Virginia Grape Disease Updates

--> I just visited another vineyard down in the south where Norton vines were in full bloom, and when I came back, our Chardonnay vines were in trace-bloom.  As we discussed at this year’s VVA meeting, bloom is a very critical time for disease management.   Downy mildew, powdery mildew, and black rot tend to show up around this time of the season, and berries will be susceptible to these diseases until 4-6 weeks after bloom.    In addition, Botrytis, ripe rot, and bitter rot can cause infection on flowers.   Management of Botrytis, ripe rot, and bitter rot at bloom time can be important because these fungi can infect flower, and come back later when berries are maturing.   As you know, development of a disease depends on so many factors such as availability of inoculum (~ disease history), past and future weather conditions, variety, canopy management, etc., thus, I will not go in to details.  However, based on the past few weeks of ...

In Winchester area, rains from May 22-23 resulted in ~19 hours of wetness with an average temperature in lower 60's. We had 5-6 hours of dry period, then another rains went through the area, starting around 5PM last night. Relative humidity remained high (>90%) until around 5AM this morning, thus it was about 12 hours of wetness. Average temperature was again in low 60F. Both were infection events for Phomopsis and black rot. A total precipitation of the last three days was about 0.7 inches. Also, in the last few days, night time temperatures were warm (in 60's) and relative humidity were high, thus, these could be downy mildew sporulation events as well.

We did not receive afternoon rains at Winchester yesterday. We were hoping for it because we are planing a new vineyard... However, we received a short rain around midnight yesterday (5/20/13). It was followed by fog, and relative humidity stayed high (>90%) for almost 9 hours with average temperature in low 60's. It was an infection event for Phomopsis and black rot.

Damages from Monday's frost event showed up a few days after.  About 5-10% of shoots (or leaves) on our Merlot vines which is located near the bottom of a hill showed symptoms.  Some are simply become necrotic, and others are showing mosaic symptoms. Also, although there were drizzles here and there, rains from this weekend were not long enough to be disease events at our location.  However, relative humidity was high (near 100%) throughout the last night.  Such a condition can promote sporulation of downy mildew.

Looks like some of us got a frost bite here and there.  I hear some people got mid-20's this morning.  It is probably too early to tell, but most of our vines seem to be OK so far.  It looks like it went just below 32 in our station, but not by much.  However, these baby vines in our nursery looked pretty frosted this morning.  We will see how it will go.  It takes about 24-48 hours for frost damage to show up, and with mild frost damages, you will see mosaic lesions.

Well, it looks like finally we have some break from rains. In Winchester area, rain started around 9AM on the 6th, and although there were a few interruptions, relative humidity stayed above 90% most of the time until around 9AM this morning (the 9th). Temperature varied from upper 40's to low 60's and most of time, it was low 50's. This rain event was long enough for Phomopsis and black rot infection events. A few people asked me "What should I do now?" Well, it depends on when you have sprayed last, and also, what type of history of black rot you had in the past. If you missed the spray before the rain (i.e., your last spray was more than 10-14 days ago), and your vineyards had an issue with black rot (which is not a common), it may be a good idea to consider either a DMI (SI) or a QoI (Strobilurins). We want to keep DMI for later use, so, unless you know for sure that the rain from the past few days will give you an issue with black rot, my recommen...

Looks like we are expecting three days with high chance of rain from Monday.  I sprayed our vines with mancozeb (3 lb/A) and sulfur (3 lb/A) yesterday. At this point of the season, my target fungal disease is Phomopsis since this pathogen can be active in relatively cool rain event, and we have relatively old vines (~23 years old) right next to our newer Chardonnay.  Mancozeb comes in handy because it also has efficacy against black rot and downy mildew.  These diseases typically develop little later in the season, but it is much easier to protect your vines from them than try to counter-act to the developing diseases.  I also added sulfur to add an insurance against powdery mildew. 

The total hours of wetness was about 18h with the average temperature at 50-56F.  It was long and warm enough for both Phomopsis and black rot.  Also, these spring rains will help powdery mildew spores (ascospores that are from winter survival) to be disseminated to the air.