Tuesday, August 16, 2016

Last stretch!!

I heard that some of early cultivars are ready to be picked on 7-10 days.  Looks like no tropical storms to worry, yet... (crossing my fingers!)

There have been frequent thunderstorms went through our area, and some of them (like the one we had last night) resulted in a significantly long wetness event (8 hours). These wetness events, especially the one happens overnight, are preferred by downy mildew pathogen since they produce spores in dark, moist conditions.

Please check my previous post on late season downy mildew. As usual, protection is the best approach.  If you have seen some downy mildew on foliage, it would be best if you can mix a protective material (e.g., copper, captan) with a phosphonate (Prophyt, Phostrol, etc). If there is heavy downy mildew already happening, please do not use Ridomil (metaxel) products because they are known to be overcame by the downy mildew pathogen.

Some of reds are probably going through veraison, which would be a good timing for your Botrytis application. Make sure to rotate mode of action since Botrytis is known to develop fungicide resistance. Please refer to my previous post for details on Botrytis management.  Also note that as with Botrytis, a key for sour rot management is the protection of fruits from any wounding events (birds, insect, human activities, etc). These pathogens tend to take advantage of existing wounds.


Friday, July 29, 2016

Another extensive rain event recorded

As many other parts of VA, we have been experiencing thunderstorms running through our area in the past week for so. Yesterday, we had about 9 hours of wetness with an average temperature of low 70's, then followed by a very humid night with an average temperature of mid-60's. These conditions are conducive for downy mildew development. Please check the previous posting for more information on downy mildew management.

Also, as veraison approaches near (or happening for some cultivars), please check 12 July posting on late season disease management.

Tuesday, July 19, 2016

A quick reminer on downy mildew

At Winchester, we had 8 hours of wetness observed on last Thursday, then there are several short rains/thunderstorms. Moreover, the night time relative humidity has been fairly high (> 95%) in the past few days. These conditions favors downy mildew development because downy mildew pathogen prefer to produce spores under dark humid conditions.

At this point, you do not need to worry about downy mildew infection on clusters; however, they can still infect leaves. Often time, you will see infection on the top of the canopy around this time of the year because younger leaves are more susceptible than older ones. Once infection gets severe, it can defoliate infected leaves.

As usual, it is much better and easier to have a preventative program than try to play a catch up game. There are many good protective materials for downy mildew, such as Revus products, Zampro, Forum (Please note that these three share the same mode of action), and Ranman. In our small trial, we found that fixed copper worked pretty well against downy mildew when we compared with captan. Captan worked too, but a fixed copper product lasted longer than captan. Some wine makers do not like to see captan or copper used late in the season, so, this time of the season maybe the best timing to use copper and captan for downy mildew. You can also use a phosphorous acid product as well. For some cultivars, you may still use zirum which has a 21-day PHI. Please mix and match to rotate among mode of action groups.

Tuesday, July 12, 2016

Seasonal disease management considerations toward the end of the season.


Most of us are about to finish critical time when clusters are susceptible to infection by downy mildew, powdery mildew, and black rot.  This critical time varies by varieties, but in general, 4 to 6 weeks and 3-4 weeks from bloom for V. vinifera and V. labrusca species, respectively.  After this critical period, you should be able to relax a bit because these pathogens no longer able to cause disease on berries.

So, what’s next?  As usual, disease dynamics really depends on environmental conditions, cultivars grown, and cultural practice you employ, but in general, this is the moment when you will be thinking about late season diseases such as Botrytis, ripe rot, bitter rot, and sour rot. 


The spray timings for Botrytis is at bloom, bunch closure, and veraison.  The pathogen seems to be active throughout the season. The main reason we recommend application of a Botrytis specific material at bunch closure is that this most likely to be the last opportunity for you to deliver the material into the clusters, especially if you grow tight cluster cultivars, such as Chardonnay. 

Also, please keep in your mind that Botrytis is very well known for its capacity to overcome fungicides.  Thus, a rotation of mode of action groups is very important aspect for Botrytis management.  Mode of action for a particular fungicide can be found as a FRAC code, which you should be able to locate on the label.  Here’s a short list of materials for Botrytis and their FRAC code in parenthesis: Elevate (17), Scala (9), Vangard (9), Switch (9+12), Inspire Super (9+3), Luna Experience (7+3), Aprovia (7), Meteor/Rovral (2), Endura* (7), Pristine* (7+11). 

The material with an asterisk has a high concern on resistance development. Please note that the group 7 is considered a high risk, but I did not put an asterisk on Luna and Aprovia. I am not indicating that they are not high risk materials; I just simply do not have enough data. However, since they belong to the same mode of action group, I would be very careful on the usage.

When you are in doubt, please rotate the FRAC code because two different fungicides with two different chemical names may have the same FRAC code as you see on the list above. Also, tank-mixing with other material, such as captan, which has weak to fair activity against Botrytis, will help lowering the risk of fungicide resistance development.  If you think one of fungicides you are using is not providing sufficient control, please contact Dr. Baudoin or me.

Another management tip for Botrytis is about wounding on berries.  Although Botrytis is capable of causing disease by penetrating grape’s fruit skin tissues, it prefers going after wounds.  Also, at bloom infection by Botrytis tends to develop into disease when grape fruit skin is damaged, but when the skin is intact, the risk of disease development will be much lower.  Thus, management of the source of wounds such as insects (esp. grape berry moth), or birds, or powdery mildew at early in the season, can results in the management of Botrytis.  Moreover, sour rot pathogens also really like going after wounds; so, the same wound management would be the key for the sour rot management as well. 

Both ripe rot and bitter rot are considered as a warm season diseases, and in fact, people in the south tend to suffer more from these diseases; however, we can find ripe rot in Northern VA year after year.  In addition, I have heard from growers from PA and MD that they are having issues with ripe rot as well.   


Unfortunately, ripe rot in particular, has been misdiagnosed more than other rots.  It shows up as if the berries are sunburned, thus you will see a round dark brown lesions on the top portion of an infected berry.  The difference would be the tiny dots you will see within the lesion, which are fruiting bodies of the pathogen.  As the time progresses, the lesion expand into the whole berry, and infected fruits become shriveled raisin-like fruits (please see the picture above).  With a severe infection, you may see the majority of berries on a cluster shriveled down.  Unlike sunburn, it does change the taste of the fruit and wine.  Unfortunately, the change is in the negative direction, and a study showed that a consumer panel could detect only 3% contamination in the mast.

Both ripe rot and bitter rot seem to be able to infect berries from time of flowering to the harvest.  Thus, protection is important, especially if you have experienced issues with these diseases in the past.  Mancozeb, ziram captan, and QoI (Strobirulin) fungicides are currently recommended.  Please rotate among these mode of actions because some of isolates causing ripe rot are not sensitive to some of those materials. Our recent research effort showed that fixed copper materials have a good efficacy as well, thus, it maybe a good idea to tank mix these materials with a fixed copper. (Note: due to the 66-day PHI of mancozeb, you may not have enough days remained to spray mancozeb at this time of the season.)

Tuesday, July 5, 2016

An update on yesterday's post

Just an update on yesterday's post: At Winchester, AREC, a total leaf wetness hours was 29 hours, with an average temperature of 67F. Thus, this rain event was warm and long enough for both downy mildew and black rot. Also, it was long enough for Botrytis as well.

For the details of downy and black rot kick back activity fungicides, please refer to yesterday's post.

Monday, July 4, 2016

An extensive wetness event recorded (and still going)

A cool and wet Fourth of July weekend resulted in a very long leaf wetness hours. So far, more than 12 hours of wetness has been recorded at Winchester AREC, and at this rate, it will go on over the night (= potentially it will be a longer than 24 hours of wetness event).

As noted in my previous post, the April frost event(s) probably resulted in a prolonged bloom period. Because of that, I have a feeling that some portions of clusters of many of cultivars are still under a critical period for downy mildew, powdery mildew, and black rot for at least one more week.

If your vines were protected with materials such as mancozeb (for downy, and black rot), Revus, Forum, Zampro, Presidio, or Ranman (for downy), Luna Experience, Rally, or other DMI (for black rot) or Abound, Pristine, or other QoI (for black rot), you should not worry too much about it. However, if your vines were not well protected, (e.g., last application was more than 12 days ago, or you have missed some of protective materials in the previous application), then you may need to use a fungicide with a kick-back activity to stop the potential infection which maybe going on right now. For downy mildew, you may use a phosphite (Prophyt, Phostrol, etc), or a Ridomil product. For black rot, you may use a DMI (Rally, tebuconazole), or a QoI (Abound, Pristine, etc). (Note: a DMI works better than a QoI in terms of kick-back activity).

Also, while I am talking about "kick-back" activity, please note that the kick-back or curative activity means that you may stop the on-going infection process 3-5 days after the initiation of infection, and while it is not showing symptoms. It does not work on existing downy or black rot symptoms. In fact, none of fungicides we use truly eradicates existing symptoms. Thus, often time, it is the best to have a good protective program, rather than trying to spray after the fact. Especially during the critical period for cluster infection, which is from bloom to 4-5 weeks after bloom.

Tuesday, June 28, 2016

Another reminder on seasonal diseases


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, but I saw delayed bloom until the middle of June, thus, I will keep my schedule relatively tight until early to mid-July (i.e., rather than the Fourth of July weekend, I would aim for the next weekend). Since we are having several rain events in the past few days, I would watch the forecast carefully, and prepare for good protection against, downy (e.g., Revus (Top), Zampro, Ranman, mancozeb, Phosphite, etc), black rot (DMI (Rally, Revus Top, Luna Experience, etc), mancozeb, QoI (Pristine, Abound, etc)), and powdery mildew (Vivando, Luna Experience, Quintec, Tanos, sulfur, etc). Please note that some of mixed materials may have efficacy on multiple diseases.

Also, we will have a vineyard meeting at Arterra Wines in Delaplane, VA from 11:00 AM tomorrow. I hope to see you there!