Michelle Moyer reminds growers who had powdery mildew problems last year to prepare for disease carryover this year.

Michelle Moyer reminds growers who had powdery mildew problems last year to prepare for disease carryover this year.

Botrytis bunch rot is a different beast than grape powdery mildew, says Dr. Michelle Moyer, Washington State University viticulture extension specialist. While it’s not a problem pathogen every year, exceptionally wet weather can trigger the disease, so Washington grape growers should consider dual-acting fungicides.

A broad-spectrum fungus, the bunch rot pathogen will go after just about anything, she said. The pathogen overwinters in the vineyard on mummified berries, moldy bunches, and vineyard debris, so there’s no real way to eliminate it. “You’re really only managing it, not eradicating it.”

Bunch rot likes cool, wet conditions, and has an optimum temperature range of 53 to 86°F, slightly cooler than grape powdery mildew. Also, Moyer pointed out that it needs 15 hours of leaf wetness to begin growing. Bunch rot can result in shoot blight in warm, wet springs.

Because it’s a weak pathogen, it uses open wounds and injuries to enter the fruit.

Early in the season, the pathogen colonizes dead or dying tissue in or near the bunch. Common points of infection include: stamen; rachis; berry caps; aborted berries; bunch debris; and wounds on fruit or shoots from mechanical or physical means (bird or insects). Severe powdery mildew incidence can enhance bunch rot, allowing infection to enter berries through the tiny openings from the powdery mildew injury.

Later in the season, when sugar accumulates in the berries, the latent disease reactivates to infect the fruit through wounds and injuries.

Timing of chemical applications for bunch rot is similar to powdery mildew. Fungicide sprays should target the bloom-to-bunch closure stage. A second application window—from veraison to harvest—targets latent infections on clusters. Wet weather during this time can initiate infection on ripening clusters.

Two new fungicides were recently approved that control both botrytis bunch rot and grape powdery mildew: Inspire Super (difenoconazole and cyprodinil) and Adament (tebuconazole and trifloxystrobin). Another fungicide, Pristine (pyraclostrobin and boscalid), also provides protection against both diseases. The dual fungicides should be applied between bloom to bunch closure. However, Adament and Pristine should be avoided if mildew is present because they have a strobilurin component.

Growers who have vineyards with significant levels of powdery mildew should avoid using fungicides repeatedly that are in high-risk categories for disease resistance. In Washington, the most resistant-prone fungicide group is the strobilurin (Qol) category. Use of such fungicides must be carefully rotated and kept to a minimum.