Dr. Walt Mahaffee, Research Plant Pathologist USDA-ARS-HCRL, has prepared the update below. Other researchers include Tara Neill, USDA, Brent Warneke, Oregon State University, and Lindsey Thiessen, North Carolina State University. This project receives funding from the Oregon Wine Board as part of its viticulture and enology research grant program.

Objectives
The main objective of this project is to optimize fungicide application timing during bloom and early berry development to control powdery mildew on grape berries and thereby improve the efficiency of wine grape production. A secondary objective is to determine the ability of fungicides to redistribute around grape tissues. The last stage of the project is to implement the findings from the first two objectives into commercial vineyards.

Process
To investigate the interaction between fungicide chemistry and application timing, a small plot experiment was conducted at the Botany and Plant Pathology Research vineyard in Corvallis, Oregon on Pinot noir grapes in the 2015 and 2016 growing seasons. Five fungicides (Flint, Luna Privilege, Toledo, Microthiol and Quintec) were applied at three different grapevine flowering stages (inflorescence elongation, 50% bloom and berry set) and the resulting disease levels on leaves and fruit were monitored.

Those trials were followed up by 2017 trials in research and commercial vineyards. In the research vineyard, the experiment consisted of rotations around the two most effective treatments from the 2015 and 2016 trials. In the commercial vineyards, the two most effective treatments were integrated into commercial spray plans and the resulting disease levels were compared to vineyard standard programs.

Laboratory and greenhouse experiments examined the ability of fungicide products to absorb into grape tissues and redistribute after application. Detached leaves and grape inflorescences were used to investigate this activity.

Findings
Field research in this project has demonstrated that fungicide selection and application timing during bloom plays a large role in cluster disease control efficacy. Applications of Flint, Luna Privilege and Quintec made at berry set and two weeks later provided the most efficacious mildew control on grape berries. In 2017, trials further validated that two applications initiated at berry set were most efficacious, and that one application at berry set was not enough to get the maximum effect. The commercial trial showed that applications of Quintec and Luna Privilege timed to berry set integrated into standard spray plans provided similar disease control to vineyard standard spray programs. Lab research found that the synthetic fungicides investigated were able to absorb and redistribute, which appeared to play a role in their field efficacy. Disease control on clusters will likely be maximized if Luna Privilege, Quintec, Flint or similar fungicides are applied during berry set and early berry development.

See the previous update for this project, published March 2017.