No. 4 — April 15, 2019
Phomopsis Cane and Leaf Spot
With bud break taking place over the next week depending on the cultivar and location, your concern should turn to protecting young tissue after bud break from Phomopsis (Phomopsis viticola) infections. Phomopsis infections occur during cool wet weather. The cool wet weather is when spores release for infection. The optimum temperature for shoot and leaf infections is between 60 to 68 degrees F with at least 6 hours of tissue wetness for infection. Average April high temperatures are near 60 degrees F and this year temperatures are trending 2 to 3 degrees above the average normal high temperature. These temperatures are ideal for Phomopsis infections, especially when tissues remain wet. Next week there is a lot of wet cloudy weather forecast. Remember cloudy weather often extends the wetting period of grape tissue, thereby extending the infection period.
To protect grape leaves and shoots from Phomopsis infections a protective fungicide should be applied at ½- to 1-inch shoots. Fungicide products that provide good protection contain the active ingredient mancozeb. Mancozeb also provides excellent protection to black rot and downy mildew. Some common trade names of mancozeb fungicides are, Dithane M45, Manzate Pro-Stick, and Penncozeb 75DF.
Figure 4. Phomopsis infections on young shoots are often on the first three nodes. The lesions are elongated, raised above the surface of the tissue and a crack runs down the center of lesions (see inset). Photo credits: D. S. Volenberg