Virginia Grape Disease Updates

Recent cooler weather seems to have slowed down vine development, but it looks like vines in the south are about to bloom. Bloom is a start of the critical time for cluster infections by downy mildew, powdery mildew, black rot, Botrytis, and ripe rot, because pathogens of these diseases can infect flower parts and develop symptoms later. What I recommend often is the use of protective materials to protect tissues for 4-6 weeks for V. vinifera varieties, and 3-4 weeks for V. labrusca varieties, which should translate into 3-4 sprays for V. vinifera , and 2-3 sprays for V. labrusca .  If you have hybrids, they are somewhere in between, so, 4-5 weeks to be protected.  As usual, please make sure to rotate mode of action (= FRAC) groups. Since we have not seen many rain events, I think powdery mildew will be the primary target for many of us. But please check your local weather to make sure, some downy mildew susceptible cultivars may still show downy mildew, especially, if there...

Some shoots of our vines are about to reach the second catch wire, but I saw many that were still about 10-12 inches too. It looks like series of rain during the month of April drove developments of black rot and Phomopsis. Although it was not severe, I found several vines with leaf lesions of black rot, and a few vines with Phomopsis. Tan-colored lesions are symptoms of black rot. If you click the image, it will open a larger file where you can observe fruiting bodies (tiny black specks) within these lesions. It is a bit difficult to see, but, please note the holes on the upper picture (in the blue circled area). The lower picture shows more characteristic leaf spots of Phomopsis cane and leaf spot. For management tips, please refer to my  around bloom disease management consideration post.  Please remember that clusters will be susceptible to black rot from bloom to 4-5 weeks after bloom. Looks like Winchester area is expecting chances of rain on Friday, M...

I noticed that some of our 'Cheloir' vines are in trace bloom and also it has quite a bit of black rot. As I suspected, the cold rains we received were long enough for pathogens beyond Phomopsis to be able to infect vines. If you have not seen black rot lesions on leaves and shoots, the pictures below are good examples. If you click on them, it should open a larger picture. Please note the small dots on these lesions. These are fruiting bodies that contains spores. Once again, most of us will be seeing bloom in the near future, and it will be a start of critical period for so many diseases . Please be on top of your game (shoot thinning, canopy management, and of course, fungicide applications). Mancozeb, as well as both DIM and QoI fungicides are very effective against black rot pathogen. Looks like we are expecting some rains again next week! Black rot on leaves  Black rot on shoots and rachis

I have received several emails about black rot recently.  With the frequency of rains we observed so far, it is not surprising to see small amount of black rot even if you had a very good coverage of fungicide. Typically black rot starts off as brown discoloration on the berry, and associated with production of fruiting bodies which are often look like black dots (the size of ground black pepper?).  The picture above is not the best, but you see black dots on the one at the top of the picture.  Eventually, the infected berry will become hard and dry mummified berry with a lots of fruiting bodies, which you can see here and there in the picture. If you are seeing something like the picture above, chances are the infection happened at least two weeks ago when berries were susceptible to the infection by black rot pathogen (and so as by powdery mildew or downy mildew pathogen).  I can say that because it takes about 2 weeks after infection for black rot pathoge...

Well, it looks like finally we have some break from rains. In Winchester area, rain started around 9AM on the 6th, and although there were a few interruptions, relative humidity stayed above 90% most of the time until around 9AM this morning (the 9th). Temperature varied from upper 40's to low 60's and most of time, it was low 50's. This rain event was long enough for Phomopsis and black rot infection events. A few people asked me "What should I do now?" Well, it depends on when you have sprayed last, and also, what type of history of black rot you had in the past. If you missed the spray before the rain (i.e., your last spray was more than 10-14 days ago), and your vineyards had an issue with black rot (which is not a common), it may be a good idea to consider either a DMI (SI) or a QoI (Strobilurins). We want to keep DMI for later use, so, unless you know for sure that the rain from the past few days will give you an issue with black rot, my recommen...

I received three emails from different vineyards (Southern, Northern, and Central VA) with black rot pictures.  Yes, it is a little early for us to talk about black rot, but we had very warm period at the end of March, with rain event (on 24-25th) that are warm and long enough for black rot.  I counted three black rot infection events at our location so far, thus it is not a total surprise to see it. If you happen to have black rot now, what can you do?  Unlike Phomopsis, black rot pathogen can produce spores after 2 weeks of infection under optimal conditions (> 70F), and if you see the symptoms now, chances are it is ready (or almost ready) for spore production.  Thus, even though we are not in the critical period for berry infection, the spores from this generation will be there for berries.  (yes, it is odd to talk about bloom in the middle of April, but at this rate, it will come soon. Hopefully cooler weather will hold off a bit.  We will see...)...

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 thunderstorm on 7/01/09 lasted a little over an hour (0.73 inches); however, RH remained high for about 9 hours with an average temperature of 65F. Thus the event was long and warm enough for Phomopsis, downy mildew, and black rot infection, and powdery mildew ascospore discharge. Also, we had nights with relatively high RH (>80%) in 6/29 and 7/2. If these are isolated events, you do not need to worry about; however, as we know, the early part of June was very wet and we had many nights with a favorable condition for downy mildew sporulation. Sure enough, when I checked vines yesterday, we had a lot of sporulations under side of leaves. If you have seen these spores on many of your leaves, you may want to think about protecting healthy leaves and berries from next infection event, or use materials with a curative activity (Phosphorus acid, Ridomil etc). The other disease I found quite a bit in our untreated vines are black rot. This is one of diseases which you really ne...

We had a short thunderstorm event on Friday (26th), but it lasted about 3.5 hours with an average temperature of 73F and a total precipitation of 0.33 inches. It was sufficient for powdery mildew ascospore discharge, and downy mildew infection. We are expecting see some more thunderstorm later this evening. As I mentioned in the previous post, most of us are still in 4-5 weeks after bloom which is a critical period for berry infection by black rot, powdery mildew, and downy mildew . I'm not advocating excessive application of fungicide here, but just reminding you that this is the critical time. If you have already protected your berries, you do not need to be panic. So what will happen when you failed to protect your berries? Here are some examples from unsprayed vines in my plot. Powdery mildew on berries Black rot on berries Black rot and powdery mildew on berries...

Thank you for those of you who made to the meeting at White Hall. My handout can be downloaded from the previous post. The rain is gone for now, and in the last three days, we had about 1.2 inches of rain, and wetness event lasted over 68 hours with average temperature of 58F. As you can imagine, it was good for Phomopsis, black rot, and downy mildew infection, and powdery mildew ascospore discharge. In addition, we had three consecutive nights with temperature above 55F which promote downy mildew sporulation. We are expecting even more rains next week. If you are in the same situation, please be prepared. Depends on when you applied the last application, what chemical you applied, how much rain you received, which variety you have, and the history of your vineyard, the situation will be different. However, it is blooming-postblooming time when the all major fungal disease can cause damages to your vines. I'm not an advocate of the intensive management, but these series ...

There was a sprinkle in the morning, but so far, we haven't experienced a significant rain event yet at Winchester. Our Chardonnay and Cabernet sauvignon have been moving its growth stage. It was about 8-90% with 8-9 leaves unfolded with elongation of rachis (E-L 15) for Chardonnay, and about 3-50% E-L 15 for Cabernet sauvignon. Cab sauvignon has more variation among vines, which is consistent since bud break. I start to see development of black rot lesions on untreated vines. It takes 2-3 weeks to develop fruiting bodies (black specks within tan colored lesion), thus, rain events during 5/5-5/7 or 5/11 are probably responsible for the infection. I hope you do not see many of these symptoms in your vineyard. If so, you need to protect your berries very well so that it won't affect your yield. Mancozeb, captan, and azoxystrobin (Abound) works well as a protective fungicide application, mycrobutanyl (Rally) works well as a curative fungicide, but I doubt that it will kil...