As shoots begin to elongate in many Wisconsin vineyards, we have seen several Phomopsis and black rot infection events across the state; check out recorded and forecasted events in your region using NEWA’s Grape Disease Models.

We are quickly approaching the beginning of a very critical period for protecting grapevines from fungal pathogens – pre-bloom to 3 to 4 weeks post-bloom. During this period, the most susceptible parts of plants include leaves, flowers, and developing fruits. It’s important to apply fungicides during this time to shield these vulnerable tissues from infections caused by black rot, Phomopsis cane and leaf spot, powdery mildew, downy mildew, and Botrytis gray mold. A comprehensive fungicide program early in the season will significantly aid in controlling diseases later on in the season. Many disease symptoms observed later in the season come from infections that occurred earlier but remained dormant until the fruits started to mature, or from the survival of initial pathogen inoculum in the early season, which then produce additional infection sources. This isn’t the time to cut corners on application rates or skip treatments. Make use of the most effective fungicides available during this critical period.

During the immediate pre-bloom to early bloom phase, typically around E-L stages 17-19, grapevines enter a crucial period where inflorescences have emerged, marking the onset of flowering. Fungicide treatments should occur when shoots reach a length of 10-12 inches, with a focus on combating black rot and Phomopsis. For vineyards growing varieties highly prone to black rot or those with a history of the disease, applying fungicides before bloom can offer valuable protection. Phomopsis infections pose a risk to the rachis (fruit stems) at this stage, potentially spreading to the fruit if not effectively managed with fungicides. Continuous protection against powdery mildew infections is imperative during this period, especially given the increased susceptibility of fruits post-bloom. This holds particularly true for vineyards growing susceptible varieties such as Brianna, Frontenac, Frontenac gris, and Marquette, or those where powdery mildew was problematic in the previous season, as this can contribute to substantial inoculum buildup.  In instances where conditions strongly favor downy mildew infections (temperatures exceeding 50°F, combined with rainfall and high nighttime humidity), you should include a product specifically targeting downy mildew in your treatment program. Keep in mind that many products targeting downy mildew do not target many other diseases (e.g., powdery mildew, black rot), so they should be tank mixed with other fungicides.

It’s important for growers to prioritize good fungicide coverage, especially within the fruit zone. The Midwest Fruit Pest Management Guide provides a list of recommended products and their efficacy ratings.

• A recommended fungicide treatment involves using a protectant (FRAC M) such as mancozeb or captan, in combination with one of the sterol inhibitors (FRAC 3) like Cevya, Mettle, Procure, Rally, or Tebustar.
• Rotating with a different mode of action, such as the strobilurins (FRAC 11) Abound, Flint Extra, or Sovran is also recommended to reduce overreliance on single class of fungicide chemistries.
• Combination products (pre-mixtures) such as Inspire Super (FRAC 3 + 9), Merivon (FRAC 7 + 11) Pristine (FRAC 7 + 11), Quadris Top (FRAC 3 + 11), and Revus Top (FRAC 3 + 40) are effective for controlling several key fungal pathogens.
• Certain products containing difenoconazole (Inspire Super, Quadris Top, and Revus Top) may cause phytotoxicity on V. labrusca, V. labrusca hybrids, and other non-vinifera hybrids. Read the label for warnings.

These upcoming pre-bloom to post-bloom sprays are key for producing high-quality, disease-free fruit!

This article was posted in Grapes and tagged disease, disease control, Grapes, Leslie Holland, pre-bloom.