Disease control after spring freeze injury in grapes: Keep on top of Phomopsis

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Since the extent of injury due to the recent spring freeze is quite variable between vineyard sites, it is important to get an estimate of the number of live buds left before making any drastic decisions with respect to disease management. A more accurate estimate of yield will not be possible until after fruit set. In previous years, the crop turned out to be harvestable after growers had given up on it. However, this year it is not clear how well the vines will be able to compensate for damage incurred. To reduce the risk of Phomopsis in the meantime, it is important to maintain some level of disease control, especially if the vineyard has a history of this disease.

The most economical control would be to apply one or two sprays of an EBDC (Dithane, Penncozeb or Manzate) before bloom to keep the foliage and developing cluster rachises protected. Tank-mixing an EBDC at a reduced rate (e.g., at 2 lb/acre) with a phosphorous acid fungicide (e.g., ProPhyt or Phostrol at 1.5-2 pt/acre) provides both protectant and systemic activity against Phomopsis cane and leaf spot (the foliar phase of black rot will also be controlled by these fungicides). Furthermore, the systemic component maintains its efficacy during rainy periods as it is rainfast. Since Phomopsis spore production has been shown to peak in mid-May in rainy years, right now would be a good time to apply an EBDC fungicide or a tankmix of an EBDC + phosphorous acid fungicide for protection.

Phomopsis spores can be released during most rain events until about bunch closing. The amount of inoculum can be estimated from the number of lesions on current-season shoots and leaves by early-season scouting. The EBDCs are cost-effective materials for use prior to bloom, and Ziram can be used after bloom. Phosphorous acids are also very effective against Phomopsis and good against black rot. For growers that have already applied dormant sprays, you can expect a substantial reduction (roughly 30-50 percent in Phomopsis and black rot pressure). The only other sprays that may be needed are one or two EBDC sprays pre-bloom and an Abound spray at first postbloom. In most years, we have not seen a benefit in terms of Phomopsis control from sprays beyond the first post-bloom spray. Pristine is another fungicide that can be used at first postbloom; it is apparently safe to spray on Niagara grapes, but should not be applied to Concord grapes due to potential phytotoxicity. Alternatively, Elite + ProPhyt (or Phostrol) + Ziram provides the same spectrum of activity as Abound or Pristine.

During the bloom and post-bloom period, black rot and Phomopsis are the main cluster diseases to control if there is sufficient fruit to harvest, especially if the vineyard has a history of these diseases. Black rot control should be focused around bloom with the first and second post-bloom sprays being most important. There is generally no need to protect the fruit beyond the second postbloom spray, because the berries become naturally resistant to infection about four to five weeks after bloom. Remember that berry maturation occurs over a more protracted period in vineyards that were heavily damaged by the frost. Elite + Ziram or even Elite alone will suffice. Other options are other SIs (Nova, Orius) and strobilurins, such as Abound, Sovran, and Pristine.

Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.

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