Virginia Grape Disease Updates

I noticed that some of Chardonnay and Merlot started blooming on Friday.  I ended up spraying on Saturday for pre-bloom treatment.  Once again, from bloom to 4-5 weeks after bloom is critical periods for black rot, downy mildew and powdery mildew cluster infection.  Please stay on top of your program during this period.  Once you successfully protect your vines through this period, you can relax a bit. I saw a little bit of downy mildew development in our vineyards, but nothing major.  I am wondering how recent hot and dry weather conditions affect their population.   We will see... Speaking of hot weather, I've noticed that there is air quality alert issued to Fredrick county.  I don't know what would be the effect to grape diseases, but it does affect us.  Make sure to hydrate yourself and keep eye on your body conditions.

Winchester area received thunderstorms yesterday evening around 7 PM; however, air dried out quickly, so, it was probably not long enough as disease risks.  Then we received a light rain around 1 AM.  Once again, it was short and very light, thus, it was not enough for disease infection risks.  However, as I mentioned yesterday, these isolated thunderstorms may have brought longer rains at your vineyards.  Please check your local weather. Another thing that I noticed is that we have been having two consecutive warm nights with high relative humidity (>90%).  The pathogen of downy mildew tends to sporulate under these night time conditions.

Isolated thunderstorms went through the area and Winchester area received a short shower last night from 12:55AM.  The relative humidity remained high (>90%) until 6:55AM, thus, it accounted for 6 hours of an estimated wetness period with an average temperature of 65F.  It is long enough for light infection for Phompsis, and infection for downy mildew.  These isolated thunderstorms are difficult to predict and as the name implies, it could affect only a limited area.  For example, I was watching dark cloud passing through last evening, but we did not receive any rain at my place.  Please check your local weather closely and make an adjustment to your spray program, if needed. Many of us are about to have bloom soon.  Please keep in your mind that from bloom to 4-5 weeks after bloom is a critical period of cluster infection by downy mildew, powdery mildew, and black rot.

We had a few short showers passed yesterday morning and afternoon; however, none of these are long enough to be disease risks.  On the other hand, other regions in VA seems to received longer showers than we did.  If you are concerned, please check your local weather record.  You can access your local weather information through the link to the "weather underground" on the right hand side.

Thunderstorms went through Winchester area soon after the midnight (~12:35AM, 0.09 inches), and the relative humidity remained high (>90%) until 8:55AM.  Thus, we experienced an estimated wetness period of 8 h 20 min with an average temperature of 67F.  This event was long enough for Phomopsis, black rot, and downy mildew.  Boy, we have a lots of rain this year!

After a short dry period, we received rains from around 8:15 PM last night (18 May), and although the rain stopped within a few hours, the relative humidity was high (>90%) until 11 AM this morning (+ we had a short shower in the morning).  Thus, we experienced about 14.5 hours of the estimated wetness with an average temperature of 57F.  It was long and warm enough for Phomopsis, black rot, and downy mildew infection. We had a nice sunny afternoon; however, I see several thunderstorms coming in this way.  We will see... Speaking of black rot, we observed the first black rot symptom today.  (the picture is fresh out of the vineyard!)  Since it is already producing fruiting body,  infection probably happened much earlier, but I did not have much time to spend in the vineyard due to rains. Having sporulating black rot colony before bloom means that we need to be concern about berry infection.  Please scout your vineyard for disease symptoms so t...

The rain started around 9:20 PM on 16 May in Winchester, and since then, there were several rain and thunderstorms went through the area.  The relative humidity remained high even between the rains, and it was around 10 AM on 18 May when things were started to dry out.  Thus, it was about 38.5 hours of estimated wetness event, with 0.08 inches of rain and average temperature of 62F.  As expected, it was long and warm enough for Phomopsis, black rot, and downy mildew. We were not affected, but some of you have experienced hails during this week's series of wet weather events.  Hopefully, it did not cause a major damage to your vines. It seems that the chance of precipitation is low for tomorrow, Saturday, and Sunday.