Frontenac Mold Issue

Asked June 29, 2020, 7:46 PM EDT

My Frontenac grapes from UM looked great for the first 3 years. Now they look diseased. Some leaves have yellow spots, some crinkle up and fall off. Most look healthy. They grapes look fine until they reach full size, then they turn black and fall off. Is this a single fungus or multiple? Should I spray with Vanguard WG or another product? Should I start now since this harvest is already a bust? I don't want to spray the grapes I hope to eat next year... Thanks, Ted Grabau

Marshall County Minnesota

1 Response

These fungal diseases can cause complete crop loss in warm, humid climates. Infection can be seen on leaves, petioles, shoots and grapes. Prune grapevines during dormancy and position shoots during the growing season to allow exposure of fruit to sunlight and good air flow through the canopy. Pruning and training are also helpful in controlling Botrytis bunch rot.

For black rot, grapes are susceptible from bloom until they begin to ripen.

  • Infected berries first appear light brown.
  • Black spore-producing bodies develop on its surface.
  • Later, the berries shrivel and turn hard and black to become mummified.

Botrytis fruit rot can grow on dead blossom parts in the cluster.

  • Before grapes begin to ripen, it moves from berry to berry within the bunch.
  • Botrytis occurs most commonly on ripening berries, where infection and rot spread rapidly throughout the clusters.

Here is a link to an extensive article on grapevine diseases. After each symptom they list the chemicals and other treatments to handle it. As far as when to spray, read the label. The label is law and the manufacturer has spent a lot of time and money to develop these instrucitons.