Wednesday, July 17, 2019

Berry touch/bunch closure disease management reminders

Our Chardonnay clusters are getting larger and berries are starting to touch each other. It feels a bit early, but here are grape disease management reminders at berry touch/cluster closure stage.

Botrytis gray mold: The best management practice is canopy management. Botrytis pathogen likes high humidity, thus, the poorly managed canopy that traps humidity will help them to thrive. This pathogen also needs wounds to develop spores, thus, grape berry moth and other insects plus birds need to be managed too.

There is a number of Botrytis materials such as Rovral and Meteor (FRAC 2), Elevate (FRAC 17), Vanguard and Scala (FRAC 9), Luna Experience (FRAC 7 plus 3), Kenja (FRAC 7), Miravis Prime (FRAC 7 plus 12), Switch (FRAC 9 plus 12), etc. Botrytis pathogen is well known for its ability to develop fungicide resistance, so, please rotate FRAC codes!!

QoI fungicides (FRAC 11) are no longer the effective material for us due to the development of QoI-resistant Botrytis isolates throughout Virginia. Pristine (FRAC 11 plus 7) has been compromised as well. The next timing of application for Botrytis management is at veraison.

Ripe rot and bitter rot: At this point of the season, captan (M4) and QoI (Strobirulin, FRAC 11) fungicides are good options. In addition, we found that Switch (9 plus 12), copper (M1), tebuconazole (3), Aprovia (7), and Ph-D (19) are somewhat effective. However, none of these materials consistently provided satisfactory suppression of ripe rot when we applied these materials by itself in a series of field studies. Thus, please mix two FRAC codes, especially if you have ripe rot issues in the past. The timing of applications will be the same as Botrytis.

Sour rot: A tank mix of the insecticide zeta-cypermethrin (Mustang Maxx) and the antimicrobial hydrogen dioxide (OxiDate 2.0) before a symptom appears (before veraison) to suppress the fruit fly population is the key. In NY, there are reports of Mustang Maxx-resistant fruit flies, thus, please do not overuse it. Two applications should be sufficient. If you don't have OxiDate, Switch also lists sour rot (suppression only), and other broadspectrum fungicides such as captan and copper, probably have some efficacy too.

Speaking of sour rot, please take a short survey below. We try to gain more information from growers so that we can come up with research priorities. It will be closed in 5 days, so, please participate today. It is not only for Virginia. Thus, if you grow grapes in other states or country, we need your opinion too!

Sour rot survey (will open a new window)

Monday, July 8, 2019

Sour rot survey

My colleagues and I are seeking information on sour rot so that we can develop research priorities (and hopefully obtain grants). Many of us suffer from sour rot (remember the last season?), and we need to get our heads together. If you could fill in this short survey, we really appreciate. It should not take more than a few minutes to complete. It is not just for Virginia, so, please feel free to complete the survey if you grow grapes outside of Virginia.

Sour rot survey (will open a new window)

Thank you!!

Downy mildew reminder

Of course, once I mentioned the lack of rainfall, we (i.e., northern VA) were hit by a series of rain. To make things more complicated, we had at least three consecutive nights of warm and humid (RH > 90%) condition prior to the rains. Humid nights encourage downy mildew pathogen to produce spores, and rains will splash spores to healthy tissues. Thus, the risk of downy mildew during recent rain events was very high. (Note: Central VA also had two humid nights in 7/5-7/6 at the Charlottesville airport, but the chance of rain is not high until this Thursday.)

There are a number of materials can be used for protection: captan (FRAC = M4), mancozeb, ziram (FRAC = M3), Revus, Forum (FRAC = 40), Zampro (FRAC = 40 plus 45), Ranman (FRAC = 21)). Mancozeb products have the 66-day PHI, but ziram's PHI is 21 days. Another good option is a copper (FRAC = M1) material, which is more economical than other materials. Copper has good efficacy against downy mildew, and tends to do well under frequent rain condition. There are several newer copper materials that cause fewer phytotoxicity issues even on relatively copper sensitive cultivars (e.g., Cueva, Champ, etc). If you have already having downy mildew issue, use copper, ziram, or captan as the backbone of your spray program.

Both a phosphite material (FRAC = P07) such as Prophyt and Phostrol, as well as Ridomil products (FRAC = 4 plus M3 (MZ) or + M1(Copper)) have a kickback activity against downy mildew (i.e., it can stop the ongoing infection process). Ridomil MZ contains mancozeb, so, we cannot use it at this time of the season due to its 66-day PHI. Ridomil Gold Copper has a 42-day PHI, so, you may able to use it, depending on the cultivar. Please note that Ridomil is known to have resistance issues, thus, let's limit the use of Ridomil to twice a season at the most. We also have seen multiple Revus-resistant isolates in VA.

So what to do, if you are not sure your previous fungicide application was sufficient (e.g., applied more than 10-14 days ago and/or received more than 2 inches of rain)? 

Options are:
A) a phosphite or Ridomil Gold Copper plus captan or ziram
B) a phosphite or Ridomil Gold Copper plus Revus/Forum or Zampro or Ranman. 

These mixes provide both protective and kick-back activities. 

Friday, July 5, 2019

A quick reminder about powdery mildew

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 rotate FRAC groups. Try to limit the use of a particular FRAC group to twice a season with an exception of FRAC group starts from M.

Fungicide resistance issues: QoI (FRAC 11, e.g., Abound, Flint, etc.) = widespread in VA. I would not count on the QoI material for powdery mildew management. Quintec (FRAC 13) = limited occurrences, but there is a recorded case in VA. There is evidence of DMI-resistant powdery mildew isolates in VA, thus, please do not overuse or rely heavily on DMI products.

If you have an outbreak that cannot stop, please use sulfur or copper (i.e., do not spray the material with FRAC group starts with a number). Also, Kaligreen, Armicarb, and other potassium bicarbonate (FRAC M) are effective; however these materials tend to be expensive. Stylet oil (FRAC M) can be used on on-going powdery mildew too; however, please note that you cannot mix (or use within 14 days) oil and sulfur (the same is true with oil and captan).