Disease control after spring freeze injury in grapes: What are the options?

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.

It is unfortunate that the Michigan grape industry has been hit with widespread freeze injury once again. In addition, there was hail damage in the Lawton area. A more accurate estimate of yield will not be possible until after fruit set. However, based on the number of surviving flower clusters and proportion of secondary buds, an initial assessment can probably be made. In previous years, the crop turned out to be harvestable in some vineyards that growers had given up on. One thing that is clear is that growers will need to reduce inputs, including fungicides. There are basically three different reduced cropping scenarios:

1) There is no crop worth harvesting and you don’t care about inoculum build-up.

In this case, protecting the fruit from infection by black rot or Phomopsis is not necessary. If you’ve had good black rot control in previous years, you probably won’t have too much disease. If you’ve had black rot problems in the past, one more year of inoculum production won’t make much of a difference because you already have an “inoculum bank” in the vineyard. We have shown that even under conditions of high disease pressure, it is possible to produce an excellent crop with a standard spray program, which you would implement next year. As far as foliar diseases are concerned, vines with a low crop can tolerate more disease than vines with a full crop. Powdery mildew may infect Concord and, to a lesser extent, Niagara leaves, but if there is no crop, the vine can tolerate quite a bit of disease without ill effects. However, there is a risk that a severe downy mildew outbreak may defoliate Niagara, which may predispose vines to winter injury, even if there is a low or no crop. If downy mildew comes in early in the season (based on scouting) and if it looks like the weather will continue to favor downy mildew, a fungicide spray may be needed to knock back the disease to the point that it does not lead to serious defoliation.

2.) There is no crop worth harvesting, but you want to limit inoculum build-up.

In this case, we don’t want to protect the vine to preserve fruit quality as much as we aim to apply fungicides at a few critical times to knock back diseases to prevent large amounts of overwintering inoculum production. In this case, we can also opt for less expensive fungicides that have good to excellent disease control efficacy. This would include at least one protectant fungicide application (e.g., before a rainy period) to protect the young shoots and exposed flower clusters from Phomopsis. An SI spray could be applied at first postbloom if you are concerned about black rot. Scouting-based management of downy mildew in Niagara would occur as described above. If powdery mildew becomes severe on Concord leaves, you may consider an eradicant spray (e.g., JMS Stylet Oil) to knock down colonies and cleistothecium formation.

3.) There is a harvestable but reduced crop.

In this case, protecting the fruit from black rot and Phomopsis is the most important activity and will require a few more sprays than the two scenarios above, e.g., one or two pre-bloom protectant sprays to protect against Phomopsis, one or two postbloom sprays to protect against black rot and Phomopsis (while also controlling powdery and downy mildew), and curative/protectant sprays against foliar powdery and downy mildew only if scouting indicates a need.

To cut input costs, you can use lower-cost fungicides (e.g., generics, older protectant fungicides, phosphites) and reduce the number of fungicide applications only to critical times. Watching the weather and stretching spray intervals during dry periods also helps to lower the number of sprays. It is important to also take labor and fuel costs for applying fungicides into account. The fewer times you have to drive through the vineyard, the better. A way to reduce the number of applications is to tank-mix fungicides with insecticides (most growers are already doing this), apply products at higher rates, or apply products with longer-lasting residuals for extended coverage. Adding a sticker-extender (e.g., NuFilm) can be a low-cost way to make a fungicide last longer and obtain better coverage. Ensuring thorough coverage by spraying every row at an appropriate spray volume (at least 50 gpa after the canopy fills in) will increase the “bang for your buck” of the fungicides you use. This is especially important for protectants like Ziram, Captan, and Manzate.

Scenario 1. No crop worth harvesting, don’t care about inoculum build-up

ACTION RECOMMENDED
Bud break to pre-bloom Bloom to 4 weeks after bloom Mid- to late season
None Scout Niagara for downy mildew – if lesions found and there is threat of severe outbreak and defoliation, apply systemic fungicide* with curative action. Continue scouting until harvest.

* Phostrol, ProPhyt, Revus Top, Ridomil Gold Copper, etc.

Scenario 2. No crop worth harvesting but want to limit inoculum build up

ACTION RECOMMENDED
Bud break to pre-bloom Bloom to 4 weeks after bloom Mid- to late season
A spray of an EBDC* or Ziram or Captan (can be tank-mixed with ProPhyt or Phostrol) to reduce Phomopsis cane infection at a critical time (e.g., at 4-8 inches of shoot growth).
  • SI** + Ziram at 1st postbloom to control black rot
  • Add Phostrol or ProPhyt or Ziram for Phomopsis and downy mildew control if needed.

 

  • Scout Niagara for downy mildew – apply systemic fungicide*** if threat of severe outbreak.
  • Scout Concord for powdery mildew; to limit cleistothecium formation, apply JMS Stylet Oil to knock down existing colonies. An SI will prevent new infections but will not visibly reduce existing colonies.
  • Continue scouting until harvest

* EBDC= Dithane, Penncozeb, Manzate. ** SI = sterol inhibitor, e.g., Elite, Tebuzol, Orius, Rally, Vintage, Mettle. *** Phostrol, ProPhyt, Revus Top, Ridomil Gold (MZ or Copper).

Scenario 3. Harvestable but reduced crop

ACTION RECOMMENDED
Bud break to pre-bloom Bloom to 4 weeks after bloom Mid- to late season
1 or 2 sprays of an EBDC* or Ziram or Captan or ProPhyt or Phostrol to reduce Phomopsis cluster infection at critical times (e.g., 4-8 inches of shoot growth, 12-16 inches of shoot growth)
  • SI** (+ Ziram) or Revus Top at 1st and 2nd postbloom to control black rot, powdery and downy mildew
  • Tank-mix Phostrol or ProPhyt for Phomopsis control as needed.

 

  • Scout Niagara for downy mildew – apply systemic fungicide*** if threat of outbreak.
  • Scout Concord for powdery mildew; if it comes in early, apply inexpensive SI** to prevent new infections. JMS Stylet Oil can be used to knock down existing infections.
  • Continue scouting until harvest

* EBDC= Dithane, Penncozeb, Manzate. ** SI = sterol inhibitor, e.g., Elite, Tebuzol, Orius, Rally, Vintage, Mettle. *** Phostrol, ProPhyt, Revus Top, Ridomil Gold (MZ or Copper).

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

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