I thought I would wait until the rain stops to update the blog, but rains are keep coming and even when we do not receive precipitations, the air has been so damp that the relative humidity has been above 90% for a while. It started late night on 9/4, and still going. So far, I counted about 90+ hours of estimated leaf wetness. The temperature has been between low 60’s to mid 70’s. This has been ideal condition for both Botrytis and downy mildew to develop. As for downy mildew, your berries should be resistant to infection, thus, concern is on foliage infection. Vines need healthy leaf areas for accumulation of carbohydrate into main trunk in order to survive the winter. With a concern on the PHI, one of Phosphorous acid or Phosphonate materials should be a good choice. They also provide a good kick-back activity too.
I have covered Botrytis earlier in this blog, so, please click here and here for more information. If you decided to make an application, please be aware about the PHI, some Botrytis materials have a 14-day or longer PHI.
You may also concerned about sour rot. In general, sour rot control need to be based on general vine management because it is typically initiated with a wounding event (insect, bird, hail, etc). Thus, the first question is whether they have issue with bees or birds in their vineyards. If they do, management of bees and birds will be the first priority.
The other potential causes are early infection by either powdery mildew or Botrytis. When powdery mildew infect berries early in the season, skin can erupt as berries getting larger. When Botrytis infects flower, it can resides in fruit tissues and cause disease when berries are mature.
As for protective chemical, a product called Switch lists sour rot and Botrytis. Switch has a 7-day PHI. Other options are Pristine, captan, and ziram. It seems that all of them provide some level of protection against sour rot, and rotation on captan and ziram showed a good efficacy on one of trials done in PSU. Since we may be potentially dealing with a fungal complex (general yeast, Penicillium, Alternalia, etc), I tend to recommend use of captan or ziram because they are less prone to have fungicide resistance. Ziram has a 21-day PHI, so, it is probably not the best item to be used at this time of the season. Captan has a 0-day PHI (with a 48-hour REI).