Prebloom disease management and frost prevention (?)

May 28, 2015

by Terence Bradshaw

Grapes in the Champlain Valley will soon be approaching the ‘immediate prebloom’ stage which is a critical time for management of multiple disease, including black rot, downy mildew, powdery mildew, phomopsis, and anthracnose. Spray applications may be made any time now to help prevent these diseases. Remember, you cannot generally treat these diseases successfully after you see symptoms. A combined application of an EBDC fungicide (mancozeb, polyram, etc.) and a sterol-inhibitor (Rally, Procure) at label rates will help to manage these diseases at this critical time.

For organic growers, the options are much more limited: copper products have a small amount of efficacy against black rot but are potentially phytotoxic (cause damage to the plants), require multiple applications season-long, and are a heavy metal that may accumulate in the soil. Sulfur products also may have phytotoxicity issues but are generally effective against powdery mildew. If using an organic program, strict vineyard sanitation is absolutely critical. This means removal of all overwintering berry mummies from the canopy and burying or burning them. Dead or otherwise infected wood also must be removed from the canopy to reduce phomopsis inoculum.

Other vineyard activities that should be performed at this time: shoot thinning to 3-5 shoots per foot of cordon on healthy vines, weed control (avoid herbicide application to bases of vines with leafed-out renewal shoots), tying vines and trellising.

There has been some recent discussion among growers about frost control measures after the May 22 cold snap that affected some vineyards. Questions about irrigating for frost control came up, and I’ll give my take on the subject: it is rarely worth it. Unless the irrigation system is carefully designed specifically for frost control (i.e. capable of outputting sufficient water to provide protection to the whole canopy), is run during the entire freezing event, and conditions such as low dew point or wind do prevent effective heat release from forming ice, then the significant effort likely won’t pay off and may cause even more damage than doing nothing at all. Frost fans are more commonly used in larger production regions, but they are very expensive and require their own specific conditions to be effective. I’ll echo comments made by others on the matter: the best frost control is good site selection, followed by good vine management. Row covers may be effective in mitigating frost conditions, but have their own infrastructure needs particularly a wire suspended above the canopy at 7-8 feet on which to hang the cover, and significant labor to apply and remove. I do not have experience to suggest a fabric type that would work best.

Hoping your vineyards are frost and disease-free,