Monday, August 31, 2009

Late season management topics

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 humidity event, Botrytis can penetrate tissues to cause infection.

As I mentioned in last week's viticulture note, it is good idea to protect your berries with Captan or Captan + Botytis material (Vangard, Elevate, etc), especially when you are expecting rain. Unfortunately, we do not have any curative materials against Botrytis, unless you apply materials within 24 hours, which is not a realistic number. Also, once sugar level go above 20 Brix, nothing much we can do in terms of management.

Downy and Powdery mildews
At this point, your berries are resistant to infection of downy and powdery mildew fungi, thus, your focus will shift to foliage management for both sugar accumulation to berries for harvest and to trunks for winter survival. Typically, foliage is easier to manage than fruit zone and you can extend your spray schedule to 10-14 day interval, especially you do not receive much rain.


Leaf roll survey
As I mentioned earlier, I'm conducting a survey for leaf roll virus (pictured above). Idea here is to have a better picture of leaf roll infestation in the state of Virginia, and also, by monitoring several years, I'd like to know how well it can spread within a vineyard. It is a free diagnostic opportunity for you (funded by the VWB), and there are spots available as of today. If you have any suspicious vines, please let me know so that I can arrange a visit to your vineyards. Other organism that I'm interested in is mealybugs (pictured below). Mealybugs can serve as a vector of leaf roll (virus). As you see in the picture, the bug is very tiny, but you can see them with naked eyes. If you have seen the bug, please let me know as well.

Monday, August 24, 2009

Finally rain has come...

We had fairly dry month so far and grasses are start to getting brown, but a thunderstorm dropped about 0.5 inch of rain last Friday around Winchester area. It stared little after 5:30 pm then continued for two hours or so. The relative humidity was high until next morning (10:30 am) with an average temperature of around 70F. This accounted for infection event for Phomopsis and Botrytis on berries and Downy mildew on leaf.

I listed Phomopsis here simply because it can infect berries at any time; however, often time infection around this time is not common. It is probably due to less inoculum (this fungus mainly creates the fruiting body in the spring), and it favors cooler climate. Thus, your risk depends on how chronically you have Phomopsis in your vineyards. If Phomopsis appears year after year, then your risk is higher. If you just see them only when we have a wet spring as we did this year, you probably have less risk.

As for downy mildew, I started to see foliar symptoms on my sprayed vines here and there. Probablty they have spread using dew periods. Since last week, there were many nights with high relative humidity which promote sporulation of downy mildew, and with a combinaiton of wind and a dew, they can infect some leaves. If day time weather is dry, it is not an issue, but as I mentioned in the last post, you need to protect your foliage to some extent to keep general health of vines. Thus, it is probably a good idea to scout around your vines, and apply curative application if necessary. (i.e., if you have many leaves with downy already, and received as much rain as we did, you probably need some action.) PHI for Ridomil Gold copper and Ridomil Gold MZ is 42 days and 66 days, respectively, and that for Prophyt is 0 day.

Wednesday, August 19, 2009

It was a very nice workshop!

(A graft union infected by fungal organism(s))

We had a very nice workshop this Monday. Now everybody become experts of various trunk diseases! As Philippe discussed, most of fungal pathogens that cause trunk diseases often requires a point of entry. In addition to pruning wounds, accidents with a weed-whacker, etc that were in the discussion, due to cold temperature during winter, some of varieties are more susceptible to have cold injury which also can serve as a point of entry for microorganisms. Site and variety selection can play a big role in prevention of these diseases.

As for other diseases we face, we had several nights with high relative humidity (8/14-16) which favors downy mildew sporulation. Then we had thunderstorm passed by on 17th and 18th. Thus, if you had visible downy mildew colonies on your leaves, the risk of new infection was high during these thunderstorms. (I'm talking about leaf infection, not berries.) At Winchester, last night's thunderstorm did not last very long, but since it came late at night (9:20 pm), leaves were probably wet until 7:40 am or so. This accounts for > 10 hours of wetness, and the risk of Botrytis would be high during this time.

Friday, August 14, 2009

See you on Monday at Jefferson vineyards

We will have Virginia Vineyards Association's summer technical meeting at Jefferson vineyards on August 17th (this Monday). Dr. Philippe Rolshausen and Lucie Morton will be talking about trunk diseases. I will be there to provide some help and learn more about trunk diseases. If you would like to talk to me about other diseases or discuss about your spray programs, please let me know.

Wednesday, August 12, 2009

I'm back!!


Hi all,
I'm back from the meeting. It was a meeting for plant pathologists, and it was a very good meeting for me to meet new people (since I'm a new kid on the block and all), as well as my other colleagues. I visited Washington's Yakima Valley grape growing area after the meeting to visit a collaborator on grape leaf roll disease project. There are very exciting area to study, and I will talk about it more in detail within a few days. The picture was taken from their field and showing mealybug infestation.

In anyway, I took a quick look at what happened (weather-wise) during my trip. It seems like there are very short rains on 7/31, 8/1, and 8/5, and these were not so significant in terms of major fungal disease infection. We had 4.5 hour of wetness on 8/2 which probably accounted for downy mildew infection (primary on leaves, at this point), and 9.5 hours of wetness on 8/6, which probably accounted for downy mildew and Botrytis (and maybe black rot, if your berries are very late and still susceptible). Your berries are probably at or after veraison at this point. I hope you did not forget your protection against Botrytis.

Saturday, August 1, 2009

I'll be out of office until 8/11

I'm at a conference until 8/11. In the mean time, we will have a vineyard meeting at Winchester AREC on 8/5. I cannot attend, of course, but I prepared a note (click here to view) and Tony will go over it.

Since I may not able to update this blog, I summarized the disease risk at this point. I hope it will be helpful.

Disease risks at this point (early August)

The critical period for black rot, powdery mildew, and downy mildew fruit infection should be over (4-5 weeks after bloom).

Black Rot: Berries should not susceptible any longer.
  • For the next year, make sure to remove all berries from your trellis at or after harvest. These infected berries can produce spores all season long if they are left on the trellis
  • Phomopsis: Berries are susceptible all the time.
  • Since we had such a long period of rain in the spring, you may see some berry infections. Usually, the damage done by Phomopsis is not significant, but if you have reasons to be afraid of Phomopsis, captan can be applied to protect berries.
Downy Mildew and Powdery mildew: Berries should not susceptible any longer.
  • At this point, your focus is shifted to management of foliar health for accumulation of sugar to berries (for wine quality) and to trunk (for winter survival)
  • Thus, periodical application of fungicide for downy and powder mildew is necessary; however, you do not need to have an intensive schedule as you may do during the critical time. 10 to 14-day interval should be enough to keep vines healthy unless there are many rain events.
  • Fixed copper is a relatively inexpensive materials that can control both downy and powdery mildew. However, application within 30 days before harvest may decrease the quality of juice for wine making.
  • Other materials to consider, especially within 30 days of harvest, are Potassium salt materials for powdery and Phosphorus acid materials and captan for downy mildew. Pristine can be used up to 14 days prior to harvest too. Please keep your eyes on PHI.
Botrytis: It favors humidity as well: canopy management is important.
  • It is good at take an advantage of wounds. Successful insect management, such as against grape berry moth, helps reduce risk of Botrytis outbreak.
  • There is no formal study done yet, but a combined application of fungicide at just before bunch closing plus at veraison is known to reduce the risk of Botrytis outbreak. Dead grape tissues (flowers, aborted berries) can be a medium for Botrytis, thus, good penetration of spray materials to inside of the bunch is important (an application prior to bunch closing), and the number of Botrytis spores in the air is high around veraison (an application around veraison).
Other late season rot:
  • As with Botrytis, wounded berries can be susceptible to other microorganisms that cause “sour rot” or “bitter rot”.
  • Thus, if you experience damage to berries due to hail, strong thunderstorms, excess moisture etc, you may need to react to the situation. Captan or Captan + Botrytis material is often recommended.
  • When Brix is high (>20 or more), it is very difficult to control growth of microbes.

Other notes about chemical management
  • Mancozeb products (Penncozeb, Dithane, etc) have a 66-day Pre-Harvest Interval.
  • Sulfur applied under high temperature condition (> 85F) can cause damages on vines.
  • Do not mix sulfur (or captan) with oil. It can burn grape tissues. Some insecticide formulations contain oil as a carrier!!