No. 6 — April 29, 2019

Focus on Phomopsis, Black Rot and Downy Mildew

With wet, cool, and cloudy conditions prevailing through next week, Phomopsis viticola is a concern. Additionally, black rot and downy mildew are the other two diseases that a threat. All three of these pathogens need the grape tissue to be moist in order for infection to occur. Phomopsis needs 6 hours of wetness with air temperatures between 60 to 68 degrees for infections to occur. The fungus overwinters on infected canes or rachises.

Photo: Leaf, top down, to show phomopsis lesions
Phomopsis lesions on Vignole leaf from the New Haven, Mo., on May 3, 2019.
Photo credit: D.S. Volenberg

Black rot overwinters on mummified fruit or as cane lesions. Rainfall events triggers the release of spores from the mummy berries or cane lesions. The spores can travel to susceptible grape tissue by splashing raindrops and wind. Black rot spores can infect all green tissue, infection is dependent on the air temperature and the duration of wetness of the tissue. Only 9 hours of tissue wetness are needed when air temperatures are between 60 to 70 degrees for infection to occur.

Similar to Black rot, Downy mildew Plasmopara viticola zoospores are released by rainfall and can infect susceptible grape tissue when moisture is present. Disease development is dependent on temperature with the optimum temperature being between 64 to 76 degrees F. Downy mildew overwinters on infected leaf tissue.

In summary, all three of these pathogens have similar environmental requirements for infections to occur. Rainfall is needed to dislodge spores, grape tissue needs to remain wet for a period of time and air temperatures range from 60 to 76 degrees.

Sanitation should be part of your management program to control these three diseases. Canes infected with Phomopsis should be pruned from vines and destroyed during the dormant pruning period. To manage Black rot, all rachises and mummy berries should be removed from grapevines during the dormant pruning period. Protectant fungicides such as mancozeb should be applied starting at 1/2" to 1” shoots, especially if wet periods are forecast.

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