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.

Throughout the state, there is a chance of rain tomorrow.  In the central and eastern regions it varies from 50-90% chance depends on the location and in which forecast you believe.  It seems the chance is higher around Charlottesville area.  In the northern regions it is around 50%.  Please check your local weather if you need to protect your vines.

I was feeling uncomfortable when the rain came around 6 pm, thinking "it came too early!", there were several rain events last night, but none of the rain did not last very long at Winchester area (< 1 hour around 6 pm, then about 20 min around 8:30pm, and < 1 hour around midnight).  It was so windy that the relative humidity did not stay high, but stayed around 70%. Thus, this series of rain events was not creating a risk of fungal diseases.  I'm more worry about damages done by wind.  I hope your vineyards were OK.

It looks like a line of thunderstorm may pass northern VA tonight.  A chance of rain is 30% during the day and 60% at night.  Since the forecast is a very windy warm night, I am hoping that the rain will dry out soon and won't cause any infection event.

It is sad to share that a tree fruit horticulturist at Winchester AREC, Dr. Rongcai Yuan passed away 4/14/10.  For his obituary, please visit this link .

At Winchester, the rain started around 10:40 am Tuesday, and although it stopped during the night, the relative humidity stayed high (>90%) until 9:20 am today.  During this wet time, the average temperature was around 47F.  This is a borderline case for Phomopsis infection.  If you did not have issues with Phomopsis in the past, the risk was probably close to none.  If you had seem a lots of Phomopsis in recent years, you may have some infection, depends on the temperature you observed during the rain.  If it was mid to low 40's the fungus has a difficulty infecting tissues.  If it is more around 50F, 17 hours of wet period will results in moderate level of infection (15-25% severity with a very strong presence of spores). We (northern VA) may have thunderstorm in this Friday, the rest of the state seems to have several sunny days.  Please check your local weather forecast.

Now it has been changed to 70% chance of rain, starting from 11 am.  Please check your local weather.  The chances of rain in Central and Eastern VA are still in lower percentages.

Those of us in Northern Virginia will have a 40% chance of rain tomorrow (Tuesday, 4/13/10) afternoon.  The temperature expected will be in 50's.  It will take about 7-9 hours for Phomopsis to have an moderate level of infection. Black rot will take about 12-24 hours to infect at that temperature range, so, if it rains and persists, there is a chance for Phomopsis infection.

 It seems that we will observe sunny days with 60-70F for next few days.  Hopefully, the night time temperature won't drop too low.  Our Chardonnay vines are showing a little bit of deformation on leaves, probably due to the low temperature during the night.  Unless we will have more cold nights, it should overcome the stress.  Most of our early varieties (e.g., Chardonnay, Merlot) are in this stage (1-2 inches long).  Please note that the flower cluster is already exposed at this point.  Fortunately, we are not expecting a rain event soon, but at this point, the shoot is susceptible to Phomopsis.  If the fungus successfully infects rachis tissue, it may cause a premature fruit drop later in the season.  The picture on the right was taken last year.  As you can see, a part of rachis became necrotic due to Phomopsis infection, and berries were dropped because the rachis is dead.  The fungus survives in canes and trunks that are ...

We hit 90 F this afternoon.  This warm weather made things go little faster than we would hope.  About 50-60% of our Chardonnay and Merlot are in the stage of bud break, my Cabernet Sauvignon is still holding tight, but I saw Cabernet sauvignon in the other vineyard is about 40-50% bud break stage.  (FYI, last year we hit 50% bud break around 4/25.) Some of us are expecting to see some showers on this Thursday to Friday. It will depend on the history of your vineyard and also the condition of the rain, but if you: 1) experienced severe Phomopsis outbreak in the recent years; 2) see your early varieties are already breaking buds; and 3) are expecting that the rain will sustain for a long time (5-6 hours), then you may need to think about a treatment against Phomopsis. For example, at Winchester, it looks like we have 60% chance of rain from 2 pm Thursday and the temperature is forecasted to be in 70's.  It will be in the same condition until the middle of the ni...

Our Dr. Keith Yoder just announced that he has a new blog about his disease updates on tree fruits .  If you are curious about diseases on apples, pears, and other tree fruits, please take a look at it!

Tony sent out March-April issue of Viticulture Notes yesterday. Along with the newsletter, he sent my fungicide spray guideline for 2010 . If you haven't received it, please let me know so that I can put you on Tony's email list. On that guideline, Ms. Lucie Morton found that I stated the rate of wettable sulfur was 5-10 lb/A and notified me that it should be lower. Although the label allows you to go that high, I agree with Lucie. It really depends on the formulation, so, I cannot give you the exact number for each, but we used Microthiol Disperse at 3-4 lb/A last year and it provided a good control. As I stated at the beginning of the guideline, it is merely a guideline. You need to adjust for your particular situations. Also, please follow the label for the rate, because I cannot cover all possible combinations of chemicals, especially the one with many generics, such as sulfur.