No. 11 — June 3, 2019

Post Bloom Disease Management

close-up of a Vidal blanc leaf showing lesions
Photo: Phomopsis viticola lesions on Vidal blanc on June 6, 2019. Photo credit: D.S. Volenberg

The grape berry crop is vulnerable to a number of disease-causing pathogens until a period of time passes after bloom. Currently, most grape cultivars are approximately 10 days post bloom, and grape berries are buckshot to pea size. The grape berries are susceptible to black rot, downy mildew, powdery mildew and anthracnose. There is reduced risk from Phomopsis infections at this time since the spore load declines around the period of fruit set. Post bloom disease management should continue until the grape berries have developed age related resistance to downy mildew, powdery mildew and black rot. The berries develop resistance to these pathogens approximately 4 to 6 weeks after bloom. Although the berries develop resistance, the leaves, shoots, tendrils and other green tissues remain susceptible to infection throughout the growing season. The period from immediate pre-bloom through post-bloom is when you should use the most effective fungicides to protect the crop.

There are number of fungicides that will provide protection during the post-bloom period. Consider the strobilurins (Abound, Flint, Sovran) or the Sterol inhibitors (Rally, TebuStar) or combination fungicides (Inspire Super, Pristine, Quadris Top, Revus Top). Take the time to familiarize yourself with the effectiveness of these fungicides on each pathogen (See page 91-92 in the 2019-2020 Midwest Fruit Pest Management Guide).

The weather forecast through June 14 likely will provide ideal environmental conditions for powdery mildew infections. The probability of precipitation will be low and both daytime and nighttime temperatures will be below normal for this time of year. These below normal temperatures will be ideal for powdery mildew infection and disease development. Ideal temperatures for powdery mildew infection are between 68 to 77°F. Powdery mildew disease infection and development often begins in shaded areas within the canopy. When scouting be sure to examine shaded leaves within the interior of the canopy.

With increased canopy growth and development coupled with fruit set means it is time to evaluate your spray coverage. This can be accomplished using some water sensitive cards stapled to leaves within the canopy. Also check your nozzles and nozzle orientation. Nozzles should be replaced when the flow rate exceeds the recommended flow rate by 10% or more.

In summary, the grape berry is at a critical developmental stage and needs to be protected from infections from pathogens. The two most critical protective fungicide sprays are immediate pre-bloom and the post bloom protective spray. These two protective sprays are when you should apply the most effective fungicides to control critical plant pathogens. As the canopies grow be sure that your sprayer is providing adequate spray coverage. If spray deposition is poor then consider re-calibrating your sprayer. Until the grape berries are 4 to 6 weeks post bloom and develop age related resistance, protective fungicides are imperative.

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