As we near harvest season, many Wisconsin vineyards are experiencing the effects of a rainy season on fruit clusters. Several commercial growers have reported dealing with downy mildew infections on rachis and fruit. After last year’s drought and powdery mildew issues, it’s not surprising to see a rainy year bring significant downy mildew outbreaks. The frequent rain made timely pesticide applications challenging during critical periods of downy mildew susceptibility. With wet conditions continuing, some growers are now noticing late-season infections from downy mildew. At first glance, we suspected another type of fruit rot such as anthracnose or black rot, due to the lack of classic downy mildew symptoms, but upon further investigation downy mildew was diagnosed. The clusters showed no visible signs of downy mildew infection but were shrunken, soft, and discolored. We also noticed brown discoloration of the rachis near the berries.

After consulting Dr. Katie Gold’s (Cornell University) publication on grape diseases (found here) we concluded that these symptoms likely resulted from infections that occurred after the berries became resistant (i.e., ontogenic or age-related resistance) to downy mildew infection but before the other green tissues (i.e., peduncle and rachis) did. Infections of the peduncles and/or rachis can result in the berries shriveling as it slows or prevents the movement of nutrients and water to those berries. We confirmed the presence of downy mildew with microscopic observations.

Downy mildew can escalate late in the season, particularly with heavy morning dews. As harvest continues for some table grape varieties and approaches for many wine grape varieties, our fungicidal options for control are limited due to pre-harvest intervals (PHI).

If you opt for fungicide control methods, target products with short PHI. Consult the Midwest Fruit Pest Management Guide for options: https://ag.purdue.edu/department/hla/extension/_docs/id-465.pdf

Fungicides in FRAC groups 11, 33 and 40 are very effective against downy mildew. However, they pose a high risk for fungicide resistance development when used alone. Tank mixing or use of premixtures such as Pristine (7+11) or Revus Top (3+40) are effective against many of the fungal pathogens, including downy mildew, that we are trying to deal with this time of year. We’ve also observed an increase in powdery mildew infections in several vineyards, likely driven by the recent break in rain combined with the high humidity over the past few weeks.

Managing both downy mildew and powdery mildew are crucial, as premature defoliation can impair the vine’s ability to ripen fruit and compromise its long-term health. Defoliation weakens the vine, affecting its cold hardiness and reducing nutrients stored in the vine, which can lead to weaker growth and lower yields in the following season.

Resource:

UW Plant Disease Diagnostic Clinic: https://pddc.wisc.edu/sample-collection-and-submission-2/

This article was posted in Disease, Grapes and tagged Courtney C Meeks, disease, Downy mildew, Grapes, Late Season Downy Mildew, Leslie Holland.