Vine growth is really ramping up in Vermont vineyards, which means that the next two months are critical to set the stage for a quality vintage for 2021. I often say that, once established, grapes are weeds, and I’ll stick to that assertion. However, there’s a difference between a plant that can grow profusely and managing it to provide optimal crop quality. It’s our jobs as farmers to manage the vines for our needs- that what makes us different from foragers of the wild grapes that indeed do grow like weeds all around us.
Top of mind right now should be two practices: shoot thinning and disease management. Shoot thinning is critical to channel the vine’s energy into an appropriate number of shoots to maintain vine balance, and to high quality shoots to ensure consistency of ripening. We typically target 4-6 shoots per foot of canopy. So, vines on six-foot spacing should have about 24-36 well-spaced shoots on them, assuming they have filled their space. Adjust that as needed- vigorous vines get more shoots, less vigorous ones fewer. Now, since they haven’t lignified, you can just pop them off with your fingers. Be sure to select the most consistent and healthy shoots to leave behind. On many cultivars, the secondary shoots, which will be behind in development and thus smaller than the primary shoots, may emerge from the same bud at the base of a more desirable shoot- break those off. Remember thought that breaking off shoots is likely to remove the basal buds that could form next year’s shoots, so you only want to break these off that are in a position where you do not want a renewal spur for next year. This is a great time to strip shoots off the trunk from the fruiting zone down to the trunk base, but leave a sucker or two at the bottom as a renewal option in case the trunk freezes out or is otherwise damaged. I explain this process in a video here (15 minutes, pardon the wind noise); my colleague at Cornell, Dr. Justine Vanden Heuvel, describes in more succinctly using Vinifera cultivars in slightly different training system here.
This is a typical time to start thinking about a spray program to manage disease. The primary disease of concern at this point is phomopsis, as rachis infection at this point in the season is may cause significant fruit loss at harvest. Anthracnose may also be active at this point , given the warm/hot weather are expecting later this week. Vineyards that have had recent problems with those diseases or organic growers using copper or other less-effective materials may consider treating this week; if you haven’t had major problems with those diseases, treatment can wait until the 5-8” growth stage as long as you are using a highly effective contact fungicide like mancozeb or captan.
As a reminder, a refreshed version of the Initial IPM Strategy for New Cold Climate Winegrape Growers is available at: https://www.uvm.edu/~orchard/fruit/pubs/Factsheets/UVMFRT004_initialIPMStrategy.pdf
Organic growers are in for a bit more work. The standard fungicides, copper and sulfur, have only fair efficacy against this disease at best, and in a couple of weeks when black rot becomes the next disease of concern, those materials will have even less efficacy against that disease. The first line of defense in an organic vineyard is a strict sanitation program. This includes removing all mummies still in the canopy (not dropping on the ground, but actually removing them from the vineyard) as well as any obviously diseased wood. Phomopsis and anthracnose both overwinter largely on infected wood in the canopy, and removing this wood during dormant pruning or now is essential to reducing disease pressure. Stubs left at the ends of spurs should now be removed since you can see where this year’s shoot growth will resume (at the developing shoot)- these stubs will die and may become infected with phomopsis this season (or were last season) .
Removing stubs at end of retained spurs.
It is worth noting that both copper and sulfur (including lime sulfur) can cause phytotoxicity on certain cultivars. Dr. Patty McManus summarized her research on copper and sulfur sensitivity in cold-hardy grapes in the 2/8/16 Northern Grapes newsletter, and I’ll summarize it to say that Brianna should receive no copper; and Frontenac (all types), La Crescent, Leon Millot, Marechal Foch, Marquette, and St. Croix should receive no more than 2-3 copper sprays per season. Save those for later when black rot and downy mildew become bigger concerns. Sulfur sensitivity was observed on several cultivars, and its use (including lime sulfur) is discouraged on Foch, Millot, Brianna, and Louise Swenson; with limited (2-3) applications suggested on LaCrescent and St. Croix.
Despite the rain we just received, most sites in Vermont are still running very dry. Established vines on most soils should be okay, but consider watering young vines. I’ll discuss soil fertility later this week.
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