Grape disease management and shoot thinning

By Terence Bradshaw

I’m probably a tad late on getting this out for some growers. The growing season continues to be cool and wet, and grapevines at the UVM Horticulture Research & Education Center are showing about four to five inches of shoot growth. We are beginning to enter a critical time for disease management. Growers should consider applying a protective fungicide soon that has activity against phomopsis in particular, although anthracnose and, to a lesser degree, black rot may be active, especially in high inoculum vineyards. 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. Most growers would do best to cover early this week with a contact fungicide like mancozeb or captan.

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), LaCrescent, 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.

So, if you have removed all diseased wood and are ready to cover your vineyard for protection against phomopsis and anthracnose, the best choices is likely lime sulfur applied at two quarts per acre in sufficient water (25-30 gallons should do it) to wet the canopy. Lime sulfur is hot stuff: caustic, corrosive, and noxious. Use appropriate personal protective equipment and spray in cooler weather to reduce phytotoxicity. Powdered sulfur may also be a good choice, I would suggest 3-5 pounds per acre at this stage.

Organic growers should absolutely begin application of something at this stage in combination with keeping up sanitation of all dead or infected wood , rachises, and other grapevine debris- sulfur, one of the bicarbonate materials (e.g., Armicarb, Kaligreen, etc.), or a biofungicide like Serenade, Sonata, Regalia, or Double Nickel may be used, although there is little good efficacy data on that last class of materials.

I mentioned sanitation- now is a good time to get out and clean up the ‘nubs’ left at the ends of spurs after pruning that will die out and serve as reservoirs for phomopsis and other diseases. While you’re at it, this is an especially good time to thin shoots. Cold hardy grapes trained to a high-wire trellis and in good health can support about six shoots per foot of canopy; select the best developing shoots and break off the others now while they are easily breakable with your fingers. Maria Smith and Dr. Michela Centinari at Penn State recently wrote a good summary of shoot thinning available here. I suggest reading it on the deck this holiday weekend with a nice glass of wine, and getting out in the vineyard next week to set this year’s crop on the right track.

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