Monday, August 24, 2020

Prepare for a potential rain event for this weekend.


Looks like a tropical storm Marco is moving westward, but Laura may hit our areas during this weekend. The current forecast shows (opens a new window with a link to Weather.com) potential movement to Virginia and surrounding states around this Saturday (8/29). Many white cultivars will be either ready to pick or getting very close to harvest, and red cultivars may have several weeks to go, depends on where you are what you grow.

At this point in the season, the target diseases are late-season fruit rots (Botrytis, ripe rot, bitter rot, etc.) and downy mildew (on leaves).

For whites, it may be a decision of when to pick, but not what to spray. If you decided to wait until the storm is over, there are several materials with a very short PHI that you may want to consider, such as Oso/Ph-D or Elevate for Botrytis and a phos acid for downy mildew. FYI: when we tested in the field, Oso plus Double Nickel combination worked well against sour rot.

For reds, it will depend on when and what you have sprayed recently. As noted in the previous posts and meetings, it may not be a good idea to spray captan, sulfur, and copper near harvest due to their potential negative impacts on the fermentation and wine quality. However, you probably need to assess the situation at both your vineyard and winery. 

The links here are the presentation slides from the last viticulture meeting that covers some of the diseases mentioned above and a list of short PHI materials for Botrytis, powdery mildew, and downy mildew, which may help you decide what to do.


Friday, August 14, 2020

Downy, Botrytis, sulfur burn, and open house

With recent rain events and warm and humid nights, the risk of downy mildew and Botrytis are probably high in many areas of Virginia. For example, I counted seven days (at Winchester, VA) and eleven days (at Rustburg and Scottsville, VA), with downy mildew infection events in the past two weeks according to the NEWA model (http://newa.cornell.edu/). It is very important to keep your vines protected, and if you think you need to have a kick-back activity against downy mildew to counteract the recent rain event that your previous spray might not have provided good coverage, add a phosphorous acid material such as Prophyt or Phostrol. 

Please refer to this list of materials with short PHI since some of the cultivars are getting very close to harvest. Note: although both sulfur and copper have a short PHI, we typically do not recommend spraying these materials within 3-4 weeks of harvest due to its potential negative impacts on the fermentation or wine quality. 

When some of us ended up spraying when the weather condition is not favoring (i.e., leaves are wet), you may see chemical burn from sulfur. It tends to happen when sprayed leaves are still wet and the temperature shoots up to mid-90F or higher soon after the spray. A typical symptom is brown burn marks in the middle of leaves where you would expect spray residues stayed wet. The picture below is an example from my vineyard where I had to spray in drizzle last Sunday morning. (In my defense, the forecast did not call for rain and I did not even see it coming on the Doppler radar...) Unlike diseases, you will likely see symptomatic leaves uniformly happening throughout the vineyard. Or you may see in spots where you tend to slow down your tractor and thus spray more amount than other places.


Last, but not the least, we will have a virtual open house tomorrow starting noon. Please join us if you have time. You can visit us while eating lunch!

Virtual Open House: Virginia Tech’s Alson H. Smith Jr. Agricultural Research and Extension Center (AREC), Frederick County, VA

In lieu of public gatherings, the faculty, staff, and students at Virginia Tech’s Alson H. Smith, Jr. AREC in Frederick County will present a “virtual” open house on August 15, 2020 from 12:00 to 2:00 pm. The AREC is a unit of Virginia Tech’s College of Agriculture and Life Sciences and is primarily focused on research and educational programs that serve the tree fruit and wine grape industries. Center personnel will explain and illustrate their research and outreach efforts to combat invasive pests such as brown marmorated stinkbug, avoid spring frost damage to tree fruits and grape, evaluate new grape varieties for table and wine use, and much more. The public is encouraged to “attend” via a Zoom video conference which you must pre-register for: (https://tinyurl.com/y6aljklf). After registering, you will receive a confirmation email containing information about joining the meeting.  For more information, contact Debra Marple: 540-232-6031 or dmarple@vt.edu. Please visit our website to learn more about our current research on tree fruit and wine grapes. http://arec.vaes.vt.edu/arec/alson-h-smith.html

Facebook Event Site: https://www.facebook.com/events/1791448907672990/

Virginia Cooperative Extension programs and employment are open to all, regardless of age, color, disability, gender, gender identity, gender expression, national origin, political affiliation, race, religion, sexual orientation, genetic information, veteran status, or any other basis protected by law. An equal opportunity/affirmative action employer. Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

If you are a person with a disability and desire any assistive devices, services or other accommodations to participate in this activity, please contact Debra Marple: 540-232-6031 or dmarple@vt.edu during business hours of 9:00 a.m. and 4:00 p.m. to discuss accommodations 5 days prior to the event.

*TDD number is (800) 828-1120.

Thursday, July 23, 2020

Recent rain events and downy mildew spray

Central and northern VA received much-needed rains in the past two days. Looks like the chance of thunderstorm is high in today and tomorrow too. Several people asked me whether they need to jump on their tractor now and spray for downy mildew or not.

Here are snapshots of downy mildew risk events in three locations during the past two weeks.

Southwestern VA, Floyd: none
Central VA, Afton: 7/11, 7/22, and 7/23
Northern VA, Winchester, 7/11 (a very short one) and 7/22

Since we have been experiencing a very dry season, I do not think we need to react to every rain event, especially if your previous spray was less than 7-10 days ago. If you are due to spray soon anyway, it may be a good idea to have a Phos acid material (e.g., Prophyt, Phostrol, Agri-Fos, etc.) in the tank-mix as insurance.

For example, my previous spray (a fixed copper material) was on the 11th and my plan is to spray my vines this Saturday (the 25th) with a Phos acid, sulfur, and another PM material (Vivando, Quintec, or Torino). Although my spray was made 12 days ago, I am not too concerned about downy mildew because we scouted vineyard recently. We had a downy mildew trial this year, and out of nearly 2,000 leaves I assessed, there were only two leaves with symptoms. Not good for us, but good for everybody else!

Speaking of scouting, I am developing a new scouting form. If you are interested in, please check the instruction and Excel sheets (VT version (preferred) and a modified-MSU version for those of you who have been using this form). If you use this form, please give me a feedback so that I can improve.





Friday, July 10, 2020

Disease risk events from last two weeks and quick reminders

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 can cause phytotoxicity. Since powdery mildew thrives under shaded conditions, canopy management is important too (as you can see on the pictures above).

You may need to consider an application against Botrytis and ripe rot at bunch closure, which is good timing for you to make sure your spray materials will be delivered to inside of the cluster structure. For the list of materials against powdery mildew, Botrytis, and ripe rot, please refer to my previous presentation slide.


Sunday, June 21, 2020

Rains and fungicide sprays

It looks like we will see a series of rain events in the next few days. As noted in the previous virtual viticulture meetings, we had several black rot and downy mildew infection events in the past two weeks, and some people start to see the development of these diseases.

Some growers asked me how long does it take for fungicide materials to be dry enough to be rainfast. It depends on your sprayer, droplet size, weather conditions, etc., but typically, I hear 1.5 hours to 2 hours are required. Some fungicide labels show the expected dry time.

The other common question is about how much rain is enough to wash off fungicide materials from the plant surface. Please check the excellent articles from Dr. Annemiek Shielder (link 1, link 2). She talks about the rain fastness of some products using a rain simulator. It looks like there is a sharp drop only after 0.04 inches of rain, but after that, the material tends to stay on the surface, even if there are more precipitations. In the article in the link #2,  she discusses the efficacy of the product (= disease development after wash off). She indicated that even after the wash off, the products she tested were still effective since the rate we are applying is often time much higher than the minimum effective dose. Thus, the old rule of thumb still applies, which is "2 inches of rain or 2 weeks, whichever comes first".  The rule won't apply to early-season when shoots are proliferating.

If you start to see downy mildew in your vineyards, here's what I would do.

First, scout your vineyards and get an idea of disease incidence (yes/no). Check 10 random leaves at a spot and record how many leaves have downy mildew, repeat at 5-10 random locations, and get a percentage.

- 1-10% (sporadic here and there): mancozeb, captan, copper PLUS either a Phos acid or Ridomil. (There is a product called Gavel which has mancozeb plus another DM material too.)  Of these three, I have been impressed with copper against downy mildew, but one drawback is that you cannot mix a copper product with a Phos acid.

- 10-25% (notable outbreak): mancozeb, captan, or copper. There is a product called Zonix, that claims it kills zoospores. It may be a good mixing partner, but I don't have any hands-on experience. Some growers told me that it worked well for them. After things settled down, i.e., after seeing hot and dry weather to slow downy mildew, I may start using a Phos acid, mixed with either mancozeb or captan. Also, there is a new product called OxiPhos, which is a combination of Oxidate and a Phos acid, which may come in handy to have an extra kick to a Phos acid. We have not seen resistance with Phos acid, yet, but there are cases on hops downy mildew, so, please use it with caution. I would not spray Ridomil when there is an outbreak (i.e., above 10-15%). It is too risky.

- more than 25%: I would stick with mancozeb, captan, or copper in a tight schedule to protect healthy tissues.

Thursday, June 18, 2020

Slides from a virtual vineyard meeting, 18 June 2020

Thanks for all who were able to make it to our virtual meeting this afternoon!

The file below is the slide set I used today, the link will open a pdf file in a new window.

Critical time *very quick* grape disease management reminders - please see the previous two presentation slides for more information! 1) June 4th meeting, 2) PSU meeting.