Virginia Grape Disease Updates

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 a...

Due to the rain event over the weekend, many people are concerned about protection for next week or two before the harvest.  At this point, the tools are rather limited due to PHI as well as wine making consideration. For downy mildew (especially for foliage, your berries are protected at this point), you can use Phosphonate (= phosphorous acid) product or captan.  (Both have 0-day PHI) For late season rots, especially Botrytis, you can use captan, Elevate (both have 0-day PHI), Switch (7-day PHI), Rovral (7-day PHI), Scala (7-day PHI), Vanguard (7-day PHI), or maybe Endura, Pristine or Luna products (these three have 14-day PHI). For ripe rot and bitter rot, a QoI (Abound, Pristine, Sovran, Flint, etc = 14-day PHI) and captan seem to be the key product.  Our lab is investigating the efficacy of a phosphonate too. As usual, please consider the situation at your vineyard as well as weather forecast.  If you do not see any issues at your place, picking in 7-day...

I was away for several weeks for a series of meetings, and of course things happen when you are not around... 7/28: 9 hours of wetness, around 66F (low Botrytis and downy risk event) 8/6:   12 hours of wetness, around 64F (low Botrytis and downy risk event) 8/7: 13 hours of wetness, around 57F (probably not much of risk event) 8/11: 19 hours of wetness, around 60F  (low Botrytis and downy risk event) 8/13: 13 hours of wetness, around 60F  (low Botrytis and downy risk event) 8/20: 14 hours of wetness, around 65F  (medium Botrytis and downy risk event) 8/23: 6 hours of wetness, around 65F (low Botrytis and downy risk event) 8/24: 10 hours of wetness, around 62F  (low Botrytis and downy risk event) I will have a few more updates coming up...

As we expected, Winchester area had a wet weekend.  We had to major rain events, which resulted in 11 hours and 10 hours of wetness, both are in upper 60F to lower 70F range.  These are downy mildew and Botrytis risk events.  Once again, humid nights we are experience can be a precursor event for downy mildew risk events because these nights favors spore production of downy mildew pathogen.

Winchester area received a shower yesterday, resulted in 5+ hours of wetness in lower 70F.  It was an infection risk event for downy mildew, and very low infection risk event for Botrytis.  In addition, we observed dews in the last two nights, indicating high humidity, which can trigger downy mildew to produce spores.  As usual, late season downy tends to appear on younger leaves on the top of the canopy.  Please keep eyes on them!  There is a chance of rain on Sunday and Monday.

Winchester area had a wet weekend.  We ended up having 9+ and 16+ hours of wet events, and both of them are in upper 60F.  Needless to say, these were disease risk events for downy mildew and Botrytis.  These were events for black rot too, but the clusters should be resistant to infection at this point.  Looks like we are expecting some rain during this week too.  Please keep your eyes on downy mildew.

After 13+ hours of wetness on the 13th-14th rain event, we had another wetness event starting from last night.  It resulted in 11 hours of wetness with an average temperature near 70F.  It was infection event for downy mildew, black rot, Phomopsis and Botrytis.  With the back to back rain event and high humidity during the night, I would keep eyes on downy mildew development.  It tends to show up on younger leaves (at the top of the canopy) first.  Also, many of varieties are about to be berry-touch/bunch closure stage, which is one of timings for Botrytis application.