Protect grape clusters from all major grape diseases at this time
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Powdery mildew and downy mildew
This past week, we observed the first active downy mildew and powdery mildew infections in unsprayed grapevines in Lansing, Fennville, and Ontario, Canada. Sporulating downy mildew lesions were seen on leaves and tendrils of Chardonnay, on leaves and clusters of Chancellor, and on leaves of wild grapes (Vitis riparia). On Niagara vines, sporulating lesions were found on leaves close to the ground, which are subject to splashing soil which contains germinating oospores. Last year, we first observed downy mildew in Chancellor in Fennville during the first week of June. It is safe to assume that these pathogens will continue to be active during the warm, humid weather that is forecast for the coming weeks. Growers are therefore strongly advised to protect flower and fruit clusters from infection using effective fungicides as soon as possible if the vines are not already protected. Also, continue to monitor vineyards for signs of infection. At this stage, the young clusters are highly susceptible to all major diseases, including downy mildew, powdery mildew, black rot and Phomopsis. Black rot and Phomopsis lesions have been seen for several weeks and indicate that the pathogens are active. The risk of infection is especially high if we have multiple or big rain events, like we’ve had recently, and moderate to warm temperatures during this time. Prolonged wet conditions during bloom can also allow Botrytis to get a foothold in the clusters of susceptible varieties by promoting growth on senescing flower parts.
If active infections are found, use fungicides with post-infection activity at the highest labeled rate. For downy mildew, Ridomil Gold MZ or Ridomil Gold Copper are the strongest fungicides, followed by phosphorous acid fungicides like Phostrol and ProPhyt. Strobilurin fungicides have limited post-infection activity and should preferentially be used in a preventive mode. New(er) fungicides for downy mildew control are: Presidio, Revus and Revus Top (don’t apply Revus Top to Concord or Noiret vines due to phytotoxicity concerns), Gavel (contains mancozeb), Forum, and Tanos. While some of these new fungicides have post-infection (curative) activity, they are best applied on a preventative basis. They are excellent for integration into a fungicide resistance management program as they represent new and different chemistries.
It will be especially critical to protect clusters of susceptible varieties from powdery mildew at this time. Sterol inhibitor (e.g., Elite, Rally) and strobilurin (e.g., Abound, Flint) fungicides have the ability to cure early infections but will not eliminate already established colonies. JMS Stylet Oil and potassium bicarbonate fungicides (Kaligreen, Armicarb, MilStop) can be used to eradicate visible powdery mildew colonies. Make sure that coverage is thorough (use sufficient spray volume), as only those colonies contacted by the fungicide will be killed. Since strobilurin-resistant powdery mildew isolates have been found in Michigan vineyards (mostly MSU experimental vineyards and wine grape vineyards with a history of strobilurin use) and we have circumstantial evidence for sterol inhibitor resistance, we recommend adding a protectant fungicide like Sulfur or Ziram to the tankmix when using either type of fungicide. Sulfur is the most cost-effective option for non-sulfur sensitive grape cultivars. Over the past two years, we have noticed that Ziram as a tank-mix partner did improve control of powdery mildew in a spray program. Also, alternate with fungicides with different modes of action, for example Quintec, Endura, Serenade, Sonata, Regalia. Revus Top is a new fungicide for powdery and downy mildew and black rot control in grapes. However, the ingredient that is active against powdery mildew is difenoconazole which belongs to the sterol inhibitor class. This fungicide may be phytotoxic on Concord grapes, so do not use on Concords.
Protect clusters for at least 4 to 5weeks after bloom, keeping in mind that due to spring frost injury there are clusters of different ages in many vineyards. Make sure to continue protecting the youngest clusters. As the berries develop, they become naturally resistant to black rot, downy mildew, and powdery mildew and the need for protection diminishes after the susceptible period ends. This happens quite rapidly for downy mildew (2-3 weeks after bloom), whereas for powdery mildew it is about 4 weeks after bloom. Concord grapes become resistant to black rot 4-5 weeks after bloom, but some wine grape varieties may remain susceptible to black rot for up to 8 weeks postbloom. However, be aware that the cluster stem (rachis) and berry stems can remain susceptible longer than the berries in most cases. The only disease to which berries remain susceptible throughout their development is Phomopsis, but the risk of infection diminishes after bunch closure because inoculum levels drop off then. Botrytis is just the opposite in that berries actually become more susceptible as they get closer to harvest, especially in tight-clustered varieties. In general, aim to protect the clusters from the major diseases from immediate pre-bloom until 4-5 weeks after bloom.
Small black rot lesions have been seen on grape leaves in various locations. Temperatures in the high 70’s and low 80’s are perfect for black rot. At these temperatures, only 6-7 hours of wetness are needed for infection. Black rot is a tricky disease because infections can remain latent (invisible) for a long period of time, so you won’t know that you have the disease until is it too late to do anything about it. However, one can scout for the small, round leaf spots – a lot of black rot leaf lesions indicate high disease pressure from ascospore inoculum and also contribute to fruit infections. In a field with a history of black rot, old fruit cluster remnants left hanging in the trellis are major contributors to infection. Fruit infections can take place anytime from bloom onwards, but only become apparent sometime between bunch closure and veraison. The period from immediate pre-bloom through early fruit development is crucial to protect grapes against black rot infection.
The approach to black rot control now focuses primarily on protecting the clusters from infection. EBDC sprays applied earlier in the season for Phomopsis will also control black rot leaf infections, and therefore no sprays are recommended specifically for black rot on the foliage early in the season. In five years of trials in New York, good black rot control was achieved with one immediate pre-bloom and 1 to 2 post-bloom fungicide sprays. A second post-bloom application is strongly advised if black rot has been a problem in the vineyard the previous year, and should be considered prudent if wet weather is anticipated. During three years of fungicide trials in a ‘Concord’ vineyard in Fennville, MI, just two post-bloom applications of SI fungicides (Nova, Elite) have provided very good control under high black rot pressure.
Sterol-inhibitor fungicides (e.g., Nova and Elite) continue to provide outstanding control of black rot, and provide several days of post-infection activity. Currently there are various “generic” tebuconazole products on the market, e.g., Orius and Tebuzol that may be more cost-effective. When using SI fungicides on a post-infection schedule, use the highest label rates, because post-infection activity is strongly rate-dependent, particularly when extended “kickback” activity is required. The strobilurin fungicides (Abound, Flint, Sovran, Pristine) and Revus Top are also excellent protectants but provide only limited post-infection activity. Flint, Pristine, and Revus Top should not be used on Concord grapes because of potential phytotoxicity.
Cane and leaf lesions have been showing up in high numbers in susceptible varieties. Each rainfall event will lead to spore dispersal and can also lead to successful infection if the tissue remains wet for a sufficient amount of time. The optimum temperature for infection is 59-68ºF, at which time about 6-10 hours of wetness are needed for infection. The longer the tissue stays wet, the more severe the symptoms will be. At this time we should be concerned with preventing Phomopsis infection of the rachis and fruit, especially in mechanically pruned vineyards and vineyards with a history of the disease. Rachis infections are most closely correlated with yield losses due to berry drop at harvest in Niagara vines, whereas fruit infections are more of a problem in wine grapes.
If at this time you find a lot of lesions on the leaves and canes, infection pressure will be high for the fruit also. It is not too late to apply fungicides for cluster protection from Phomopsis. The best fungicide options for control of Phomopsis during and after bloom are Abound, Sovran, Flint, or Pristine (do not use Pristine on Concord grapes). Phosphorous acid fungicides such as ProPhyt and Phostrol are also good and cost-effective alternatives. These are systemic and will likely provide some kick-back activity. In trials done in Michigan, ProPhyt provided very good control of Phomopsis when sprayed on a 14-day schedule. Tighten the schedule and increase the rate if disease pressure is high. Ziram is a moderate to good protectant against Phomopsis and can be a tank-mix partner with any of the phosphorous acid fungicides. EBDC fungicides and Captan are good protectants but cannot be applied after bloom has started in grapes grown for the National Grape Cooperative. EBDC’s have a 66-day pre-harvest interval.