Virginia Grape Disease Updates

As you probably noticed, the sun is out! And grape vines are moving fast. Our Chardonnay and Cabernet sauvignon has moved quite a bit as well. About 7-90% are 5-7 inches (E-L 12) and about 5-70% are 5 inches (E-L 12) for Chardonnay and Cab sauvignon, respectively. Yesterday (5/07/09), Winchester area had a brief shower (0.12 inches) from 10:25am (64.4F) to 12:00pm (64.4F), and the RH was >90% until 1:00pm. And then a very short thunderstorm from 3:40 to 4:00pm (69.8F). The first event was bit short for Phomopsis and black rot infection events; however, it was probably enough for powdery mildew ascospore discharge and downy mildew infection event. A mild temperature range favors both mildew pathogens too. Some of varieties are probably getting closer to the bloom, and you need to start thinking about these mildews. 2-3 weeks pre-bloom is the start of critical period for downy mildew rachis and berry infection. Make sure that your spray program has something against downy...

At Winchester, yesterday's (5/6/09) rain started from 5pm (60.8F) and ended at 6:40pm (59.0F); however, the RH was >90% until this morning (8:40am). A total amount of precipitation was 0.05 in. Thus, it probably accounted for >14h of wetness with upper 50's in temperature (F). It would be enough for Phomopsis, powdery mildew ascospore discharge, black rot, and downy mildew infection. (I feel like I'm repeating myself...) Please do not get overwhelmed by my report of infection events. I'm just giving you the information about what could happen based on weather conditions observed. If you applied your protective fungicide application, it should stop infection. In addition, there are chemicals with curative (or kick-back) activity against powdery mildew, downy mildew, and black rot. Moreover, the risk of infection by these fungi depends on the history of your vineyard too. For example, if you had a history of outbreak of downy mildew year after year, you t...

Plenty of water affected growth too. About 10% of Chardonnay were 5-7 inches (E-L 12). Growth of Cabernet sauvignon was not as dramatic as that of Chardonnay. As with yesterday, 6-70% of shoots are 3-inch (E-L 9) and others are 1-2 inches.

Yesterday's (5/5/09) precipitation at Winchester was 0.27 inches starting from 4:20pm (55.4F) and ended at 9:20pm (53.6F); however, the relative humidity was almost 100% until midnight. Thus, >8 hr of wetness with mid 50's temperature probably accounted for Phomopsis light infection, powdery mildew ascospore discharge, and downy mildew infection. We had overcast weather this morning, and I hope that you had a chance for application (if needed). We are expecting more rain in next few days and next break could be during this weekend.

We experienced a whole day of rain yesterday (5/4/09). Average temperature was 52F, and amount of precipitation was 0.76 inches. The total length of rain event was over 30 hours, including a day before and early this morning. Needless to say, it was long enough for Phomopsis, powdery mildew, and black rot. Our Chardonnay had grown since the last observation. Average was about 60% 3-5 inch stage. Cabernet sauvignon still varies with vines, some vines are showing 6-70% 2-3 inch stage and others are showing 7-80% 1-2 inch stage. We are having a break from rain today in most of central and northern VA. I hope those of whom with concern on Phomopsis had a chance to apply your treatment.

The ongoing rain event since yesterday morning (5/3/09, 8 am) has been relatively cold. Starting temperature was about 52F, at mid-night it was 49F, and it has been lower 50's today. It is low, but long enough (>33 hr so far) for Phomopsis to cause light (5-15% disease severity on a cane) to moderate (15-25%) disease. In addition, as of this morning, we have received 0.84 inches of rain since last week (and it’s still raining as of 4 pm, but looks like the end is coming!!). As for powdery mildew ascospore discharge, this rain event met the requirement. A typical rule of thumb for fungicide re-application is “2 weeks or 1 inch of rain, whichever comes first” (Note: you need to adjust the time between applications based on the growth of vine). Please check your local weather for rain and temperature information. There is a banner of “weather underground” on the upper-right side where you can check your local weather information in detail by typing in your zip code (you can even...

Update on growth stage: Chardonnay was about 70-90% 3-inch growth; Cabernet sauvignon was still about 50% 1-2 inch growth with a variation among vines. Still raining as of 6:45pm...