Seasonal grape disease updates
For VVA Summer Technical Meeting at Stone Tower Winery
12 June 2019
The critical period for cluster infection: Grape clusters are sensitive to infection by powdery mildew, downy mildew, and black rot from the time of bloom to 4-6 weeks after bloom. The length of this critical time depends on the cultivar, 3-4 weeks for American grapes (V. labrusca), 5-6 weeks for French grapes (V. vinifera), and somewhere between the two for hybrids. Please make sure that you have good coverage during the critical period. In a typical year, once we pass the Fourth of July weekend with a clean canopy, we can relax a bit. We are a week or two ahead of “normal” year, so, you may able to relax sooner this year.
Downy mildew: Rainfall is not as intensive as the last year (knock on the woods!), but there are enough to drive downy mildew (plus we all probably have more than the typical amount of inoculum due to the last year). 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)). 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). Please note that Ridomil is known to have resistance issues. Luckily, we have not seen it in VA, so, let’s keep it that way. Please limit the use of Ridomil to twice a season at most. We also have seen Revus-resistant isolates in VA too.
Another good option is a copper (FRAC = M1) material, which is more economical than other materials, has good efficacy against downy mildew, and tends to do better 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, mancozeb, and captan as the backbone of your spray program.
Also, downy mildew tends to come back as the temperature goes down at the end of the summer (i.e., the last half of August). Lower temperature causes the relative humidity at night to go up, and it may end up with dews, which is an ideal case for downy mildew pathogen to produce spores and infect new leaves. Thus, make sure to use a material for downy mildew to protect your vines later in the summer.
Black rot: Rain also makes black rot pathogen very active, Mancozeb works well, but if you see leaf symptoms already, adding a QoI (FRAC = 11, e.g. Abound, Flint, etc) or a DMI (FRAC = 3, e.g., Rally, Mettle, etc) or a SDHI (FRAC = 7, e.g., Aprovia, Luna, etc) for black rot is probably a good idea. The DMI has a better kick-back action (i.e., can reach-back a few days) than the QoI, and we do not have a data on SDHI in terms of the kick-back action. Please note that both captan and copper do not have good efficacy against black rot.
Powdery mildew: Powdery mildew pathogen thrives on shaded leaves (i.e., overcrowded canopy). We have a fair number of options for powdery mildew management. Sulfur (FRAC = M2) is an economical option, and there is a range of products with different modes of action (= FRAC group). Examples 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. Also, please note that you can not mix (or use within 14 days) oil and sulfur (the same is true with oil and captan).
Unfortunately, we have widespread of QoI (FRAC 11, e.g., Abound, Flint, etc.) fungicide resistance powdery mildew isolates in VA, so, I would not count on the QoI material for powdery mildew management. Also, there is evidence of DMI-resistant powdery mildew isolates in VA, thus, please do not overuse or rely heavily on DMI products.
Botrytis gray mold: Development of Botrytis depends on what type of varieties you grow, as well as your canopy management strategies. White-fruited varieties with tight cluster architecture tend to be more prone to Botrytis. I.e., a red-fruited variety with loose clusters have fewer issues with Botrytis, especially if the canopy is well maintained. Since Botrytis pathogen likes high humidity, poorly managed canopy that traps humidity will help them to thrive. Bloom time is important for Botrytis management because this fungus can infect flower and flower debris, and come back later when berries are maturing. 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.
As with powdery mildew, QoI fungicides (FRAC 11) are no longer the effective material for us due to the development of QoI-resistant Botrytis isolates throughout VA. Pristine (FRAC 11 plus 7) has been compromised as well. Botrytis pathogen is known to develop resistance to fungicides very quickly, so, please make sure to rotate FRAC groups! The next timings of application for Botrytis management are: at bunch closure (to make sure the materials gets to the inside of clusters), and veraison.
Also, research from Penn State showed that when Botrytis infect flowers, symptom development requires wounds on the berry skin. One of the causes of wounds on grape berry skin is powdery mildew which damages skin cells to cause splitting. Grape berry moth also causes wounds on the berry skin tends to show up in the vineyards around this time of the year. Unlike diseases, you can wait until you see webbing on the clusters to make a decision on spraying against grape berry moth.
Ripe rot and bitter rot: Unfortunately, we do not have a lot of options against them. Mancozeb (M3), ziram (M3), captan (M4), and QoI (Strobirulin, FRAC 11) fungicides are currently recommended. 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. Moreover, we documented that resistance to the QoI fungicide is common among ripe rot pathogens in VA.
Thus, if you have a severe case of ripe rot, what I recommend is the use of mancozeb as long as the 66-day PHI allows, plus add a QoI or Switch or Aprovia or tebuconazole at bloom. Then use either copper or captan (0-day PHI for both) or zirum (21-day PHI) and mix with a QoI (14-day PHI) or tebuconazole (14-day PHI) or Switch (7-day PHI) for the latter part of the season. (Aprovia has a 21-day PHI.) We have resistance issue with the QoI fungicides, so, do not apply by itself. I am still recommending the QoI as a mixing partner since we are dealing with several different fungal species, and some are still sensitive to the QoI. The timing of applications will be the same as Botrytis, so, include ripe rot material when you apply for Botrytis.
Sour rot: A study from Cornell University reported up to 80% reduction of sour rot when they use a tank mix of the insecticide zeta-cypermethrin (Mustang Maxx) and the antimicrobial hydrogen dioxide (OxiDate 2.0) weekly starting prior to the appearance of sour rot symptoms (~ veraison). You probably do not need to spray weekly, but if your cultivar is prone to sour rot, the fruit fly management before symptom development will be the key. As with fungicides, please rotate the mode of action of insecticide. Switch also lists sour rot (suppression only), and other broadspectrum fungicides such as captan, probably have some efficacy too.
If you have not, please check my blog (grapepathology.blogspot.com) or Twitter (@grapepathology) or Facebook page (Grape pathology at Virginia Tech), if you prefer them.
GrapeIPM.org: If you have not, please sign up for our new pest management support system. It will help you keep track of pesticide inventory, planning, etc. It is free!
NEWA: Virginia is part of an Agricultural weather network called NEWA (http://newa.cornell.edu/). We have about a dozen weather stations scattered around the state. You can check daily disease and insect pest risks. Please check it out. If you want to join NEWA (i.e., purchase a weather station), please let me know. I am the state coordinator for VA.
Virus kit: VT’s plant disease clinic and I am offering a free virus (grapevine leafroll virus-2 and 3, and Red blotch viruses). Please see me so that I can give you a sampling kit.
Good luck with the rest of the season!!!