The most important time of the year for fruit disease control in grapes is from pre-bloom to 4-5 weeks past fruit set. The potential for fruit infection drops significantly 4-5 weeks post-bloom. Important diseases to control during this time include black rot, downy mildew and powdery mildew. Fruit of the most commonly planted varieties is resistant to black rot, downy mildew and powdery mildew, but rachises (stems) and leaves remain susceptible. Therefore, protection against these fungal pathogens is required throughout the growing season for these parts of the plant.
A list of recommended products can be found in The Midwest Fruit Pest Management Guide. It is important to get good coverage in the fruit zone. Also, beware that downy and powdery mildew pathogens are especially prone to fungicide resistance, so avoid back-to-back applications of any one systemic fungicide class (i.e. FRAC 3, 7 or 11).