minimization of perennial wood is thought by some to be a management tool to help reduce vegetative growth of big vines (I’m not convinced that it would work in our humid conditions, but there is some good logic to the argument).
• Ability to avoid, and more easily compensate for, wood-rotting fungi such as Eutypa and Botryosphaeria canker.

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V. Other news:

Wine Grape Production Guide for Eastern North America: This 330-page book finally goes to the print shop in December 2008 after a very long gestation. I’ll call more attention to the book when it’s in the distribution channels; however, a few last-minute reminders:

• the pre-print cost of the book ($44) is much lower than the after-printing cost ($75).
• The book will not be available online.
• The pre-print savings period has been extended to 23 December.
• The book is described and can be ordered online at: http://www.nraes.org/nra_winegrapecontent.html.

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VI. Upcoming meetings:

• Entomology research updates, Doug Pfeiffer, Virginia Tech.
• Grape root borer research update, Chris Bergh, Virginia Tech.
• Regulation of Cabernet Sauvignon vegetative growth via cover crops and root restriction, Tony Wolf and Tremain Hatch, VA Tech
• Virginia Wine Marketing Office updates, Annette Ringwood Boyd
• Safety and/or legal issues for pesticide recertification, Michael Lachance, Nelson County Cooperative Extension
• Impact and importance of leafroll virus on grapevines and potential wine quality, Marc Fuchs, Cornell University
• Selection criteria for clean, high-quality nursery stock, Helen Waite, University of Melbourne
• Dealing with adversarial neighbors and legal compliance with pesticide laws, Tom Burke, Pesticide Services, VDACS
• A vision for Virginia’s grape pathology program and review of Phomopsis cane and leaf spot management, Mizuho Nita, VA Tech
• Chardonnay grower and vintner panel
• Trade show and vendor reception

Calendar of Regional Viticulture and Enology Events for 2009. This calendar is prepared each year by Mark Chien of Penn State, to whom I am grateful for taking on the task.

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