As the harvest approaches (plus recent rain events in some part of VA), I am receiving several emails on sour rot. It is often confused, but not all late season rots are sour rot. Sour rot is basically a disease caused by bacteria that form acetic acid, such as Acetobactor and Gluconobactor. It is often associated with wounds (birds, insects, hail, too much rain, early season powdery mildew, certain very fair-skin varieties, etc), high Brix level (13-15 or higher), warm temperature range (high 60F to mid 70F).
Since these bacteria cannot penetrate berry skin tissues by themselves, what we typically recommend is wound management (e.g., bird netting, grape berry moth management, etc.), and many people mention that it works. However, with a potential role of Spotted Wing Drosophila (or lesser extent, African fig fly), which may able to penetrate skin, there is a possibility that we may see more sour rot in the future.
Luckily, Dr. Wayne Wilcox’s group at Cornell University has been working to see if we can combine an insecticide and copper (they also tested potassium metabisulfite, or KMS, but please note that KMS is not legal to be used in the vineyard in the US + very toxic when you get it on your eyes or inhale it). A good news is that it looks like a combination of copper + Delegate provided as good protection as KMS treatments. Although the report is not really concerned about spotted wing Drosophila, the combination of an insecticide and copper resulted in ~50% control of sour rot. If you are interested in his work, here is Dr. Wayne Wilcox’s report on sour rot. Please note that as Wayne indicates in the report, it is based on only one year of data, we need to see whether they can repeat the results in 2014. Also, they intentionally sprayed with 7-day schedule, and it maybe an overkill to spray every 7-days at the end of the season. (They mentioned about lowering the frequency once they prove that the treatment works.)
Also, the experiment is based on the case where you do not have wounds to begin with. If you have already seen wounds on berries, I am not sure how effective the treatment would be. Since fruit flies tend to be attracted to open berries, I would think it works too, but that something we need to investigate.
Although copper has 0-day PHI, it has potential residue issue that may affect fermentation process. However, as I mentioned in the last post, sometimes it may come down to whether you will have a crop or not, and at that point, having an option certainly helps.
Lastly, based on a few trials we conducted, we noticed a trend with a better powdery mildew management at bloom time resulted in lower Botrytis and sour rot. Thus, if you have a variety that is very prone to sour rot issue, make sure to have a very good control on powdery mildew too.