Vineyard and Winery Information Series:
Vol. 20 No. 4, July-August 2005
Dr. Tony K. Wolf, Viticulture Extension Specialist
August is often our slowest summer month in vineyard management, and it's a good time to re-group and look ahead at harvest and post-harvest activities. 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 °Brix will essentially represent final weight, with some variation due to rain or moisture deficits. 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. Given a forecast of increased tropical storm activity this year, we reduced crop levels on our VSP-trained vines from around 5.0 tons per acre to about 3.5 tons per acre in our training system study at Winchester (somewhat higher crops on the Smart-Dyson and GDC-trained vines). Will the lighter crops ripen faster than would heavier crops? In reality, if the leaf area to crop ratio is adequate (above 15 cm2 per gram of crop), there may be no perceptible difference in the rate of fruit maturation on high vs. lower-cropped vines, when thinning is done post-veraison. Differences might be more apparent with young or stressed vines.
Pest management: If you've done a good job with powdery mildew (PM ) control, you can "coast" through harvest; if not, you may still have a fight on your hands (see the following article on dealing with powdery mildew outbreaks). 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, 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 delay of sugar accumulation in treated fruit due to temporarily depressed 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 or Vangard. Pristine is also reported to offer some botrytis protection if used at the higher label rate (8 to 12.5 oz/acre). 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 or mid-October, whichever is later. Fungicide options once you are within 66 days of harvest, are captan, Abound **, Pristine and the phosphorous acid compounds such as ProPhyt and Aliette. 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. Although effective against both downy and powdery mildew, I would tend not to use Abound at this point in the season for reasons of cost and for conservation for the more critical timing around bloomtime. Mancozeb (e.g., Penncozeb, Manzate, etc.) would be an option on young or otherwise non-fruiting vines, or as a post-harvest treatment on vines harvested early in the fall.
** We have some concerns about possible resistance of downy mildew to strobilurin fungicides. They would not be my first choice during weather that is favorable for down mildew development. More will follow on this.
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.
Ground cover management: We have used under-trellis ground covers Ð either natural weed development or intentionally-planted grasses Ð in the past two years to impose some stress on vines in mid- to late-summer in the hopes of suppressing vegetative growth. To be clear, this is being done where vine size is large and continued, late-season (post-veraison) shoot growth is a normal feature. It is not being done with young vines or those whose vigor is already insufficient. Several commercial vineyards are also conducting under-trellis ground cover trials and the results are Ð mixed. They do appear to restrict vine vigor, which is the intent. The downside can be excessive vine growth suppression, so careful management and a willingness to burn back the ground cover with contact herbicide is needed. Careful monitoring of vine nutrition and potential insect and mite pests is also necessary. There is good reason to suspect that under-trellis vegetation may increase problems with climbing cutworms in the spring, grape root borer, and possibly grapevine yellows disease. And, the reductions in vine vigor must translate to increased fruit or wine quality for under-trellis ground covers to be a justified management tool. With those caveats, there may be some merit in lightening up on your late-season, under-trellis weed control. In one vineyard block, we simply let the annual crabgrass colonize the under-trellis area. The seed heads are not that tall and the grass forms a thick mat on the soil surface. On our more vigorous Cabernet Sauvignon and Chardonnay site, we have sown creeping red fescue under the trellis. Again, the preliminary results are encouraging but more experience and research data will be needed to fully understand the implications of this tool.
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Hopefully most vineyardists are awaiting harvest with clean fruit hanging in the vineyard. There have, however, been a few reports of powdery mildew (PM) sporulation on grape clusters in our region, as there are in most years. Although full blown infections of PM are not reversible, immediate action at the first sign of PM spores could reduce the chance of total crop losses. Clusters become resistant to PM infection when soluble solids reach about 8 (Brix) or greater, thus the focus may be on red grapes for this time of season. Use of strobilurins (Flint, Sovran, Pristine) or sterol inhibitors (Nova, Elite, Rubigan) is common for prevention of PM but they should NOT be used on existing infections. High spore populations may increase the development of resistant PM strains. A few alternative options are covered here.
Canopy Management: Removal of heavily infected clusters is recommended prior to treating infected vines. It would not be possible to remove infected leaves, however, leaf pulling around the clusters prior to spray treatments will enhance coverage on uninfected clusters. Action should be taken to eradicate infections as soon as possible. It is not possible to reverse the damage of an existing infection. It is possible to reduce powdery mildew spores produced by the current infection and thus reduce spread of the disease within a vineyard.
*Sulfur: A high rate of sulfur is required in order to eradicate spores and slow the spread of PM in vines. Read the label to determine the "cut off" point for high rates of sulfur. Some rates are as high as 10 lbs/A, but such rates may be VERY RISKY in mid-summer heat (90 F or greater). An upper limit of 6 lbs/A would be safer, but once again, check the label of your sulfur source as formulations may vary. As a reminder, do not apply sulfur above 85 F. High humidity will further increase risk of sulfur phytotoxicity. Also check the temperature forecast for the following 3 days after your anticipated application since high temperatures may affect volatilization of sulfur even if temperatures were below 85 F at the time of application.
Sulfur alternatives/tank mixes: Nutrol (monopotassium phosphate) will also eradicate PM spores. Nutrol, however, is not very effective against full blown infections. What percentage of your clusters are infected? OxiDate (hydrogen dioxide) will eradicate spores and must be applied "back-to-back" at the labeled high rate requiring multiple passes through the vineyard (1-3 consecutive for eradication followed by 5-7 day intervals). OxiDate may be tank-mixed with DF or liquid sulfur according to the manufacturer (BioSafe Systems). Some growers have reported using Armicarb 100 or Kaligreen (potassium bicarbonate) at the high rate (tank mixed with sulfur), though Armicarb may be more expensive than Nutrol. It is not clear if either Armicarb 100 or Kaligreen are more effective than Nutrol. Do not tank mix Nutrol with OxiDate. Do not exceed a rate of 2.0% Nutrol. These products do not offer forward protection (eradicant only). Not all of these products are compatible with copper.
Horticultural oil: If you have already sprayed sulfur, you will need to wait 2 weeks until it would be safe to apply a horticultural oil, such as JMS Stylet oil. You would also not be able to apply sulfur again for 2 weeks after an oil spray. Please note these time intervals between oil and sulfur sprays in order to avoid phytotoxicity. JMS Stylet oil has shown to be very effective for PM spore eradication at 1.5-2.0% applied in 10-14 day intervals (Penn State). The oil acts as a barrier preventing infection by spores thus good coverage is essential for oils to control PM outbreaks. JMS stylet oil also provides up to 4 days of forward protection (Nutrol, Armicarb 100, Kaligreen and Oxidate do not provide forward protection).
Options: Rotate OxiDate into your current sulfur spray program. Sulfur sprays could continue at high rates if temperature and PHI (*for wine quality) permit. Addition of Armicarb as a tank mix may enhance the effect of the sulfur. Is the powdery continuing to spread? If sulfur is not an option, a switch could be made to Stylet oil 14 days after the last sulfur application (see above). Oxidate would be a safe option during the transition period between sulfur and oil. Be aware that Stylet oil will slightly reduce photosynthesis (thus slightly reduce Brix) and may not be suitable for post-veraison use on varieties that ripen late or do not consistently ripen on a given site. Remember that consistent scouting and early detection is essential to prevent further infections by PM.
*Note on sulfur use: Using sulfur 4-6 weeks prior to harvest may increase H2S concentrations in wine. The following is a quote from the July-August 1998 issue of Dr. Bruce Zoecklein's newsletter "Enology Notes" "Volatile sulfur containing compounds are known to impart distinctive aromas to wines such as rubbery, skunky, or like onion, garlic, cabbage, kerosene, etc. The objectionable odor of hydrogen sulfide, generally described as rotten-egg-like, also has been observed. If no correction is made, hydrogen sulfide may undergo reactions with other wine components to yield mercaptans, which can have detrimental effects on wine palatability and may be difficult to remove. Only 5 mg/L of elemental sulfur in the must is enough to produce H2S concentrations which cannot be removed. Therefore, sulfur sprays should not occur less than 35 days prior to harvest." http://www.vtwines.info/
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Growers in Loudoun County know that Jason Murray resigned his Cooperative Extension position and is now working as a vineyard consultant and as a vineyard manager for a commercial vineyard in Fauquier County. Loudoun County is advertising to refill the horticulture position vacated by Mr. Murray.
We are currently reviewing candidates for our viticulture pathology position to be based in Winchester and hope to have the selected candidate on board early this fall. This position was authorized as a result of the Commonwealth Staffing Initiative (CSI) begun in 2003 and partially funded by the 2005 General Assembly.
Also at the state level, Sharron Quisenberry, Dean of the College of Agriculture and Life Sciences at Virginia Tech, recently announced that Dr. Mark McCann was appointed interim Director of Virginia Cooperative Extension. Dr. McCann was previously department head of Animal and Poultry Sciences at Virginia Tech. Dean Quisenberry is also hosting a grape and wine industry "listening session" in Charlottesville at the end of August. This meeting is intended to provide a forum for wine and grape industry members to express their needs from the College and from Virginia Cooperative Extension. If you are interested in attending the session on August 31, please contact Tony Wolf (firstname.lastname@example.org or call 540-869-2560 x18).
Best wishes to readers for a stress-free harvest.
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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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