Virginia Grape Disease Updates

Thunderstorm and rain event started around 5PM on 29th resulted in about 10 hours of estimated leaf wetness event with an average temperature of 68F.  It was an infection event for Phomopsis, black rot, downy mildew, and Botrytis. It seems that many of us are experiencing uneven flowering this year.  For example, some of our Merlot is almost BB size while others are still in bloom.  If that is the case for you make sure to protect bloom against Botrytis.  Also as you are aware, from bloom to 4-5 weeks after bloom is a critical period for black rot, downy mildew, and powdery mildew berry infection.  It is very important to protect your clusters around this time of the season!

It did not rain much at my place (about 10 miles East of the station), so, it did not register to my mind, but Winchester area received a series of thunderstorms on Sunday night.  It stared rain around 10AM and an estimated leaf wetness period was 10 hours with an average temperature of 64F.  It was a risk event for Phomopsis, black rot, downy mildew, and Botrytis.

Winchester area received rain from a thunderstorm last night around 7:30PM.  An estimated leaf wetness period is 14.5 hours with an average temperature of 63F.  It was a risk event for Phomopsis, black rot, downy mildew, and Botrytis. Our Chardonnay is in full bloom and Merlot is about 60-70% bloom now.  Hopefully this weather won't affect fruit set.

Winchester area received a series of thunderstorms from around 11PM last night.  It accounted for a 11 hours of estimated leaf wetness event with an average temperature of 60F.  It was risk event for Phomopsis Black rot, downy mildew, and Botrytis infection. Also, we are having several warm humid nights in a row due to rains.  Downy mildew pathogen prefers this type of condition for production of spores.

We are about 60-70% bloom on our Chardonnay.  Merlot is still in trace bloom stage. Winchester area received a series of rains from around 3:00 AM this morning, and relative humidity stayed above 90% until around 3:00 PM.  Thus, we had 12 hours of an estimated leaf wetness period with an average temperature of 63F.  It accounted for black rot, Phomopsis, downy mildew, and Botrytis infection event (note: Botrytis typically takes more time (>15-17 hours of wetness) to infect than the model says).  The forecast is calling for more rain tomorrow and Wednesday.  We will see how it goes.

I received comments from Anton on the previous post about Botrytis.  He is more concerned about Rovral (=Meteor), Vangard, and Scala than I indicated. I wasn't very clear on the previous post about how the information has been developed.  There are two components in this story.  One is the probability of having Botrytis with resistance to some of the Botrytis materials.  So far, it seems that probability is very high for Flint (or other QoIs), high for Endura and Pristine, moderate for Rovral (=Meteor), and unknown for Vangard and Scala.  The other component is the level of resistance.  It is high for Flint, moderate to high for Endura, and Pristine, moderate for Vangard and Scala, and low for Rovral.  The key point that Anton helped me understand today is that even though the resistance level against Rovral is "low", it still means that the labeled rate won't be as effective as it should be.  Thus, if there is a certain percentage of Botrytis...

There are still a chance of an afternoon rain, but here are estimated disease risks from rains from yesterday to this morning.  We had about 16 hours of an estimated leaf wetness period with an average temperature of 63F.  It was infection events for Phomopsis, black rot, and downy mildew.  I think most of us are getting close to or at bloom.  Please remember that a period between bloom to 4-5 weeks after bloom is a critical time for fruit infection by powdery mildew, downy mildew, and black rot.  Also, at bloom, we recommend protection spray against Botrytis because it can infect flower parts and either resides in it or stay with flower debris until later in the season. Many of you probably remember a report from Dr. Anton Baudoin about fungicide resistance of Botrytis pathogen, Botrytis cinerea .  This pathogen can infect wide variety of hosts and it is known to develop resistance to a fungicide or multiple fungicides relatively quickly.  Thus, A...

Winchester area received rains from 11AM yesterday.  Although rain was over by the evening, the relative humidity stayed high (>90%) until this morning.  Thus, an estimated leaf wetness period was 21 hours with an average temperature of 64F.  It was an infection event for downy mildew, black rot and Phomopsis.  Also, the humid warm night promote downy mildew to sporulate.

Winchester area received a series of rain from the morning to the afternoon yesterday.  Overall, we had 6 hours of an estimated leaf wetness event with an average temperature of 62F.  It was a light infection event for Phomopsis. In addition, we are having two consecutive nights with a very high relative humidity (>90%) with warm air (>60F).  Downy mildew can produce spores under such conditions.

Winchester area received a precipitation from 3AM on May 1st, an estimated wetness period was 5 hours with an average temperature of 59F.  It was a low risk event for Phomopsis. We are having sporadic rain events today as well.  We will see how it goes.