Frost injury this spring has created a situation where grapes have a few primary shoots, some secondary shoots, and many non-count basal and latent shoots. This puts shoots at various stages of development and creates problems for growers. Regardless, some varieties are are at or nearing bloom on surviving primary shoots, which is a key time to control important diseases such as black rot, downy mildew, and powdery mildew. The three or four sprays made from immediate pre-bloom to 4 weeks post bloom are critical for controlling fruit infections. Since bloom will likely be prolonged this year, growers may need to make a couple of additional sprays during this time.
Growers should pay extra attention to coverage, especially in the fruit zone, and use the best fungicides available. The Midwest Fruit Pest Management Guide lists recommended products. A protectant (FRAC M) such as Mancozeb, Captan or Ziram, plus one of the demethylation inhibitors (FRAC 3) such as Mettle, Procure, Rally or Tebuzol is the recommended fungicide treatment. Rotating with a different mode of action, such the strobilurins (FRAC 11) Abound, Sovran, or Flint is a good option as well. The combination products such as Pristine, Inspire Super, Revus Top, Quadris Top, and Luna Experience are also effective for broad-spectrum disease control. Be sure to read the warnings about phytotoxicity with fungicides containing difenoconazole. These next few sprays are critical to producing sound, clean fruit. Pay close attention to your sprayer output to be sure you’re getting thorough coverage. This is the most important time of the year for fruit disease control. Once we get 4 to 5 weeks past fruit set, potential for infection of fruit drops significantly. — By Bruce Bordelon, Professor of Horticulture, Purdue University