Vineyard and Winery Information Series:
Vol. 17 No. 2, March-April 2002
Dr. Tony K. Wolf, Viticulture Extension Specialist
II.Increased threat of Pierce's Disease in Virginia?
Dr. Jeffrey Derr, Virginia Tech¹s weed specialist for fruit crops, recently mentioned that Surflan herbicide was still in short supply, following a disruption of production in 2001. He suggested that the comments on alternatives provided last year be repeated, as follows:
Surflan (oryzalin) is a pre-emergent herbicide registered for use in both young (including first year) and bearing vineyards. Prowl would be a good alternative to Surflan in nonbearing vineyards. Chemically, Prowl is very similar to Surflan and will provide similar control. Prowl, like Surflan, works best on annual grass and small seeded broadleaf weeds. The manufacturer has never registered Prowl for use on bearing vineyards. Prowl should be applied as a directed spray to dormant grape vines. Prowl can stunt growth and cause abnormal leaves to form if sprayed over the top of plants after budbreak. Another substitute for Surflan would be Solicam, but only if the vines have been in the ground for 2 seasons. Watch the rate of application on sandy soils. Vines growing in heavier soils can tolerate higher rates of Solicam. The injury symptom from Solicam is bleaching of foliage, which is more likely to occur in sandy soils. Plants will outgrow any bleaching that occurs. Solicam works well in combination with Princep. Solicam controls annual grasses and certain annual broadleaf weeds, and will suppress nutsedge. Combining it with Princep will provide better broadleaf control.
ps: At the Nelson County meeting on 3 April, Dr. Derr mentioned that FarmSaver (http://www.farmsaver.com/products.asp) has filed application to distribute Oryzalin 4 AS in Virginia. This is essentially the same product as Surflan (Oryzalin is the active ingredient of Surflan, the patent for which has expired). A check of FarmSaver's website shows that Oryzalin is now available in Virginia, and can apparently be ordered directly from their web site at about $40/gallon. It'd be worth checking the cost of application against Prowl before you buy.
Question from the field: A northern Virginia grower posed the following comments and question to Dr. Greg Evanylo, Agronomist with Virginia Tech. Both agreed to my reprinting the question (Q) and Dr. Evanylo's response (A). Many of you met Dr. Evanylo during the course of the 14-16 February Technical Program in Charlottesville, where he spoke on the benefits of compost application to horticultural crops, including grapevines.
Q: I enjoyed your talk at the VVA seminar in Charlottesville this past February. I have an established vineyard and am planning to try compost this year on a new planting as well as some of my six-year-old vines. A comment you made about the addition of gypsum to raise the pH level of our acidic subsoil's sparked my curiosity. I have gone on line and found some information about the use of gypsum used in conjunction with lime to help raise soil pH. The pH of my subsoil is about 5.3 to 5.4. Could you make a specific recommendation for the addition of gypsum and lime to raise this subsoil pH to as close to 6.0 or higher as I can get? If this process works, how often would it need to be repeated? I have long felt that the grape growing community has neglected research on our soils and the relationship they have with our vines. It seems that many people feel that the soil is just a place to anchor a vine and that if we just fertilize each year and have good drainage that should be enough. The relationship of the soil and its biotic life is almost completely ignored.
A: Firstly, allow me to state that amelioration of subsoil acidity with gypsum is only beneficial if the amount of acidity in the subsoil is truly reducing crop vigor, yield, and quality of fruit. I am not a grape expert, so I can't comment on the plant's ability to withstand subsoil acidity, but a pH of 5.3-5.4 is not bad. Aluminum (Al) toxicity (the major problem in low pH soils) begins to become limiting as the soil pH drops below 5.2.
Some background info on the chemistry of limestone, gypsum, and acid subsoils follows:
1. Limestone (calcitic = CaCO3; dolomitic = CaMg[CO3]2) raises soil pH by disassociating in the soil solution to produce base (OH-) which neutralizes acidity (H+). [Ca=calcium,Mg=magnesium, SO4=sulfate, CO3=carbonate, OH=hydroxyl, and H=hydrogen proton.] Unfortunately, limestone is slowly soluble and only reacts upon contact with acidity. Therefore, once limestone reacts with and neutralizes the acidity in the surface soil horizon in which it is placed, it ceases reacting and only moves into the subsoil to neutralize acidity slowly. The quickest way to get limestone into the subsoil is to place it deeply into the soil by means of subsoil tillage.
2. Gypsum (CaSO4*2H2O) is a neutral salt, which means it produces neither acidity nor base. The process by which gypsum can ameliorate subsoil acidity involves its interaction with the chemical and mineralogical constituents that are inherent in Virginia subsoils (and other states in the southeast whose soils are highly weathered and acid). These soils are characterized by increasing clay concentrations that are dominated by soluble Al (aluminum). These soils also often have a sizeable content of hydrous oxides (with chemically bound OH-). Gypsum disassociates into Ca2+ and (SO4)2-, which are soluble and can move vertically through the soil profile to the subsoil, where the high acidity resides. Al3+ is the major cause of acidity in our soils because it reacts with H2O to form a stable mineral (Al[OH]3) and 3H+ (acid). The Ca in gypsum can replace Al3+ from the cation exchange sites of acid soil, and the (SO4)2- can replace the OH- from the positively charged hydrous oxides. Al3+ and the 3OH- then form an insoluble mineral (Al[OH]3) that eliminates the source of acidity (Al3+) in soil solution.
Research by Farina and Channon, published in 1988, demonstrated that a one-time application of 4.5 tons of gypsum/acre to corn fields progressively (i.e., with time) increased subsoil pH, Ca, Mg, and SO4 and decreased Al concentrations. The beneficial effects (cumulative corn grain yield increase of 54 bushels/acre) weren't fully observed until the fourth season after application. The benefits appear to be derived from the increase in root proliferation into the formerly acid subsoil with a concomitant increase in water uptake (and drought stress reduction). This has been demonstrated by many others. The pH of subsoils in which acidity was limiting growth and yield ranged from 4.2 to 4.8.
The required rates of gypsum application for subsoil acidity amelioration is variable, depending on desired depth to affect and the amount of acidity to be neutralized; however, benefits have been observed from application rates as low as 0.75-1.5 tons/acre. There doesn't appear to be any reason to exceed 5 tons/acre.
One drawback to high gypsum rate applications is that the high concentrations of Ca can replace and push other essential cations (i.e., Mg, K) through the soil out of the immediate zone of most efficient nutrient uptake. This would require replacement of these nutrient elements following gypsum treatment. If you are certain that your subsoils are reducing grape production because of low pH (i.e., < 5.2) in the depth between 12 and 42 inches (and I'm not certain that they are), you might overcome this problem by applying 1 to 2 tons gypsum/acre. Gypsum should be broadcast, not banded, because vine roots will extend into the inter-row area. Apply the gypsum at the end of a growing season (October) and permit the winter rainfall to move the amendment into the soil. You should then check soil Mg and K each of the next two seasons and make appropriate supplemental applications to prevent insufficiencies of these nutrients. You can monitor the effects of the gypsum by measuring pH of the subsoil with time.
I see no reason to raise the pH of your subsoil to 6.0. Most plants will do fine at a pH of 5.5 in the topsoil as long as nutrient concentrations are adequate. A high concentration of H+ (which pH measures) is only toxic at very low pH's (i.e., below 4). It is the increased solubility of Al (and sometimes, Mn) at low pH that causes plant growth reduction. Al and Mn are natural constituents of soils. In fact, Al comprises about 7 to 10% of all soils. Very sandy soils with very little clay (a source of Al) and/or soils having a high concentration of organic matter can support good plant growth at lower pH's than low organic matter, high clay soils in the south. The problem with soils whose pH is less than 6.0 is probably due to factors other than pH (e.g., actual P, Ca and Mg concentrations) because an adequate pH is usually maintained with applications of limestone (a source of Ca and Mg).
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Briefly, PD is caused by a bacterium (Xyella fastidiosa), which is transmitted from vine-to-vine and from alternative host plants to vines principally by leafhopper insects. The bacteria are limited to the water-conducting xylem tissues of the plant, hence the Genus Xylella. In disease-susceptible vines, the bacteria can multiple to such levels that they cause vascular blockage, either directly or via defensive gums produced by the vine. The vascular blockage ultimately leads to the characteristic disease symptoms - leaf scorching, wilting, defoliation - all evidence of reduced water transport. PD is often lethal to the vine and control is difficult to achieve. Attempts to eliminate the vectors with insecticides are generally ineffective because the vectors are quite mobile and their rates of transmission are high. Some control can be provided by minimizing the amount of alternative host vegetation within and around the vineyard. Some control can also be achieved by choice of grape variety to grow in regions at risk of PD. Among Vitis vinifera, Chardonnay is highly susceptible to PD, while Chenin blanc is more resistant to disease symptoms. The muscadines (Muscadinia rotundifolia) are not immune to PD, but appear to tolerate the disease. Hybrids and other native American grapes, such as Vitis aestivalis (e.g., 'Norton') may have variable tolerance, but this has not been fully evaluated in Virginia. The interested reader may find much more information about PD in the popular press (e.g., Purcell, 1993), on the web (www.cnr.berkeley.edu/xylella) and in the primary research literature (e.g., Purcell, 1977).
Virginia, North Carolina, and possibly Maryland are frontier states in terms of the northern distribution of PD in the eastern USA. Historically, the actual 'frontline' of PD appearance in Virginia has corresponded very well to a theoretical winter isotherm that has been proposed as defining risk severity. By way of background, cold winter temperatures are thought to limit the extent of bacterial development within the vine, such that areas with "cold" winters are less at risk of PD than are areas that enjoy "warm" winters. How cold works is not precisely known, but it is believed to be related to the bacteria's lack of survival or multiplication in the vine rather than having a direct effect on vectors. Recent research from Dr. Alexander Purcell's lab (Fell and Purcell, 2001) has shown that the PD bacterial population within grapevines decreases as vines are held at about 41°F. Dr. Purcell has proposed that areas that have an average minimum January temperature of 30°F or less, are much less at risk of PD than are areas that have warmer winter temperatures (http://www.CNR.Berkeley.EDU/xylella/). The isotherm is obtained by taking the minimum temperature for each day of January, and then deriving a single average from those 31 values. If we look at the historical, 30-year January average minimum isotherm (Figure 1), we see that our documented cases of PD fall to the south (warmer) of this line in Virginia, while areas to the north and west (colder) have remained apparently free of the disease. It is important to emphasize here that the 30°F isotherm of Figure 1 is based on a 30-year record. Ominously, if we recalculate the 30°F isotherm based on just the last five years' data (Figure 2), the isotherm has moved significantly further north - a reflection of the warmer winters that we've enjoyed since 1996. If we accept the relevance of the 30ƒF isotherm, we should take a very cautious approach to commercial viticulture in a large portion of Virginia's Tidewater and southside, as well as a much greater portion of North Carolina. In the latter case, data were presented at a meeting in Asheville NC this past January by staff at North Carolina State University that revealed PD-positive grapevines in north-central North Carolina, just below the Virginia border. The threat to "new" areas of Virginia is tangible.
What's in the future? Firstly, we have growers in documented PD-positive areas of Virginia who continue to profitably grow vines; however, their experiences might reinforce the notion that prudent variety choice (e.g., minimizing Chardonnay), and aggressive management of alternative host plants are keys to sustaining profitability. Nevertheless, these growers suffer increased costs and loss of production when infected vines die.
Figure 1. Average January minimum of 30ƒF isotherm based on 30-year (1970-2000) record.
Secondly, it's difficult to fathom whether the recent spate of warm winters is a long-term trend or just a brief anomaly. Taken together, the risks of PD should temper the enthusiasm for viticulture based on sensitive varieties along Virginia's southern border, as far west as the Blue Ridge Mountains. On the bright side, we will all undoubtably gain from the intensive PD research programs that have been implemented in California and elsewhere as a result of the introduction of the glassy-winged sharpshooter into California.
Figure 2. Average January minimum of 30°F isotherm based on the last 5 winters (1997-2001).
Fell, H, and Purcell, A.H. 2001. Plant Dis 85:1230-1234.
Purcell, A. H. 1977.Plant Disease Reporter 61:514-518.
Purcell, A. 1993. Practical Winery and Vineyard, March-April, p 13-16, and May-June, p 50+.
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10 April Vineyard meeting: Barboursville Vineyards. 11:00 - 1:00 pm
8 May Vineyard meeting: Dexter Vineyard, Roseland VA-Nelson Co. 11:00 - 1:00 pm
22 May Vineyard meeting: Naked Mountain Vineyard. 11:00 - 1:00 pm
29 May Vineyard meeting: Thomas Vineyard, Clifford, VA - Amherst Co. 11:00 - 1:00 pm
11 June New Grower Workshop: Farm and Home Center, Lancaster, PA. An intensive one-day session covering economics, vineyard development, rootstocks, varieties, equipment andmuch more. Taught by Dr. Joe Fiola (Univ. MD Coop Ext), Dr. Tony Wolf (VA Tech U) and Mark Chien (Penn State). Information and registration can be obtained at http://winegrape.cas.psu.edu/calendar/calendar_pa.html
12 June Vineyard meeting: Horton Vineyard (town of Orange). 11:00 - 1:00 pm
22 June Vineyard Establishment class at Linden Vineyard in Linden, VA. Topics include variety and site selection, planting, trellis, early vineyard care and economics. Classes cost $75 per person and run from 10:30 to 4. Space is limited, pre-registration is required. Call 540 364-1997 or email@example.com.
23 June Vineyard Canopy Management class at Linden Vineyards in Linden, VA. Topics include training systems, basic vine physiology, managing vigor and quality and basic vine nutrition. Same details as previous listing.
26-28 June American Society for Enology and Viticulture Annual Meeting. Portland, Oregon. This is a fine technical meeting with many papers from viticulture and enology researchers around the world. A large trade show is also part of the event. You can find information at http://www.asev.org/
10-12 July ASEV Eastern Section American Society for Enology and Viticulture 2002 Conference, Baltimore, Maryland. Conference focus: Red Wine Varieties for the East: Merlot, Cabernet Franc, Syrah, and Chambourcin - viticultural and enological aspects of four red varieties that have become the backbone of the East. Feature presentations of international experts and commercial and academic specialists from the US, including tasting of representative wines. The conference will be held at the Sheraton Baltimore North, 903 Dulaney Valley Road, Towson, Maryland. Reservations: (410) 321-7400 or visit the hotel website at www.sheraton.com/baltimore. For conference registration and information visit the ASEV Eastern Section website at www.nysaes.cornell.edu/fst/asev or contact: Ellen Harkness, 1160 Food Science Building, Purdue University, West Lafayette, IN 47907, ph: (765) 494-6704., E-mail: firstname.lastname@example.org.
7 August Vienyard meeting: Christensen Ridge Vineyard and Winery. 11:00 - 1:00 pm
7-8 August Third Annual Eastern Pinot Noir Conference. Finger Lakes region. A technical celebration of this greatest of red wine grape varieties. Technical meeting and tasting. If you are a commercial grower or vintner of Pinot Noir, please attend. Contact Mark Chien at 717 394-6851.
21 August Vineyard Meeting: Whitehall Vineyards, Whitehall, VA-Albemarle Co. 11:00 - 1:00 pm
11 Sept Vineyard meeting: Rush River Vineyard. 11:00 - 1:00 pm
18 Sept Vineyard meeting: Wintergreen Winery. 11:00 - 1:00 pm
If you are a person with a disability and desire any assistive devices, services or other accommodations to participate in this activity, please contact the AHS Agricultural Research and Extension Center, at 540-869-2560 during business hours of 7:30 am to 4:00 pm weekdays, to discuss your needs at least 7 days prior to the event.
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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: email@example.com
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