Viticulture Notes

Vineyard and Winery Information Series:
Vol. 15 No. 5, September-October 2000

Dr. Tony K. Wolf, Viticulture Extension Specialist

Table of Contents

I. Current situation, questions, and resources:

The summer's pattern of cooler than average weather and abundant rainfall have continued into fall, the net effect being delayed fruit maturity and increased disease pressure. Some varieties, such as Sauvignon blanc and Riesling, sustained significant rot losses. Fruit sorting on the vine or at the winery was a common necessity. There were both winners and losers. I saw some very nice Chardonnay picked at 22° (±) Brix and very little rot, and I encountered reports of very nice Merlot crops. By contrast, other Chardonnay producers reported dropping up to 40% of their crop. In many cases, the relationship between grape berry moth injury and late-season fruit rots was strikingly positive.

As of this writing (2 Oct.), Chardonnay is still on vines at Winchester, at approximately 21°Brix, 3.3 pH, and a total titratable acidity of around 10 g/L. It's about where it was 2 weeks ago! Lows in the thirties are expected by the weekend. We are losing daylength and heat, and further gains in sugar/flavor will come in very small increments. The 2000 season has been cooler than average, but it's neither our coolest nor our wettest, recent year. Both 1996 and 1997 were cooler (Fig. 1), and 1996 had greater rainfall (28" [1996] vs. 24" [2000]) in the Apr.-Sep. period, where the average is 22 inches.

Fortunately, the canopies are healthy. There's little that we can do at this point other than keep the canopies clean, sit back, and hope to get some more heat before the first frost.

Zonate Leaf Spot: Among the peculiar observations of the 2000 season was a late-season (August) development of foliar, necrotic lesions on 'Norton' (Vitis aestivalis) vines grown by a commercial producer in Fauquier County. The lesions were dime- to quarter-sized, sometimes coalescing into larger areas, and frequently limited in size by major leaf veins (Fig. 2). Both older and newer foliage was susceptible, but the highest incidence of lesions appeared towards the canopy interior, and closer to the ground of the Geneva Double Curtain-trained vines. Individual lesions consisted of a series of concentric rings of varied gray/brown color (Fig. 3). The dead leaf tissue dropped out of older lesions. Approximately 10% of the leaf area was affected on some vines. We ruled out potential contact herbicide drift, foliar nutrient spray burn, or other obvious problems. Suspecting Zonate Leaf Spot, from descriptions and photo in the Grape Disease Compendium , affected leaf samples were collected and sent to the Plant Disease Clinic at Virginia Tech, Blacksburg. This service is available to all Virginia residents, and is a well used option for identifying diseases . The diagnostic lab result was positive for the Zonate Leaf Spot fungus, Cristulariella moricola. The disease can affect all grapes, but is apparently controlled by one or more of the fungicides used on most grapes for control of mildews and black rot. Norton is quite tolerant of mildews and black rot, and the grower, in this case, had justifiably chosen to discontinue post-bloom fungicide applications. In a drier year, the disease might not have been observed, but in 2000 the fungus took advantage of the unprotected vines. Keep this in mind if you grow Norton and you have a wet season; an additional captan or strobilurin spray in mid-summer might be helpful.

Question from the field: I am preparing to do a post-harvest spray and want your advice on the best approach to mop up the powdery mildew that I battled this summer. Would you recommend applying Stylet oil for the PM at this time of year, or would sulfur be the better choice? Would the spray intervals be the same as pre-harvest? Is there any point in spraying after leaf drop?

Answer: "Post-harvest" for some may coincide with first frost this season. No, there's no point in spraying once the leaves have fallen. Otherwise, it is generally considered wise to control downy and powdery mildew after harvest, at least until leaves naturally senesce or are frosted. The reason we would aim to protect the foliage is that leaves continue to produce carbohydrates after harvest, and those carbohydrates are useful to the vine to mature wood, and for storage for next year's growth. Furthermore, minimizing disease incidence this fall can help retard disease onset during the following season. Fungicides should be selected based on their efficacy against the mildews, and on the basis of what you've used throughout the season. From a cost standpoint, an application of sulfur for powdery, combined with a copper fungicide for downy would be a good, cheap choice for varieties that are tolerant of those fungicides. At this point, a single application should suffice for the balance of the growing season. Sulfur and copper will not eliminate the mildew that is already affecting your vines. JMS Stylet oil, or any other horticultural spray oil, may also be used. In addition to providing protection, oils have good powdery mildew eradicant qualities if applied at high (100 or more gallons per acre) rates, but there are also some penalties in terms of reduced leaf function at that rate, as discussed in prior newsletters.

Question from the field: Should I "hill-up" my vines for winter injury avoidance this fall?

Answer: (This answer was taken directly from last year's September-October VN). Mounding or "hilling" of soil over graft unions of grafted, cold-tender varieties is one means of avoiding potential vine loss due to cold injury. In practice, soil is ploughed up against the trunks of vines with a tractor-mounted blade in the fall, before the advent of severe cold weather. We've hilled trunks as late as mid-December. The soil must be mounded high enough to protect a 6- to 12-inch portion of the vines' trunks. This soil conducts heat from the earth and insulates the covered portion of the trunks. In the event of catastrophic cold damage to the above-ground structure, the vine can be retrained using buds that had been protected by the layer of soil. The extreme example of this practice is the complete burial of vines practiced in very harsh environments. The fall hilling operation is followed in the spring by a "de-hilling" of the trunks, ostensibly to prevent scion rooting, although I've yet to determine whether scion rooting is a liability. The "scion" (pronounced "sign") is the horticultural term applied to the upper, or fruiting portion of a rootstock/scion combination (e.g., the Chardonnay part of a grafted, Chardonnay vine). What are the pros and cons of hilling (and dehilling)? Benefits include the protection offered in the event of extreme weather. Vinifera trunks, depending upon variety and time of season, can be injured by temperatures above 0ƒF, although injury is more common at temperatures below -10ƒF. Symptoms of cold injury can include poor shoot growth in the following season, development of crown gall, and death and splitting of affected trunks. Another possible benefit of hilling and dehilling is the destruction of weeds, overwintering insect pests, and reduction of certain disease inoculum caused by the mechanical cultivation. The costs of hilling and dehilling must also be considered. First is the capital cost of equipment and annual operating costs. Grape hoes can cost from $1,500 to over $5,000, while operating (machinery and labor) costs run from $20 to $25 per acre. Annual hilling and dehilling has led to severe soil erosion problems in some older vineyards. The soil in some of these vineyards has eroded many inches below the graft union and the practice of hilling is no longer effective. Furthermore, the loss of top soil is associated with reduced vine vigor, lack of trellis fill, and unprofitable crop yields. One means of reducing soil erosion on hilly sites (aside from keeping rows oriented perpendicular to the prevailing slope) is to leave undisturbed soil "dams" every 30 feet or so along the row. Simply pull the plow out of the soil at regular intervals to avoid creating a continuous channel or trench down a row.

Another potential negative consequence of hilling and dehilling is the mechanical damage to vines. This can range from the obvious collisions with trunks to the less obvious damage to roots near the soil surface. An additional negative is the cost of equipment ($1K to $4K) and operational cost of hilling and dehilling. Each grower must weigh the pros and cons of hilling and decide for one's self if this is a justifiable practice. Your own experience will determine whether hilling of graft unions is good insurance or a wasteful practice. A grower may conclude, after 5 to 10 years without trunk cold injury, that the "insurance" is unnecessary. The most recent severe cold event in Virginia (February 1996) would have been much more destructive had it not been for the snow cover that blanketed most of the Piedmont vineyards. The snow did essentially the same thing that our mounding of soil did - it provided a thermal continuum between the earth and vine trunks. What about sawdust, grow tubes, white paint, etc? Sawdust and finely milled bark mulch would probably work, provided it retained sufficient moisture to conduct heat; however, the mechanics and cost of application must also be considered. Significantly, we have repeatedly noticed that organic mulches can lead to increased climbing cut-worm populations in the subsequent spring. Grow tubes provide no cold protection, and can even cause increased winter injury. Painting of trunks with white paint is intended to reflect sunlight and limit radiational warming of the trunks. I am unaware of this practice being used successfully in Virginia vineyards. Nurserymen generally recommend the hilling-up of young vines for the first few years in the vineyard, even with excellent sites. The occasional observation that trunks of young vines sustain greater cold injury than do the trunks of older, established vines probably relates more to cropping stress on the young vines, rather than direct age effects on cold hardiness. I do, however, recommend that you follow the advice of your grapevine supplier with respect to hilling of graft unions. It's usually easier to obtain recompense from a nursery for failed vines if you've followed the nursery's planting and care recommendations. Be careful when mounding soil against young vines. The young trunks and graft unions may not withstand the impact of heavy clods of soil. For first-year vines, we find that it's safer to turn the soil near the trunk with a grape-hoe and then use a hand-hoe to move the loosened soil against the trunks.

Some growers have found that hilling does not guarantee vine renewal in the event of trunk injury. Repeated episodes of vine kill generally dictate the need for more radical measures. If the problem is confined to a small area of the vineyard, study the area to see if it's associated with poor soil conditions, lower topography, or other features that may be contributing to vine failure. It may be best to remove these "problem" areas from production if there are site-specific limitations.

Timing: Fall hilling can be done at any time before the ground freezes, typically after harvest and before mid-December in VA. De-hilling in the late-winter or spring occurs after the threat of extreme weather, and before the application of pre-emergence herbicides; the period from late-February until mid-March is convenient.

We occasionally receive questions that reveal that the questioner is unaware of some of the printed resources that are available to growers and vintners in this region. Case in point was a recent request for advice on suitability of wine grape varieties for northern Virginia. As a reminder, VCE publication #463-019, "Commercial Grape Varieties for Virginia" (42 pages) was released in 1999, and covers our current grape recommendations. The bulletin is available ($10.00, payable "Treasurer of Virginia Tech"), from VCE Distribution Center, 112 Landsdowne St., Blacksburg VA 24060.

An annual publication that I find quite useful is Vineyard and Winery Management's "Directory and Products Guide." Cost is $40, and you can get a copy by calling 607-535-7133, plastic in hand. Or, check the web at http://www.vwm-online.com . I use the directory often. In addition to products, it lists fed and state research and regulatory agencies, and wineries. Check it out.

We occasionally observe vine decline and vine death due to the planting of vines in close proximity to walnut trees. Black Walnut and butternut trees contain juglone, a naturally occurring, allelopathic chemical in the leaves, fruit, and roots of these trees. Juglone can inhibit the growth and/or cause death of certain plants including grapevines. Affected vines will exhibit weak growth, with wilting, chlorotic leaves (Fig. 4 and Fig. 5). Severely affected vines can die. Research has not clearly shown how juglone causes the inhibition of growth and/or plant death, but the alteration or inhibition of oxygen uptake and photosynthesis are suspected. Take caution when designing your vineyard around these trees. We recommend removing trees in the area of the planned vineyard by a margin comparable to the height of the tree. For example, if the walnuts are 60 feet high, keep the vines at least 60 feet from the base of those trees. If you intend to remove the tree, do not plant vines in the area of the root system for several years. The roots will continue to release juglone until the roots are completely decomposed. More information about juglone can be found at:

www.ag.ohio-state.edu/~ohioline/hyg-fact/1000/1148.html

Return to Table of Contents

II. Coming meetings

2001 Virginia Vineyards Association and Virginia State Horticultural Society Annual Meeting When: 22-24 January 2001 Where: Williamsburg, Virginia Details: This meeting will be comparable to that held in Williamsburg in January 2000, and Roanoke in January 1999. Starting on Monday, 22 January, the "grape" portion of the meeting will run concurrently with the tree-fruits portion of the meeting through Tuesday, to be followed by a combined session with both audiences on Wednesday morning, 24 January. Plans are still being finalized, but a partial list of speakers and topics include:

Other speakers/topics are pending, but include the following topics:
Vineyard mechanization options
Update on weed control
One or more grape disease updates

In addition, the meeting will feature a trade show and a reception. Mark your calendars now and plan to attend. Registration materials will arrive under separate cover, later this fall.

Return to Table of Contents

"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

Commercial products are named in this publication for informational purposes only. Virginia Cooperative Extension, Virginia Tech, and Virginia State University do not endorse these products and do not intend discrimination against other products that also may be suitable.

Visit Virginia Cooperative Extension.

Visit Alson H. Smith, Jr., Agricultural Research and Extension Center.