Virginia Grape Disease Updates

The previous predicted rain affected sporadically around Winchester area.  The sky was dark, yet no rain at my place, but Winchester airport recorded a light rain for 30 min or so.  Since air was so dry, I think it did not accounted for any disease risks. Now we are expecting to see thunderstorms tonight and tomorrow.  The percentage of precipitation is 50% and 80% for tonight and tomorrow, respectively. 

We had a very warm day that was followed by a severe thunderstorm last night around Winchester.  As with many thunderstorms in the summer (yes, we are still in May...), it was sporadic in terms of the affected area.  For example our station did not receive any precipitation, but my place had about 0.3 inches of rain.  Based on the record from Winchester weather station, the rain lasted about one hour, and the relative humidity was not high, thus, it was not a major disease event.  However, it might have been different at your location.  Please check your local weather station. We are expecting to have another series of thunderstorms in next few days. 

We had some sporadic rain events around Winchester area on 23rd and 24th of May.  Duration of rains during these two days were very short, but the one on 24th was a back to back rain event, and resulted in 2.5 hours of wetness with an average of 66F.  It was long enough for downy mildew. We are also expecting to see more rains later this week, starting tonight.  I understand that right now is very busy for all of us, but please make sure that you are on top of the situation since we are at the critical period for berry infection for downy mildew, powdery mildew, and black rot.

Winchester received a series of rains from 2:20 PM yesterday (5/22/10), and the relative humidity stayed high (>90%) until 9:20 AM.  Thus, we had about 19 hours of wetness with an average temperature of 70F, that accounted for disease risks for black rot, Phomopsis, and downy mildew.  Also, we will have a vineyard meeting at Sans Soucy Vineyards ( www.sanssoucyvineyards.com ) from 11 AM tomorrow.  I hope some of you can make to the meeting.

It looks like the first set of thunderstorm is coming very soon.  (Actually, I see rain drops now.)  I hope it won't stay very long.  There are 30-40% chance of thunderstorm tomorrow too. I received several questions about the risk of downy mildew (DM) based on the previous rain which was a risk event for DM.  Most of questions were about whether to apply a kick-back (curative) activity fungicide such as Ridomil or Prophyt.  At this time of the year, it would be a good idea to consider a kick-back material, if you: 1) did not protect your vines, say, the previous application of DM fungicide was more than 10-14 days ago, 2) had considerable rains since the last application, and 3) had an outbreak of DM in recent years.  In the other words, it is probably not necessary, if you have protected your vines, and/or you haven't have a big out break of downy mildew in recent years.   It will take about 10 days for DM fungus to develop symptoms.  Ple...

(I'm still at a meeting, so, I will make it quick.)  At Winchester, the series of rain started around 2 AM on this Monday (5/17/10), it was not a continuous rain, but we had frequent enough rain events during Monday, Tuesday, and this morning to keep the relative humidity high (>90%) until ~9:30 AM today.  During Tuesday, there was a moment when the RH was little less than 90%, but it was still in high 80's.  Thus, I would say this rain event accounted for about 55.5 hours of wetness and average temperature during this period was about 58F.  Thus, it was long and warm enough for downy mildew, black rot, and Phomopsis.   Some of us are expecting bloom very soon.  Please remember that the critical time for downy, black rot, and powdery mildew berry infection is from bloom to 4-5 weeks after bloom.

I thought the forecast was saying rains from Tuesday, but I guess I didn't read it right.  Now two low pressure systems are coming back to back and we are expecting to see showers during next few days.  I hope you have been protecting your vines since we are getting very close or already at the critical period for berry infection by downy mildew, powdery mildew, and black rot.  If you are at bloom, there are some risk of Botrytis infection as well. We will have a vineyard meeting on 19th at Narmada vineyard.  Since I have an another meeting to attend, Tony will be there to talk about both disease and viticulture tips.  I will ask Kenner to print out my handout for your reference. I will be out of my office due to an USDA-related meeting.  No, it's not one of fun meetings. ;-)  The best way to interact with me for next few days are via email.

At Winchester, the second set of rain started at 4:00 pm on 5/12/10 and then we had several short showers during the night.  In the end, the relative humidity was high (>90%) until 11:00 am on 5/13/10.  The average temperature was 64F, starting with higher temperature (72F) and ended lower (55F).  The total amount of precipitation was 0.08 in. Thus, we had about 18 hours of wet event.  The temperature during 12th was close to the optimal temperature for downy mildew.  As I mentioned in the previous post, we (Winchester) had a potential event for downy on May 3rd, so, I think this rain gave the fungus a good opportunity.  Also, it was high risk event for black rot and Phomopsis. We are expecting to have more rain events during the next week throughout VA.  Plus, I think most of us are expecting bloom very soon.  The period between bloom to 4-5 weeks after bloom is the critical time for berry infection by downy mildew, black rot, and powdery ...

This page from Michigan State University Extension describes the temperature threshold to have frost/cold injury on Concord vines.  Hopefully we won't receive any frost at this point. Also, I updated "labels" on this blog.  Now the name of diseases will corresponds to the entry that is either 1) have a picture or 2) discussed in length.

At Winchester a series of rain started around 11:40 am and lasted until 6:40 pm resulting in 0.1 inches of precipitation.  The average temperature during this period was about 50.5 F.  Since the rain ended late in the evening, relative humidity stayed high (>90%) until 8:40 am today.  Thus, in reality, leaves were probably wet for 21 hours or so, with the average temperature of 55 F. This accounted for moderate risks for both Phomopsis and black rot.  Although there were long enough wet period for downy mildew, the temperature might have been a low side of the activity for the downy mildew fungus, thus, I would rate a low to moderate risk for downy.  Plus we had long enough wet period for the powdery mildew fungus to shoot out ascospores. We had a potential downy mildew infection period back in May 3rd.  This is about the time you will start to see the symptom development (click the picture to see a larger image).  Please check your vines closel...

Some parts of the central and northern VA had a cold morning that may have resulted in frost/freeze injuries on your vines.  You may see younger leaves with damaged tissues that are often become like scorched tissues (lower left leaves on the first picture; click the picture to see a larger picture).  Or you may see entire shoots become wilted (second picture; click to see a larger picture).  If only younger leaves are damaged, the shoots will recover over time. It seems like tomorrow's rain event will be a relatively cold one (mid to low 50's) in terms of the disease development.  Hopefully we don't see a major risk event.  We'll see...

The percentage of the chance are 90% in the northern VA, 80% in the central VA, 60% in the southwestern VA, and 30% in the southeastern VA as of noon today.  Please check your local weather for the details.

Looks like most of us did not receive any precipitation yesterday.  I checked several weather stations around VA.  Some had a trace amount of rain, which probably did not account for any significant disease event.  For conformation, please check your local weather stations. The next rain forecast is on this Tuesday, and it is 60% chance at this point (Sunday).  Tomorrow seems to be less windy than today, so, it maybe a good day for an application, if you needed.

Mmmm...  This will be a tough call.  Northern VA and also Southeastern VA will expect thunderstorms during Saturday morning.  The reasons to justify your protective spray could be: 1) vines has been grown quite a bit over this week due to warm weather; and 2) you might have sprayed more than week or 10 days ago and it is due anyway.  One the other hand, there are reasons to hold off the spray: 1) the expected thunderstorm is in the morning hour and the forecast of Saturday is sunny with high wind, i.e., leaves may not stay wet for a long time; 2) the chance of rain is 30-40% as of this morning; and 3) there are rain forecasted early next week  (20-40% during Tuesday and Wednesday for many parts of VA) and you may want to aim for that rain. Please check your local weather, fungicide record, and disease history of your vineyard to make a decision the best for you.

Thank you very much for those of you came to the vineyard meeting at Horton.  If you could not make to the meeting or could not receive the hand out, please click here to download.

We'll have a central VA vineyard meeting tomorrow at Horton Vineyard from 11AM.  Tony and I will be presenting seasonal reminders on the pest management and viticulture tips.  If you can make it, please join us. By the way, this is a picture of our Chardonnay taken yesterday.  When I checked them on Friday, it was about 5-6 inches growth, but it must have grown 2-4 inches over the weekend.  We are expecting to see some thunderstorm events again during this weekend.  Please keep your eyes on the local weather forecast.

Most of northern VA did not see precipitation until very late last night (~1:30 am).  It seems that the storm system missed most of us, unless you are in West Virginia.  At Winchester location, a series of rain started around 1:20 am, then there were several precipitation intermittently, and it resulted in a total of 0.34 inches of rain.  The relatively humidity was stay high (>90%) until 10:00 am.  Thus, if you count the very first rain, we observed around 8.7 hours of wetness.  The average temperature during this event was around 73F. This was a borderline event for black rot and Phomopsis because they requires about 8 hours at this temperature.  For downy mildew, the temperature range was optimal, thus, the infection risk was probably high.  Also, it was long enough for powdery mildew to discharge ascospores. 

I guess since I wrote about the chance of rain during the weekend, we had a nice sunny Saturday.  The regional radar is showing a strong storm system moving across the Midwest and coming to our direction, but it seems the system is pushed a litte upward than it was predicted in the forecast.  At this point (Sunday noon), there are higher chance (80%) of rain tomorrow (Monday) than today (30%) in northern VA, and the chance of rain decreases somewhat as we go south.  Please check your local weather for more details.