Vineyard management, June 25

By Terence Bradshaw

Things haven’t changed all that much in Vermont vineyards since the last posting here. Despite the showers over this past weekend and a pretty good rain last Monday, the predominant weather feature has been dryness. That’s good for most diseases, and as we shift into the postbloom period, we need to also shift our thinking of the predominant fungal diseases that are likely to affect Vermont vineyards. Phomopsis (PH) is becoming less of an issue with each passing day, and leaves and fruit will increase their resistance to black rot (BR) in the next few weeks. Downy mildew (DM) is increasing in importance, and this past wetting event is likely to have triggered infection. Botrytis (Bot) will be an issue soon, and powdery mildew (PM), which needs only high humidity and not necessarily rain (unlike most fungal diseases) will continue to affect unprotected vineyards through the summer and into harvest.

This has been a good season to stretch fungicide coverage because of the lack of rain (or to get away with more lackadaisical management than a wetter year would allow), but it is important to keep vigilant as we enter the heat of summer. This is the time that I generally suggest growers shift over to fungicides targeting PM, DM, and (less importantly, unless it’s really wet or your vines are a tangled mess or in a foggy hollow) Bot. This may mean using multiple materials if necessary. Captan is a pretty solid material to use to cover the emerging DM and the waning but still important BR and PH. Addition of a strobilurin (e.g., Flint) or DMI (e.g., Rally) would give great protection against BR and PH as well as PM. For organic growers, copper is probably your best bet now, as it is fairly effective against DM and PM.

I’ve received a few questions about potassium fertilization, which is commonly needed in bearing Vermont vineyards and is appropriate to apply now. If vineyards show deficiencies in both potassium and magnesium, then sulfate of potash magnesia (Sul-Po-Mag) is a great material that is available in both organic and non-organic forms. If potassium is needed (based on petiole and soil tests) but not magnesium, potassium sulfate is also available in both organic and non-organic forms. There is a great section in Appendix A of the Wine Grape Production Guide for Eastern North America (which every grower should have a copy of on the bookshelf, ready to grab with short reach) which helps to guide fertilizer application rates based on soil and petiole samples. If you have a copy of those to send to me, I’d be happy to help you with the numbers as well.

I think it’s still a bit too early to do much canopy management in the vineyards, as the bases of shoots are still too tender to hold up to much wrangling without breaking off. But plan on combing and shoot positioning soon, around the July 4 holiday.

Finally, for anyone looking for stimulating discussion this week, I’ll be part of a public panel with a number of my colleagues discussing Farming Practices and Ideologies this Tuesday at the UVM Davis Center. Information can be found at: https://www.facebook.com/events/187470125292645/. There is no fee for admission and we expect a lively discussion amongst the panel and the audience, so I encourage the Vermont orchard community to participate and weigh in. I’ve included specific information on the panel in another posting.

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