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Manage Late-season Diseases for Marketable Grapes

June 17, 2020
This agronomic image shows Powdery mildew in a Washington grape vineyard.

Syngenta, 2019: Powdery mildew in a Washington vineyard.

Disease management is an important step in producing marketable fruit. As the season progresses, damaging diseases like Botrytis and powdery mildew can become bigger threats to grape yield and quality. Do you have a preventive management plan ready?


Botrytis makes its presence known later in the season when it feeds on sugar and affects ripening grapes, causing the skin to split and releasing spores to infect nearby plants, thus reducing marketable yield potential.

What makes Botrytis so dangerous for growers is that once it is spotted in vineyards, it’s already a problem. The fungus overwinters as sclerotia in berry mummies. Sclerotia germinate after a rain or irrigation, producing spores and infecting flowers. The fungus becomes dormant until later in the season when sugar concentration is higher, then grows and spreads throughout the infected berry, which can split and ooze. The pathogen can sporulate on the surface and then spread to nearby berries.

Powdery mildew, another problem disease, is identified by its powdery or dusty appearance on the surface of fruit and leaves. It damages leaves, affecting their ability to photosynthesize. From there, the grape clusters will have trouble fully developing, affecting yield and flavor. Powdery mildew is most dangerous because one infection can take out entire clusters and rows of grapes.

Disease Triggers

Fruit is most at risk from Botrytis and powdery mildew when there is excessive humidity and moisture, and when temperatures are between 58-82 °F. Shade can also encourage disease growth, so grooming and managing the canopy is a best management practice. Grapes damaged by insects or birds also provide opportunities for infection.

Disease Management Tips

  • Groom the grape canopy to decrease excessive humidity and moisture.
  • Manage insect populations that feed and produce entry wounds that promote infections.
  • Apply a preventive fungicide spray before they are identified in the vineyards.

Miravis® Prime fungicide has demonstrated highly effective control of Botrytis and powdery mildew. The product accumulates heavily into the wax layer and translocates through the vines. Research trials show vines treated with Miravis Prime produce less diseased fruit, leading to higher marketable yield potential.

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