Viticulture Notes

Vineyard and Winery Information Series:
Vol. 22 No. 4, July - August, 2007

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. Current situation
   1. Virginia's 2006 grape crop report
   2. Introducing Dr. Mardi longbottom
2. Pre-harvest considerations
3. Fruit development after frost
4. Drought conditions

I Current situation

A. Virginia grape crop report:

Commercial grape acreage and production data are collected annually by the National Agricultural Statistics Service. In Virginia, this is done through the Virginia Department of Agriculture and Consumer Services. The 2006 Commercial Grape Report is now available on-line at: http://www.nass.usda.gov/Statistics_by_State/Virginia/Publications/Grape_Report/2006%20grape%20publication.pdf

Virginia producers harvested 6,200 tons of grapes in 2006, up 11% over the 2005 crop. The raw product value was $8.6 million. Albemarle County continues to lead the state’s counties in both acreage and grape production. Remarkable figures. Two decades ago, Riesling was the 3rd most extensively planted variety and Viognier was just being considered for trial planting. Today the acreage of Riesling has fallen to about 50, while there are about 150 acres of Viognier.

If you are a new, commercial producer in Virginia, and/or do not receive VDACS’s crop survey, please contact VDACS statistician Mr. Jason Jones to add your vineyard to the survey. The data are useful for illustrating growth and development of the industry, lobbying legislative support, and justifying human and fiscal support from state and federal agencies. Jason Jones can be reached at:  USDA / NASS / Virginia Field Office, 102 Governor Street, Room LL20
Richmond, VA 23219    (800) 772-0670    jason_jones@nass.usda.gov

B. Introducing Mardi Longbottom:

I’m very pleased to introduce Dr. Mardi Longbottom as our newly hired Viticulture Research/Extension Associate here at Winchester. Mardi and her husband Heath arrived in late-June and Mardi has been busy familiarizing herself with Virginia viticulture and meeting some of you through meetings and correspondence. We are indeed fortunate to have someone of Mardi’s caliber on board and I’m excited to think about some of the opportunities that her hire provides. I owe a debt of gratitude to Mardi’s PhD advisor, my friend Peter Dry of the University of Adelaide, for encouraging Mardi to accept our position. Peter has twice visited Virginia, first speaking at the VA Vineyards Association’s annual meeting in Roanoke in 1999, and again visiting last summer.

Mardi’s role at Winchester is one of both extension and research. In the former, we envision development of more web-based resources, additional viticulture training programs for Virginia Cooperative Extension agents, organization and participation in industry meetings, and trouble-shooting grower problems. Potential research areas include vine nutrition, canopy management and relationships between vine vegetative growth and grape quality – we’d like her to take some time to look at the industry needs before jumping into the research.  Mardi can be reached at mardilongbottom@vt.edu.

I’ve asked Mardi to write a bit about herself; in her words then:

“I am very excited to join the team here at Winchester as the new Viticulture Research/Extension Associate. While my home is Australia, the lure of working with Dr Wolf and tackling some of Virginia’s viticultural challenges, whilst enjoying the bounties of such a beautiful region, were irresistible.

I come to Virginia with 15 years of ‘hands on’ vineyard experience gained in my family’s 250 acre vineyard at Padthaway and numerous other large and small vineyard enterprises. I’ve also worked as a viticulturist in Coonawarra, the Adelaide Hills, Barossa Valley and McLaren Vale regions of South Australia. My formal viticultural training is from The University of Adelaide, most recently a PhD under the supervision of Dr Peter Dry and for the past 6 years I have also been teaching in the undergraduate and graduate viticulture programs at the University of Adelaide.

Throughout my formal studies I have chosen to work on applied viticultural research because I recognize the value of practical research outcomes to growers. During my Masters candidacy my research focused on canopy management, investigating the effects of sacrificial (or ‘kicker’) canes on vine balance and fruit composition in regions where vine vigour often compromises fruit quality. I understand that canopy management is a major challenge here in Virginia so I hope to assist growers with some of my expertise in that area.

Another of my areas of interest is yield manipulation and crop forecasting. My PhD research concentrated on aspects of flowering and fruitset which ultimately determine yield. A major part of that project was a nutritional focus which I also think will be valuable to Virginian grape growers.

I appreciate the demands on grape growers to consistently produce the best quality fruit possible and believe that this can be achieved effectively, efficiently, and economically using established best practice techniques and with ongoing region-specific research. I look forward to meeting you all and helping you to achieve your viticultural goals.”   -  Mardi

II. Pre-harvest considerations:

I am “re-using” this article from my July-August 2005 Viticulture Notes, with some updates for 2007.  There are some reminders for all, and maybe new stuff for newer readers (TKW)

August is often our slowest summer month in vineyard management, and it’s a good time to re-group and look ahead at pre-harvest considerations.  A check-list of such activities might include the following:

Canopy management:  Do a final check of the vine canopy. Prematurely senescing, yellowing leaves should be pulled from the fruit zone. They do not contribute carbohydrates to fruit maturity. Dead leaves retard the drying of clusters when they are in contact with clusters, and they can promote botrytis development on fruit in both direct and indirect ways.  Keep the leaf layers in the fruit zone of the canopies down to 2 or less on average (a real or imagined probe run through the canopy should contact no more than 2 leaves on average as the probes passes from one side of the canopy to the other). While there is still a chance of causing fruit sunburning by being too aggressive with leaf-pulling, in my experience, the sunburning is more apt to occur closer to the summer solstice. A majority of the clusters should receive some direct sunlight for some portion of the day. Look for congestion at the tops of hedged VSP-trained canopies.  If the hedging was not done in a timely fashion, the shoot tops might be growing horizontally along the top wires, giving rise to leafy laterals. Normal hedging can also produce several laterals where there was originally only one growing point. Collectively, this lateral growth can create very dense regions at the top of the canopy. It is often in these shaded, poorly ventilated regions that downy mildew gains a foothold on young, susceptible leaves.

Crop management:  It’s not too late to reduce crop levels on vines that are carrying a heavy crop. Clusters at 50% veraison weigh about 80% of their harvest weight and fruit at 15 to 17 ºBrix will essentially represent final weight, with some variation due to precipitation extremes. If you failed to collect mid-season cluster weight data you can still estimate crops and make downward adjustments to the crop if you feel the crop level is excessive. As I’ve used in previous communications, a good range of desired crop is about 1.5 to 2.0 pounds of crop per foot of canopy, irrespective of vine density in the vineyard.  Are you there?  What about drought and the prospects for ripening that crop (see related article in this newsletter)?  Drought will slow the ripening of grapes and the effects will be greater for heavily-cropped vines than for lightly cropped vines. If you’re seeing drought effects, and don’t have irrigation, and don’t see storm clouds coming your way, you might want to drop some crop and aim more towards 1.0 to 1.5 pounds of crop per foot of canopy.

Pest management:  If you’ve done a good job with disease control, you can “coast” through harvest; if not, you may still have a fight on your hands.  Berries are less susceptible to PM infection once they attain about 8° Brix.  Fruit may, however, continue to show lesion development from infections that occurred up to one month ago.  Low levels of PM may exist on fruit, even with apparent “good” prevention programs.  The “inconspicuous” mildew can increase fruit susceptibility to botrytis and other rots later in the season.  So, when I say “coast” I mean you should continue to maintain a prudent mildew prevention program. Powdery mildew fungicide options in the pre-harvest period are constrained by label pre-harvest intervals (PHIs) and the need to avoid sulfur residue on harvested fruit, which can lead to sulfide production and off-odors in wine. It is advisable to avoid sulfur application within 6 weeks of harvest if at all possible. The options are sterol-inhibitors and the strobilurins and quinoxyfen (Quintec), most of which have a 14-day PHI.  Other alternatives are Nutrol (monopotassium phosphate), Armicarb 100 (potassium bicarbonate), and OxiDate (27% hydrogen dioxide, aka hydrogen peroxide). These are very short-lived materials and are typically more effective as post-infection materials than as protectants.  Used on a weekly (7-day) basis, they appear to effectively control powdery. The biocontrol product, Serenade (Bacillus subtilis) is another option. Oils, such as JMS Stylet oil, offer good protection IF used with sufficient gallonage (at least 100 gallons of water/acre).  The downside of oils is a temporary depression of sugar production in treated fruit due to reduced photosynthesis.  Post-harvest oil use might be an acceptable proposition though, and oil lends itself to disease resistance management by introducing a different mode of action to that of the sterol-inhibitors and the strobilurins.  Once beyond harvest, the options for PM are sulfur or copper fungicides. Copper is only fair for PM control, but if the vineyard is clean, it has the advantage of offering excellent downy mildew control.

Botrytis:  Botrytis incidence varies from year-to-year, but we tend to have greatest problems in large, compact clustered varieties such as Seyval and Chardonnay.  Culturally, the incidence of botrytis can be reduced by removing leaves that are directly touching clusters, and opening the eastern side of N/S-oriented rows to aid air movement and spray coverage.  It’s certainly not too late to do some follow-up leafing in botrytis-prone cultivars, but avoid pulling too many leaves that could result in sunburning of fruit (see comments above under canopy management).  Fungicide options, specific for botrytis, are Elevate, Scala, and Vangard. Pristine is also labeled for botrytis if used at the higher label rates prescribed on the Pristine label and its supplemental label.  Should you apply a botrytis fungicide now if you’re starting to see botrytis?  I’d offer a qualified “yes”, simply to slow new infections. Once botrytis starts affecting multiple berries, it seems to progress quickly to adjacent berries and, in the absence of dry weather, it is both difficult to control and can progress to non-specific bunch rots.

Downy mildew: Conditions that favor the spread of downy are temperatures of 65 to 77°F and free moisture.  A summer late-day shower followed by a humid evening creates the perfect scenario for a downy infection. Fruit becomes resistant to infection as it develops; however, young leaves (such as on laterals) are highly susceptible, and this is often where late-summer infections develop. To avoid a potential defoliation, continue a downy mildew protection program through harvest.  Fungicide options once you are within 66 days of harvest, are captan, and the phosphorous acid compounds such as ProPhyt and Phostrol. Due to wine-making concerns (haze development and suppression of varietal aromas) copper fungicides are not recommended in the six weeks prior to harvest; however, copper could be used post-harvest. Captan provides excellent downy protection as well as providing control of the fruit phase of Phomopsis, and perhaps some of the other late-season rots (e.g., bitter rot [Melanconium spp.], and ripe rot [Glomerella spp.]) that we occasionally observe. We do, however, have some concerns about captan use close to harvest from a wine-making standpoint. Although Pristine (14-day PHI) has a broad-spectrum of control and minimal wine-making concerns, we are concerned that downy mildew resistance to the strobiliurin component (pyroclostrobin) of Pristine will render it ineffective against downy in many Virginia vineyards. The same concerns exist for downy mildew with the other strobilurin fungicides (Abound, Sovran, Flint). While Pristine might be effective against some other late-season rot organisms, it may not adequately control downy mildew in your vineyard.

Grow tubes:  As a reminder to anyone using grow tubes, the tubes should be removed from vines by 1 September to allow vines to normally acclimate to fall conditions.  DO NOT leave the tubes on over winter.  We have seen ample evidence that vines can be severely damaged by winter temperatures if the vines remain in tubes over winter.

III. Fruit development after frost

Virginia grape producers were confronted with widespread frost over the Easter Weekend of 2007. While the full impact of the freeze won’t be assessed until harvest is complete, the injury to early-budding varieties such as Chardonnay was substantial in the central and southern piedmont. Injury to reds appears to be less significant, although the reds will also be off their potential yields. One of the potential complications of a spring frost is a mix of primary and secondary shoots on the same vine, each bearing a population of grape clusters that are ripening along different timelines: the primary shoots have clusters that might bloom 3 or more weeks in advance of clusters borne on secondary shoots which broke bud after the frost.  Does a three-week difference at bloom translate to a three-week differences at veraison? What about harvest dates?  Where we’re trying to synchronize ripening or achieve as great a uniformity of ripeness as possible at harvest, a two-week difference in the degree of ripeness amongst the clusters in a given block could be a real nightmare.

Several growers inquired about this potential asynchrony this spring, and asked if it made sense to remove the surviving primary crop in order to have a population of uniformly ripening grapes. My experience argued against the removal of crop from vines that already had a diminished, optimal yield. It would hurt cash flow and it would further stimulate vigor on vines that might not need the vigor stimulation. My supposition too was that pronounced differences in phenological development at bloom, might not be so apparent at harvest, at least with vines that were carrying a relatively light crop. 

Mardi Longbottom and I had the occasion to discuss this scenario earlier this summer and she mentioned that she’d had some experience with a widespread frost that produced two groups of grape clusters ripening along two very different timelines.  I asked Mardi to put her observations together for this newsletter:

ML:  In spring of 1998 vineyards in Coonawarra were affected by several spring frosts. One of the worst affected blocks that I was involved with was a 20 acre block of 3 year old Cabernet Sauvignon.

At the time of the frost the shoots had 8-10 separated leaves and the inflorescences were clear. The frost burnt most of the shoot tips and many of the inflorescences were either completely or partially affected (Figure 1).

Within a couple of weeks of the frost the vines began to recover and pushed new shoots. Many of the original inflorescences on the primary shoots survived and, as expected, new secondary and lateral shoots produced another generation of inflorescences. At this stage it is usual to apply remedial measures to prevent variability in the final crop, most commonly, removing all secondary bunches. Alternatively, some growers take the extreme measure of spraying the vines with either a contact herbicide or a concentrated dose of urea to burn off all remaining green tissue to allow uniform regrowth. However, because we had budgeted for a crop from these vines and many of our other blocks were down in yield as a result of the frost, we were reluctant to remove any more fruit.

At this stage it was obvious that the differences in maturity between the two lots of fruit may potentially have a big impact on final wine quality. However, there were several important unresolved questions that influenced our decision-making at this stage.

Would removing the late crop improve the quality of the remaining fruit? Would it still be economical to harvest the remaining fruit after fruit thinning?  Which fruit should we remove – early or late ripening?

Approximately two weeks prior to the anticipated harvest date I began sampling the block, keeping the two bands of fruit separate. At the first sampling date the difference in sugar concentration between the two samples was around 8° Brix. Because vintage was well underway at this stage and this block was the last of 250 acres to be harvested, the decision about what to do with the fruit was postponed. A week later I sampled the block again and the difference between the pre- and post-frost fruit had decreased to around 4° Brix. Based on these analyses we decided to re-evaluate in another week.

On the final sampling date the difference in sugar concentration between the two lots of fruit from the frosted Cabernet vines was less than 2° Brix and we made the decision to harvest all the fruit together.

While this may not have been a great example of timely decision-making, ultimately leaving all the fruit on the vines turned out to be the best decision. We avoided the cost of thinning, we did not lose any crop and all of the fruit ripened satisfactorily.

Last season I observed the same phenomenon in frosted Chardonnay and Shiraz. After several frost events the vines were carrying two distinct generations of fruit, however the differences in fruit development decreased towards the end of the season. At harvest time differences in maturity were negligible and there was no negative effect on final fruit composition.

If you are faced with a similar decision to make there are several important things to consider:

• Removing second crop and leaving too little fruit on the vines may perpetuate the vegetative cycle of the vines, i.e. increased shoot length, lateral growth and shading. This results in low bud fruitfulness and ultimately low crop levels in subsequent years. Other consequences of overly vegetative canopies on fruit and wine composition are:

• Decreased sugar
• Decreased colour & phenols
• Decreased flavour
• Increased pH
• Increased vegetative characters
  
  
• What are the economic implications of removing fruit from the vines, that is, the cost of fruit removal and the cost of a reduced yield?
• If ripening is delayed, what impact will this have on the fruit?

IV. Drought conditions:

Rainfall throughout Virginia has been spotty at best over the last 6 to 8 weeks. Some areas look downright green while others, such as the Winchester area, are brown. We had 1.6 inches of rain in July for a month in which we normally experience about 3.5 inches. Generally, Virginia’s best wine years have been our hot, dry years. But that’s not to say that vines and grapes do best under drought. The most recent NOAA forecast suggests persistent drought through much of Virginia through early fall (see accompanying figure).

How does drought affect grapevines?  One of the first signs of drought stress is a change in the appearance of the vines. Rapidly growing shoot tips of well-watered vines appear soft and yellowish or reddish green. As soil moisture becomes limiting, the rate of shoot growth slows and the shoot tips gradually become more grayish green, like the mature leaves. Tendril drying and abscission is also a useful early indicator of vine drought stress. As stress intensifies, leaves appear wilted, particularly during midday heat. Under prolonged stress, leaves may yellow, or specific nutrient deficiency symptoms may be expressed, such as marginal yellowing (white-fruited varieties) or reddening (black-fruited varieties). Severe stress will lead to desiccation and abscission of affected leaves. Discoloration and abscission commence at the base of the shoot and progress up the shoot as stress intensifies. Water-stressed fruit exposed to the sun can sunburn and shrivel, much like a raisin. The visual symptoms of drought stress are summarized in Table 1.

In addition to visual indicators, vine water stress can be measured with special instruments. Some instruments measure the water status of vines, whereas others measure the moisture status of the soil. Hand-held, infrared thermometers can measure the temperature of vine canopies. Leaves of well-watered vines are generally cooler than the air temperature, even during the hottest period of the day. The leaves of water-stressed vines are often warmer than the surrounding air because of reduced transpirational cooling. The leaves heat because the stomata of the leaves close as the water status of the vines becomes limiting. This closure conserves the remaining water in the leaf, but the “cost” of this water conservation is decreased sugar production. With stomata closed, carbon dioxide cannot enter the leaf and the photosynthetic conversion of carbon dioxide into sugars will not occur. The impairment of the photosynthetic processes will generally occur before leaves are visibly wilted. Reduced photosynthesis can explain why fruit fails to ripen during periods of water shortage; little or no sugar is being manufactured. A point will be reached during a drought at which the daily stress of insufficient water will have an irreversible impact on the vine’s performance. By the time leaf wilting occurs, vines are severely stressed.

While we are interested in reducing some water availability to the vine in the bloom to veraison period to throttle back vegetative development, we do NOT want to impose water stress on the vines after veraison. If irrigation is available, we’d choose to supply enough water post-veraison to keep the leaves functioning optimally, but not so much water as to stimulate lateral shoot development. How much water this balancing act takes depends. It depends on evapotranspiration rates; it depends on soil depth and root system development; it depends on natural precipitation; it depends on crop load – heavily-cropped vines require more water for ripening than do lightly-cropped vines. And, of course, it depends on whether you have an irrigation system in place. More “depends” than a brief newsletter article has time to delve into. If you have the water available, supply enough to keep the foliage from heating during the mid-day heat.  Good luck. Hurricanes and tropical storms are the real wild card in this game.

"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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