Virginia Grape Disease Updates

Although the chance of precipitation is between 30-50%, thunderstorms are in the forecast from this weekend to Tuesday for northern and central VA, and during Monday and Tuesday for eastern VA.

There will be an orchard meeting at Winchester AREC on 5/6/10 from 6pm.  The reason I'm posting here is because one of the speakers is Dr. Jeff Derr from Virginia Tech and he will talk about weed management.  If you are interested in, please stop by at our center.

There is another chance of precipitation coming up this weekend.  At northern and central VA, the chance of thunderstorms are 30-40% on both Saturday and Sunday, and at eastern shore, the chance is about 30% on Sunday.  Most of northern VA probably received more than 1 inch of rain during the last series of rain, and depends on how hard it rained, your protection may have been washed out.   A typical rule of thumb for fungicide application timing is "2 weeks or 1 inch of rain whichever comes first", but during the spring when grape shoots grows rapidly, you may need to adjust the schedule based on the growth, thus my recommendation is 7-10 days schedule up to 1 month after bloom, then 14 days. Plus , there is a frost advisory issued for tomorrow morning.  I hope it won't happen...

At Winchester area, the second series of rain started around 5:20 pm on Sunday, then it continued on and off until around 8:30 pm on Monday with about 0.17 inches of precipitation.  Thus, there was about 27 hours of wetness with average temperature of 58F or so.  It was long and warm enough for Phomopsis, and Black Rot to cause infection.  Also, these rains from Saturday probably initiated ascospore release of powdery mildew.  (Powdery mildew does not require water for its infection, but it requires water to release ascopsores from its over-wintering structure.)  The average temperature was low for downy mildew, but between noon to 5 pm Monday, we observed 60-65F while it's raining.  Thus, it might be accounted for downy mildew as well.

The first set of rain (0.11 inches) came around midnight in Winchester area.  It did not last long, but since it came late at night, the relative humidity stayed high (>90%) until 11 am.  Thus, it probably accounted for approximately 11 hours of wetness with an average temperature of around 59F.  Under such condition, both Phomopsis and black rot fungi can cause disease. Then the second set of rain came around 5:20 pm, and this event is still on-going.  I will post the results once it is over.

It seems that we will have a rainy weekend.  Chances of rain are around 40-60% throughout the state.  Forecasted temperature during the hours with a high chance of rain is around mid-50's to low-60's.  This may become our first real risk of disease development.  ( Of course, since I stated that, we may not get rains. ) If you have sprayed in preparation to the previous rains (i.e, within 10 days or so), then your vines are probably protected.  It also depends on how much your vines have been grown and how much rainfall you have received.  At Winchester, many of the rain events were showers with less than 0.02 inches of precipitation, and due to more like a normal weather, shoots were not growing as vigorously as they did few weeks ago.  Please check your record, vines, and local weather history. However, if you were waiting until now and your vines are grown considerably, then you may want to protect your vines, simply because there are three days ...

It seems that the line of rain has passed around Winchester area (Folks around Richmond may see more rain later today).  It started around 7 am and lasted until 11 am or so.  The relative humidity was high (>90%) until 11:40 am.  Thus, this event probably accounted for 4.7 hours of wetness (0.06 inches) and average temperature during the event was 52 F.  It was little too cold and too short for Phomopsis or Black rot to develop.  Please check your local weather for more information.  You can click on the link on the upper right and then type in your zip code to see the record from the nearby weather station.