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...

Thanks for attending today's online Extension meeting. Please find the materials I discussed today below. 1) Presentation slides (will open a new window with a pdf file). 2) List of short PHI materials for Botrytis, powdery mildew, and downy mildew.

Here are snapshots from the NEWA risk models from the past two weeks. Floyd: no black rot or downy mildew infection events Afton: black rot and downy mildew infection event on 7/1-2. Winchester: downy mildew infection event on 7/8 Looks like we are having relatively dry conditions. Depends on where you are and which cultivar you grow, you may not need to worry too much about downy mildew or black rot for a while. Also, most of our vines are done with the critical period of cluster susceptibility to black rot, downy mildew, and powdery mildew. What I would keep eye on will be powder mildew, since the pathogen does not require water for infection. Sulfur is a good material, and you can add powdery mildew materials, such as Quitec, Vivando, Torino, DMI, and SDHI to provide extra protection. When you spray a sulfur product, make sure the sprayed material will be dried up before temperature gets above mid-90F. If the leaf surface is wet with sulfur when the temperature gets high, it ...

It depends on where you are, but in northern VA, we are having relatively "dry" weather this summer. We have some rain events here and there, but overall, the air is humid, but not wet. Such a condition promote powdery mildew development, so, I just want to provide a reminder on powdery mildew management. (Powdery mildew symptom on a cluster: the picture was taken this morning) Powdery mildew pathogen thrives under shaded condition  (i.e., overcrowded canopy). Thus, canopy management is a very important aspect of powdery mildew management. Chemical management options: Sulfur (FRAC = M2) is an economical option. Copper (FRAC M1) works too, but compared with sulfur, copper seems to be slightly less effective. Other options are: DMI fungicides (e.g., Rally, Elite, Mettle, Rhyme, etc, FRAC 3), Quintec (FRAC 13), Vivando (FRAC 50), Luna Experience (FRAC 7 plus 3), Topguard EQ (FRAC 11 plus 3), Rhyme, Kenja, and Aprovia (FRAC 7), Torino (FRAC U6), etc. Please make sure to...

Powdery mildew colonies at the center of two leaves in the picture With this wet season, what we think most are downy mildew, black rot, Phomopsis, and Botrytis. Powdery mildew is often associated with a dry season, partially because this pathogen does not require rain for its infection process. However, what they also like to do is grow on shaded leave because they are sensitive to the UV radiation. With very clouded canopy from strong growth triggered from heavy water input (= rain), there are lots of leaves shaded by other leaves. Well, at least that is true in my vineyards... Sure enough, when I took a look at these leaves, I found powdery mildew colonies.  You can see a small white fuzz (= powdery mildew colony) on two leaves in the picture above. Note that how the leaves are shaded. We have a fair number of options for powdery mildew management. Sulfur (FRAC = M2) is an economical option, and there are range of products with different modes of action (= F...

Photo: we have started to see powdery mildew on clusters. I hope you are not! Looks like series of thunderstorms are hitting various part of the state this week. Just a reminder that due to April frost events, some of us experienced extended bloom. This not only means potential lag in harvest time for these late clusters, but also, an extended critical period for seasonal diseases. Grape clusters are susceptible to black rot, downy mildew, and powdery mildew from bloom to 4-5 weeks after bloom (3-4 weeks for American grapes). Thus, this is the time I want you to be on top of the game (i.e., nice coverage, 7-10 days interval, good selection of fungicides to be applied). Many growers told me that if they are clean around the Fourth of July weekend, you don't expect outbreak of black rot, downy mildew, and powdery mildew. I generally agree with their assessments since in many years, bloom happens late May or early June. However, this year, bloom started the first week of June,...

I guess both Anton and I tend to work on Sundays...  He send me two nice pictures of flag shoots. and As you can see leaf surface is covered with white powder (= mycelium and spores of powdery mildew). He also mentioned that he has used Armicarb and sulfur together and did not see any phytotoxicity.  Thus, if your intention is to fight against established colonies (whether flag shoots or late in the season), it would be a good combination.  If Armicarb or Kaligreen is too expensive for your budget, a straight sulfur application is the alternative. The other thing about powdery mildew is the effect of cold temperature.  According to a recent study by Cornell, low temperature (8 hr of below 8C or 46F) stops the powdery mildew activities. This is another reasons why we do not see much activity of powdery mildew relatively early in the season.  Hopefully, we have enough cold (but not freezing) nights ahead of us to stop early season powdery mildew.

In reply to the previous post about powdery mildew, Anton (Baudoin of VT) made a very good comment about "flag shoots" of powdery mildew.  This is a case of infection where powdery mildew pathogen survives on buds, and infect developing shoots from the get-go.  The result is infected shoots with retarded growth, hence people call it flag shoots.   You can see a picture from this Australian department of Ag site. Why this is important? Since it bypasses the ascospore stage, the infection event I mentioned in the previous post will be irrelevant, and you will have conidia in your vineyard from the beginning of the season, i.e., mass production of spores can happen quickly. Based on conversations I have had with growers in VA, flag shoots do not occur in VA growing conditions, and I have not seen this symptom yet.  This is primarily due to low winter temperatures; however, the winter we had was very mild, plus we had a dry summer (before Irene) to promote powdery mi...

I have been receiving more than usual number of calls and emails about powdery mildew this year.  It seems that consensus is that it started to show up on clusters last week and just keep going.  We have passed critical time for cluster infection at this point, thus, you may wonder how it happened?  Since it takes 7-10 days to develop symptoms (powdery appearance on the surface, i.e., spores), and it probably takes a few generations prior to the explosive outbreak, thus, the initial infection events probably took about 20-25 day ago when berries are still susceptible. Management options on on-going powdery mildew infection is limited.  As with other diseases, we do not have real curative or eradication material against powdery mildew.  Also, it is not recommendable to apply protectant fungicides because there will be higher risk of fungicide resistance development; however, as I mentioned in the previous post, there are a few options that can help you out....

I have received several emails and phone calls regards to downy mildew and Botrytis last week.  Also, some people are struggling with powdery mildew.  Here are general options against these diseases at this time of the season. Downy mildew :  At this point, your berries are out of susceptible period.  However, because of the last minutes rain some of us had about two weeks ago, some locations are experiencing downy on clusters.  Thing is that the infection took place a while ago, and there are not much options available.   For kick-back activity, you can use Ridomil Gold Copper (note: it also have 42-day PHI), or Phosphorous acid materials such as Prophyt or Agri-Fos (0-day PHI).  However, please keep in your mind that kick-back activity is for on-going infection, but not for already established colonies (i.e., if you see downy growth, these are established colonies).  If there are only minor colonies, you may able to keep them contained in...

Winchester area had thunderstorm yesterday from 12:40 PM to 3 PM (0.27 in), the average temperature was around 72 F.  The relative humidity were low after the storm, thus it accounted for about 3.5 hours of wetness.  Then, another drizzle hit us during 3:20 AM to 4:20 AM (0.02 in), and the RH was high (>90%) until 6:20 AM.  The average temperature during this 3 hour wetness event was 59F.  Since both were relatively short and relatively warm, the risk of downy mildew is the major concern. As I noted yesterday, we are having hot humid early summer which is a conducive condition for powdery mildew.  Here are today's pictures from our untreated vines that started to show the development of powdery mildew (click the picture to see a larger image).  Please scout your vineyards especially the location where leaves are shaded.  Powdery mildew pathogen prefer to thrive under diffused sunlight conditions. This is a close-up of powdery mildew pathogen...

I've been in and out of my office with my leaf roll survey project, and it has been bit difficult to frequently update the blog. Also, at this point, not much you can do about major fungal diseases. The critical time of infection is gone. If you see downy, powdery, or black rot on your bunches, the infection probably took weeks ago, if it was not a month. Thus, I will point some key issues at this time of the year. Botrytis, downy, and other rot We had a long period of wet and cool nights during the last half of August. This conditions favors both Botrytis and Downy mildew sporulation, and potential infection on nearby leaves. (i.e., it won't spready quicky, but it probably enough to increase number of spores.) If it is followed by rain (like we had last week), the risks of Botrytis and downy mildew increase. As I mentioned earlier, downy mildew won't touch berries at this point. Berries are resistant to the infection. However, with a prolonged rain or high hu...

On Sunday and Monday morning (7/6 and 7/7/09), we had relatively high relative humidity (>80%) at night which favor downy mildew sporulation. Thus, if downy is your concern, you may want to scout your vineyard. (But remember that it requires rain for new infection.) As I mentioned several times, we have an experimental plot where some of vines are not treated with any fungicides. On these vines, black rot, powdery mildew, and downy mildew become very severe. Last time I showed black rot, so, now it's a turn of powdery mildew. Powdery mildew on upper leaf surface Powdery mildew on bunches... No, it's not truffle... Powdery mildew on a cane

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 very much for attending the vineyard meeting at Doukenie last night. If you could not make it, you can download my note from here . As I mentioned several times in here and in this note, time period between bloom to 4-5 weeks after is the critical time to protect berries from downy mildew, powdery mildew, and black rot for most of wine grapes ( V. vinifera varieties) . Please scout your vineyards and make an adjustment to your spray schedule, if needed. There are some chance of rain tomorrow and on Sunday. The average relative humidity was low (~50%) and average temperature was around 71F last night. Now we have five nights without favorable conditions for downy mildew spore production; however, we had so many nights with favorable conditions for spore production during early June, so, keep that in your mind. Also, in our untreated vines, I start to see more and more powdery mildew. Rain events in last month or so probably provided enough opportunities for powdery ...

As you may noticed, temperature has been declining since this morning. National weather service is issuing frost advisory from 4 to 9 am tomorrow (Monday) morning for Frederick county. Please check your local weather. Recent series of rain events are providing enough moisture to vines to push their shoots rapidly, and these succulent tissues are sensitive to stresses. If you are the lucky one with one of air circulation systems (propeller etc), this may be the time to use it. Hopefully, your vines won't experience frost damages. Our Chardonnay was about 80% 7-8 leaves unfolded (~10-inch or longer), and Cabernet sauvignon was about 20% 7-8 leaves unfolded (7~10-inch) as of today. (Phomopsis symptoms on a vine in our experimental vineyard, 5/17/09) As for disease concerns, we experienced rains and thunderstorms yesterday from 6:20 pm to 10 pm, then there are light rains here and there until 2 am or so. The relative humidity was above 90% until 3:20 am. Temperature started of...