Virginia Grape Disease Updates

We have not received precipitation since the 21st, but every night since then has been humid.  Dews that formed 3-4 AM stayed until 9-10 AM in some days.  Once again, these conditions favors downy mildew spore development.  If your vines are covered with a previous application, you do not need to worry about it, but if not (say, the last application was more than 10 days ago, or you had more than 1.5-2 inches of rain since the last application), then you may want to consider a protective application before next thunderstorm hits, which may happen in the next few days.  We are almost through with the critical period, though!

Winchester area was rained out pretty much whole day yesterday, resulted in two wetness events, one with ~16 h of wetness with low 60F, and another one with ~7 hours of wetness with high 50F.  These resulted in infection risk event for black rot, downy mildew, Botrytis and Phomopsis.  Once again, please try to be on top of the situation while we are in the critical period for cluster infection.

A thundershower hit Winchester area around 6PM last night, resulted in 16 hours of wetness with lower 60F in average temperature.  It was disease risk event for black rot, downy mildew, Phomopsis, and Botrytis. As with last few rains, it resulted in a warm and humid night that helps downy mildew to produce spores.  Many of us are still in critical period for cluster infection, and this is the time of the year to be on top of the game.  Please refer to previous few posts about spray ideas.

Boy, what a week! Winchester area received about 17 hours of wetness with average temperature of 20 C (68F) on the 11th, 14 hours of wetness with average temperature of 22 C (72F) on the 12th, and right now on-going wetness with 17hour + with an average temperature of low 20C (68F). These events are warm and long enough for downy mildew, Botrytis, Black rot and Phomopsis.  On top of that because of evening rain events, night time conditions have been favoring downy mildew development.  Our main concern is downy and black rot since our clusters are currently in critical period.  These berries are susceptible to the infection by these pathogens. It looks like the weekend weather is promising, and a 10-day forecast is calling for a chance of rain again on Tuesday... what can we do? For our vines, the last application was made 10 days ago, and we received a total of 1.47 inches of rain at the site.  Last application was mancozeb + sulfur + Phosphite (Prophyt, Phos...

The rain event started on the night of the 8th resulted in about 14 hours of wetness with about 65F in average temperature.  This was infection risk event for downy mildew, Botrytis, Black rot, and Phomopsis.  Also, in the last two nights, the evening thunder storms brought up night time relative humidity, thus, we should keep watching out for downy mildew development.

Many people in northern VA are about to see blooms (our Chardonnay vines are trace bloom as of June 2 nd , the one on the picture is one of the few with many open flowers), and I am sure rest of regions are going through bloom by now.   Therefore, many of us are in the critical time for cluster infections by downy mildew, powdery mildew, and black rot.   Bloom time is also critical period to prevent Botrytis, ripe rot and Bitter rot, because pathogens of these diseases can infect flower parts and come back later. We do have materials with kick-back activities against downy (Ridomil products, Phosphonates, etc.) and black rot (myclobutanil, etc.), but infection on flowers and young fruits can happen very fast.   Unless we have a very dry season, this is the time where you have to be proactive.   Thus, what I recommend often is use of protectant materials to protect tissues for 4-6 weeks for V. vinifera varieties, and 3-4 weeks for V. labrusca varieties, w...