Midsummer vineyard management

By Terence Bradshaw

Sorry grape growers, you just received my comments on orchard activities for the immediate time being. The good thing, as my predecessor Dr. Lorraine Berkett used to say when discussion apples and grapes, is that it’s the same story with different characters. So, here we go with Act One:

Weather has been as dry in vineyards as it has in orchards, although some sites got a good soaker on Monday. We are expecting rain next week, and the fruit clusters are at ‘berry touch’ stage and we’ll soon see bunch closure when it will be more difficult to get protective materials into clusters. The main diseases of concern are spot outbreaks of powdery mildew, downy mildew, and possibly botrytis as we approach veraison/harvest. The good thing is that primary infection of all of these were low, so there’s little secondary inoculum out there to cause problems. However, good IPM suggests not only reducing pesticide applications unless the biology says they are absolutely necessary, but also targeting critical management points so that diseases may be managed “one and done” and not have to fight them longer through the season. I’ll summarize that point later.

Our main insect of concern is grape berry moth (GBM), which is also difficult to manage after bunch closure. The threshold for this pest is evidence of webbing between berries on 6% of inspected clusters, but I would be a little more conservative if that window is due to close. Bt is an effective and low-impact material for use against GBM, as are Delegate, Altacor, and other lepidopteran-oriented materials.

Japanese beetles are a concern for young vines, they can be managed with numerous materials including carbaryl, Assail, and Avaunt; or Surround and Pyganic for organic growers. Note that those are more broad-spectrum materials than the ones I suggested for GBM, and I generally steer clear of them unless necessary. Really, most vineyards can tolerate any Japanese beetles that they get.

Spraying summary- I’d lean toward applying something in the next 7-10 days against GBM if you’re seeing damage, and consider a downy mildew fungicide at the same time. That might be it for the season unless things get wet and we’re seeing the mildews (downy and powdery) showing up on foliage.

Canopy management: this should be priority #1 now, get those shoots combed first, but don’t really worry about leaf removal given the high amount of sunlight we’ve been getting. This is also a time to think about final crop load, I’ve received a question or two about estimating the crop to plan harvest or to help with dropping excess crop. The concept here is that you can fairly easily guesstimate the potential crop by knowing three variables: average number of clusters per vine, average cluster weight at harvest, and actual vines per acre. This is when past records are helpful for determining average cluster weight at your vineyard. If you don’t have them, I’ve used numbers from Cochran and Smiley’s Review of Cold Climate Grape Cultivars in the past, but I’ve also way overestimated doing that. Another estimate that is used is the lag phase method, which assumes that grape clusters are about half their final weight during the midsummer window when berry growth slows. Despite its use by Oregon Pinot Noir growers, the timing of the lag phase is difficult to determine on newer cold climate cultivars and so I don’t recommend using it for estimating crop yields unless you have good empirical data from your vineyard. Ohio state has a nice guide with appropriate formulas and worksheets for crop estimation.

This is a good time to apply potassium and/or magnesium fertilizers, based ideally on petiole and soil analyses from the last year or two.

Coming up: veraison and petiole analysis…

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