What is in this article?:
- Wine grapes are still at risk of foliar and fruit diseases.
- However, the weather conditions have definitely helped to keep fungal diseases at bay.
- The main concern is if the weather changes and we will have to deal with lots of rain in the weeks before harvest, which would promote Botrytis bunch rot and sour rot.
Downy mildew, black rot
Downy mildew symptoms were seen on wild grapes in mid-June but have not been seen in juice or wine grapes. The dry weather has kept this disease at bay. During hot, dry weather, downy mildew “goes on vacation.” Were the weather to change, there could still be some downy mildew development; however, it is unlikely to be very damaging. In August and September, heavy dews can aid downy mildew development. Continued monitoring of the vines and weather is advised.
Black rot is showing up on the fruit in some vineyards where black rot was a problem last year and fungicide sprays may have been missed. Fruit symptoms include a sharply delineated brown area that expands quickly, finally shriveling up the fruit to hard, blue-black mummies. These symptoms are the result of infections that took place sometime between bloom and four to six weeks after bloom and have remained dormant until now. Any symptoms showing up now are an indication that you missed an infection period during which preventive or curative sprays should have been applied.
The susceptible period ranges from bloom to about five weeks after bloom in juice grapes and up to eight weeks after bloom in some wine grape cultivars, at which point the berries become naturally resistant to infection. Since there were so few rainfall events during which clusters would have been wetted for a sufficiently long period to get infection, it may be beneficial to look back at the black rot Enviro-weather model to see when infection periods occurred in relation to fungicide sprays. If black rot is showing up in wine grapes and there are still relatively young berries (stragglers), there might still be a chance of infection, potentially from already infected berries. With the high temperatures, six to seven hours of fruit wetness would be sufficient for infection.
Consult the black rot Enviro-weather model of a nearby weather station, and if an infection period has occurred, apply an SI fungicide (Elite, Rally, Orius, etc.) within 24 to 48 hours if possible. This fungicide spray will also aid in powdery mildew control.
Phomopsis lesions are present on canes, leaves, petioles and even some rachises to some extent in many vineyards, but levels are lower than in previous years. Fruit infections will become apparent a few weeks before harvest. During warm, dry years, there may be remaining spores that can still be released in July and August were rainy weather to prevail later in the season. In susceptible cultivars (e.g., Vignoles), it may be helpful to apply a broad-spectrum product (e.g., Pristine) to catch any late-season Phomopsis and other diseases. In wine grapes, there can be berry-to-berry spread by Phomopsis as harvest approaches as rotten berries remain attached more than in juice grapes. Therefore, later-season sprays for Phomopsis may still have benefits in wine grapes.