Immediate prebloom in grapes

By Terence Bradshaw

Grape bloom is looming in many Vermont vineyards, and this is a critical time for disease and vine management. As we mention in our initial IPM strategy for winegrapes, all of the major diseases save for late-season fruit rots are sensitive to management right now. That means that fungicide applications should be made, using a material or materials with broad range of coverage against powdery mildew (PM), downy mildew (DM), black rot (BR), anthracnose (Ant), and that last bit of phomopsis (Ph) for the season. Generally, this means a combination of materials, including a protectant (mancozeb, most effective against Ph, BR, PM, or captan (Ph, DM)) plus a systemic or more narrow-spectrum material. Those may include Vivando / Quintec (PM only); a DMI material like myclobutanil or tebuconazole ((BR, PM); or a strobilurin (BR, PM, also excellent against botrytis so save until later in the season in July if you have issues with that disease). I’ll mention other materials with excellent efficacy against DM and botrytis later when those diseases are of greater relative concern.

For organic growers, be sure to maintain your copper and/or sulfur sprays, watch for phytotoxicity, and remove diseased leaves and clusters as soon as you see them. The good thing is that it has been relatively dry this season, so disease pressure should be manageable. That said, in this vulnerable period, spray coverage should be applied prior to expected wetting events.

Insect activity is usually pretty quiet at this time of the season. Keep an eye out for bloom on wild grapes, as that sets the clock for the degree day model used to time management of grape berry moth (GBM). We typically add BT (Dipel, Javelin, etc.) or another material specifically active against lepidopteran pests soon after bloom at the earliest, so there’s time before we consider managing for this pest. GBM isn’t always a problem in every Vermont vineyard, we’ll talk about scouting for that pest in an upcoming bulletin. While we’re on the subject of insects, I haven’t seen much / any grape tumid gallmaker (GTGM) at the Hort Farm vineyard this year, but it has been a somewhat unlikely yet injurious pest in local vineyards in the past couple of years. I do not want to get into recommending treatment for minor pests of likely little economic importance, but if yours is one of the handful of vineyards which as suffered from extensive damage in recent years, consider using Movento or Assail if you see the first stages of galling by overwintering midges as they are hatching out now. There are only a very few vineyards where I’m even suggesting treatment for this- if you don’t remember this pest and its’ damage, then don’t worry about it on your farm. For more information on GTGM check the link above or my previous bit on them here.

Horticulture: There are two good times to collect petiole samples to assess vine nutrient status, bloom and veraison. Generally, you should stick with whichever timing you have been using so that you may compare to past tests. Dr. Joe Fiola at University of Maryland has a good fact sheet on petiole sampling. We recommend the Cornell Nutrient Analysis Laboratory for petiole sampling, as they have extensive experience in providing nutrient recommendations for grapes. Should nutrient applications be needed, this is a good time to apply boron, magnesium, and nitrogen as they are needed during this period of rapid shoot and fruit growth.

Other activities that I don’t need to tell you about: shoot thinning can continue, but shoots aren’t lignified at the base enough to comb them, they’ll just break off. Keep the vineyard mowed to improve airflow, but a golf course mowing regime isn’t necessary unless that’s your aesthetic choice. Keep the in-row weeds down however you can, but most herbicides should be put away for now because of likelihood of vine damage.

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