June 17, 2020
I’ve visited some vineyards from the higher-elevation interior of the state to the Champlain Valley in the past week. Champlain Valley grapes are entering or even well-into bloom, upland and inland are around 5” shoot growth stage. Bothe are at key management points, so it’s best to spend some time in the field in the coming week. If it hasn’t been done, now is the time to thin your shoots out to 3-5 per foot of canopy length. That’s a pretty gross rule of thumb, it might be better to adjust down to a per acre target and go from there, especially if your vines aren’t completely linear. I’ve seen a couple of instances where established VSP / mid-wire cordon vines are being shifted to a top wire cordon training system. Those vines have plenty of vigor to hold a decent crop, but the canopy is in a middle-stage and shoots originate from all over the place. In that case, consider a target harvest per acre and count back from there. Day you want 3.5 tons, or 17,000 lbs per acre. You have 800 vines per acre on a 6 x 10 foot spacing. (Calculating vines per acre = 43,560 square feet/acre / (in-row spacing (ft) * between row spacing (ft) or 43,560/(6*10) = 796). That means you’ll want 21.25 clusters per vine. If your clusters average about 1/3 lb each, that’s 60 clusters to keep per vine. Then, you’d thin out shoots to select the most uniform and appropriately-spaced ones to get to that number. In many cases where vines give you two clusters per shoot, that would leave five shoots per foot of canopy (or 30 shoots on our six-foot vine).
Just as important as getting your shoots and cluster numbers in order is disease management. We’re blessed to be in such a dry spell during this critical period for disease, and I’ve seen remarkably little disease even on minimally-sprayed vines. But we’re in the prebloom and soon, postbloom window where all of the major grape diseases are primed to infect. Chances of rain are increasing as we move into the weekend and beyond, so it’s important to get things covered this week. Be careful spraying in hot weather, for two reasons. First, a properly-protected spray applicator (Tyvek suit, gloves, goggles, etc) is at risk for developing heat-related health problems, so spraying at dawn or even at night can help. Second, some of the more phytotoxic materials like captan, sulfur, and copper can cause more plant damage when applied just before a heat wave. Spray material choice is dependent on your management system, I recommend our Initial IPM Strategy for Grape Growers fact sheet in general, and specifically for non-organic growers. Regardless of system (organic or non-organic), this is the time to use your best tools available. For non-organic growers, that would be a protectat and single-site fungicide combination, like mancozeb or captan plus a strobilurin (e.g., Flint) or DMI (e.g., Rally) or SDHI (e.g., Aprovia). Note for that last part of the mix, there are also some good pre-mix combinations like Luna Experience (SDHI + DMI) and Pristine (Strobilurin + DMI) that can cover a lot of bases while reducing resistance development in the target pathogen.
For organic growers, I’d also consider a two-part mix. Regalia has been getting a lot of interest lately and it does appear to have good efficacy against downy and powdery mildews. It needs to be applied 1-2 days before infection, and requires sunlight to activate resistance in the plant- spraying the night before a wetting event won’t do it. Regalia can be mixed with sulfur or copper (more on those two later). Serenade is another biological product which is approved for organic production. Its mode of action is as a microbial antagonist to the pathogen, so it must get established and colonize susceptible tissue before wetting and infection events. Because of this, Serende is not compatible in a tank mix with sulfur or copper, but it could be rotated with them in separate sprays. Serenade has most efficacy (good but not excellent) against powdery and downy mildews. However, it does not protect against some key diseases that affect grapes during this window- anthracnose (especially important on Marquette), Phomopsis, and the most difficult to manage organically, black rot. For those diseases, copper has a bit more efficacy, so this is one spray that should include it, if the vines aren’t sensitive to it. Cold-hardy hybrids with considerable labrusca in their parentage (e.g., St Croix, Brianna, other Swenson varieties) are more sensitive to copper than some others. However, copper damage can be reduced by avoiding application during slow drying conditions (there goes my recommendation to spray at dawn…). As always, spraying should be about your fourth line of defense when managing grapes organically- first consider http://www.uvm.edu/~fruit/grapes/gr_ipm/RelativeDiseaseRatings2017.pdfsusceptibility; then strict weekly sanitation and removal of all diseased tissue; maintain a training system that ensures good air flow; and finally fit a spray program to help manage within that greater IPM (yes, organic systems are IPM too) program.
Finally, this is a good time to consider applying micronutrients to the vineyard. Boron is very often deficient in Vermont soils, and is needed for fruit development at bloom. The best way to assess need and amount to apply is with a recent petiole sample analysis (last year’s, although you could sample at bloom and make a quick correction). Without that, I feel safe recommending 1 lb per acre actual boron (5 lbs per acre Solubor, an OMRI-certified boron source with 20% B) applied in your spray water. The only caution I make there is that boron and water soluble packaging materials (like Rally bags) do not mix- you’ll gum up your sprayer and ruin your day. Apply those separately. Also, this is a pretty low rate of boron I’m recommending, as you can overshoot pretty easily so I do recommend a petiole analysis to tune your application rate before adding any more.
Finally, Wild grape bloom occurred on June 5 in South Burlington, which sets the clock for grape berry moth management. The first overwintering generation is rarely significant enough to warrant a management spray except in vineyards that have had extreme damage in previous years. Each generation requires about 820 degree days (base 47°F, or DDb47°F) to complete, and the first generation typically emerges around the time of wild grape bloom. So if we use June 5 as our ‘biofix’ and track DDb47°F, we can estimate the best time to treat for the more damaging later generations.
This is made easy by the Network for Environment and Weather Applications (NEWA) system, which I coordinate for Vermont. The network includes twelve on-site weather stations (all located at orchards) and six airports, and imports data in near real-time for use in pest models. It is free to use, and growers can locate the site nearest them and develop a best-guess of phomopsis, black rot, and downy mildew infection periods as well as a sense of the grape berry moth generational development.