Grape pest management, June 19

By Terence Bradshaw

June 19, 2019

It’s always impressive to me how fast grapevines can go from zero to full foliage in a matter of a couple of weeks. Vines at the UVM Horticulture Research & Education Center are generally at 16-24” shoot growth. We’re not quite at “Immediate Pre-Bloom” stage, but not far off, either. Now is a critical time for disease 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. It’s been a wet spring, although a bit drier since grape budbreak that earlier in the year. I expect diseases will be pretty significant problems for growers this year, so stay alert.

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.

We are seeing a bit of grape tumid gall (GTGM) in the UVM vineyard this year. This is an infrequent pest that causes visually striking but (usually) relatively insignificant damage to the vines, and management practices are not recommended against them. However, serious cases can affect fruit clusters, especially in La Crescent and Marquette, so any signs of them in clusters may warrant treatment. I discussed this pest in a post on June 8, 2017:

This insect is similar to phylloxera in that the larvae feeds on leaf tissue which responds by forming a protective gall around it. GTGM is different in that it also affects rachises and fruit, and when the galls become fully engorged, can look pretty dramatic.

Figure 1. Grape tumid gall on a fruit cluster in midsummer. UVM vineyard, 2009.

GTGM is an midge insect (small fly) that lays eggs on grapevines. Flies are ephemeral and only live a day or so; management should not be targeted at adults. Hatching larvae burrow into vine tissue and are thus protected from contact insecticides. In most vineyards, GTGM is considered a minor pest or even a curiosity as the galls can be quite drastic-looking. That said, I have been hearing reports of high levels of GTGM in vineyard this year, and some of these vineyards reported them last year under different management, so I suspect that certain vineyards have increasing and potentially problematic populations.

Figure 2. GTGM on Marquette, UVM vineyard, 2010..

Vineyards should be scouted before assuming you have a problem with this pest. There is no threshold established, as most growers tolerate low levels of damage. However, I have heard reports of 50% or more leaves affected in some vineyards, and I can’t imagine how that wouldn’t negatively affect vine productivity.

Figure 3. GTGM on inflorescence. UVM vineyard, 2011.

Sanitation is an important method of reducing GTGM populations and may be enough in low-pressure situations. Galls can be crushed if seen on leaves, or severely affected leaves removed and destroyed. In organic vineyards, as always, this should be the first line of defense since spray options are minimal. If the extent of damage is beyond physical removal, an application of Movento is the best (only) recommended option I can find. Movento is a systemic insecticide with a unique mode of action and is listed as posing low risk to wildlife, and as non-toxic to fish and birds. It is best used at first sign of galls, practically speaking, as scouting for hatching larvae (which hatch from microscopic eggs) is difficult under normal field conditions.

GTGM may be distinguished from phylloxera by the gall surface, which is smooth compared to phylloxera which is bumpy or warty. Movento is effective against phylloxera as well as GTGM and has a long residual control period, so may be a good material if either or both of those pests are common in your vineyard.

More information on GTGM may be found in this Cornell fact sheet:

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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