Friday, September 30, 2016

Rain rain...

It has been pretty wet week so far. At Winchester, we observed about 11 hours of wetness on the 26th with an average temperature of 64F, 50+ hours (and counting) from the 28th to today with an average temperature of 61F. It is certainly long and warm enough for Botrytis infection.

Whether we require a fungicide application for Botrytis or not at this point depends on several potential factors. The first one, of course, is whether you had a previous application or not. If you had a previous application to cover these rain events, you probably do not need to be concerned much.

The second one is a time to harvest. If you still have several weeks to go, you may want to consider an application, but if you will harvest within a few days, I am not sure an application of fungicide will help you much since we do not have any curative materials for Botrytis. Whatever we spray at this point will be good against future infections, but not the infection happened already.

The third is cultivar. Chances are, many fruits out there at this point would be that of red cultivars, which is relatively less susceptible to Botrytis infection than whites, especially the one with a loose cluster architecture. I can see some white cultivars such as Petit Manseng can still be out there, but Petit Manseng is also less susceptible to Botrytis than many other whites.

The fourth is the environment. The rain this week was pretty severe, but the last major rain event at Winchester was 18 August. We had wetness event here and there, but these were relatively short events (many are less than 5 hours in leaf wetness), thus, I have a feeling that this dry weather did not favor spore production of Botrytis. Thus, even the environment was favoring this week, spores might not be available to cause infection. Since it has been pretty dry, the application you have made a while ago may still not be washed away too.

The other late season disease you may want to consider is sour rot. With this rain, berry skin may be damaged and that may invite sour rot pathogens to come in. Our understanding is that sour rot is caused by several different types of pathogens, thus, often time, a broad spectrum fungicide is recommended. I do not think any of fungicide works to stop the on-going infection, but you may want to stop the spread of sour rot. Something like copper or captan may be a good choice, especially if you have a few weeks to harvest. Both Switch and Fracture list Botrytis and sour rot (for suppression, not control) as a target disease. Also, polyoxin-D materials such as Ph-D or Oso list not only Botrytis, but also Alternalia, which is also known to cause late season general rot, as target pathogens.

Lastly, here is re-posting of a list of fungicide with relatively low PHI for your reference.

Wednesday, September 7, 2016

On these unusually slow ripening issues

Maybe I should keep posting that rains are coming... Each time I do, it looks like rains disappear. ;)

If you have not subscribed to the email list of Dr. Tony Wolf, who is our viticulturist, please do. It always have a wealth of information. In his latest newsletter, he explores potential reasons why some of cultivars are stalling on ripening process this year. Our Chardonnay is still stuck at 19 Brix or so too!

The below is a link to his newsletter, but it typically take several days for the newest one to appear. Thus, the best way is to subscribe to his email list (the instruction for subscription is listed on the page too).

http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/extension/VN_options_index.html

Friday, September 2, 2016

List of low PHI materials for Botrytis, powdery mildew, and downy mildew

Well, I was hoping that the rest of the season would be nice and sunny, but I was little naive. Looks like some portion of VA may be affected by tropical storm Hermine. Plus, probably due to the hot several weeks with no cooling time during the night in August, some cultivars have been very slow to ripen. I was bit surprised to see our Chardonnay is still around 17 Brix this week.

Thus, I complied a list of fungicides with relatively low PHI (7 days or less). I cannot cover every single fungicides out there, but I tried to cover common ones. Please click here to download the table from Google drive.

NOAA's precipitation prediction for Harmine as of 2 Sept. 2016

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.)