It’s cooling off for a bit, time to think about activities in the vineyard. Thankfully, extreme heat isn’t good for most grape diseases, and the dry weather hasn’t been good for them either. However, berries are sizing up and bunch closure will be here before we know it, so we should consider some upcoming potential issues that might affect fruit quality at harvest.
First is grape berry moth (GBM). This moth lays eggs in developing clusters which hatch and infest fruit, causing direct damage as well as allowing rots to develop. Now is the time to start scouting 50-100 randomly selected clusters from throughout the vineyard and inspect for the characteristic webbing of GBM larvae, six percent infested clusters indicates a need to treat. If sprays are necessary to manage them, they need to be applied before the clusters close up. Bt is effective against GBM, as is Intrepid, Delegate (also good against spotted wing drosophila, more on that next week), Entrust (SWD as well, and organic), Sevin (good against Japanese beetles), and Avaunt (also JB). Notice I’m not recommending pyrethroids like Danitol, as they tend to be harsh on predators and may increase mite populations as a result.
Next is Botrytis. This disease is increased by prolonged wet periods and moderate temperatures (60-75° F), not what we’ve had recently. Still, if you have tight clustered cultivars (anyone still growing Vignoles?) or a history of the disease, an appropriate material may be called for if a long rainy week ever appears in the forecast. Downy mildew can infest leaves and clusters, and, like with botrytis, an effective material may need to be applied ahead of any rainy period that may come up in the next few weeks. Finally, keep an eye out for powdery mildew on foliage. If you’re relatively clean, this disease may not pose a problem for the rest of the season, but if there is incidence of the disease, it may spread given appropriate weather.
All diseases will be much better-managed with an open canopy, and your fruit quality will increase as well. Soon, if not already, the bases of shoots will lignify (get ‘woody’) and will be tough enough to comb shoots downward and separate them in order to expose fruit to the sun. This is an absolutely critical practice to attain highest quality fruit, as well as to allow for sufficient hardening off of shoots and other tissues heading into next winter. If the heat continues too much, I would avoid pulling leaves around the clusters at this time, the combing should expose them enough to develop some thicker cuticles on berries that will resist sunburn. A final leaf pulling can be debone a few weeks before veraison.
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