Young fruit clusters are highly susceptible to all major diseases, including downy mildew, powdery mildew, black rot, Phomopsis and anthracnose.
If prolonged cool, wet weather prevails during bloom, Botrytis can also gain a foothold in clusters of susceptible varieties by promoting fungal growth on senescent flower parts. However, with continued dry, warm conditions, it is unlikely that bloom will be an important time for Botrytis infection. Black rot and Phomopsis lesions have been seen in the last couple of weeks and indicate that the pathogens are active. Grape anthracnose symptoms are also visible on shoots, leaves and cluster stems of susceptible varieties. The first powdery mildew colonies have been seen on unsprayed Chardonnay vines in Clarksville, Mich. Powdery mildew has also been spotted near Traverse City, Mich., but this report is still being confirmed. Downy mildew so far has only been seen in low-lying wild grapes, which often show symptoms at least a week before cultivated grapes. However, it has been relatively dry, so downy mildew development may be a bit late this year. Careful scouting is advised on a weekly basis.
It is possible to have powdery mildew fruit infection without seeing any foliar infections, so protect the fruit of susceptible cultivars even if no powdery mildew has been seen on the leaves. Often, downy mildew infections of flower clusters in cv. Chancellor are seen before leaf infections as well. In 2009, we first observed downy mildew in Chancellor in Fennville, Mich., during the first week of June and in 2010 during the second week of June. Growers are strongly advised to protect flower and fruit clusters from infection by all these pathogens using effective fungicides. The risk of infection is especially high if we have multiple rain events and moderate to high temperatures.
In general, aim to protect the clusters from the major diseases from immediate pre-bloom until four to five weeks after bloom. 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 (two to three weeks after bloom), whereas for powdery mildew it is about four weeks after bloom. Concord grapes become resistant to black rot at four to five weeks after bloom, but some wine grape varieties may remain susceptible to black rot for up to eight weeks post bloom. 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.
Sterol inhibitor (e.g., Elite, Rally, Procure, etc.) and strobilurin (e.g., Sovran, Flint, Abound, Pristine) fungicides have the ability to cure early infections, but will not eliminate colonies that are already established. JMS Stylet Oil and potassium bicarbonate fungicides (Kaligreen, Armicarb, MilStop) can be used to eradicate visible powdery mildew colonies. If you use eradicants, 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 (mostly in MSU experimental vineyards and wine grape vineyards with a history of strobilurin use) and we have circumstantial evidence for sterol inhibitor (SI) resistance, we recommend adding a protectant fungicide like Sulfur or Ziram to the tank-mix when using either type of fungicide. Sulfur is the most cost-effective option for non-sulfur sensitive grape cultivars.
(For more, see: Wine grape season shaping up for powdery mildew)
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 on the research stations where we have strobilurin resistance. Also, alternate fungicides with different modes of action, for example Sulfur, Quintec, Vivando, Luna Experience, Endura, Serenade, Sonata or 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 is phytotoxic on Concord and Noiret grapes, so do not use on these cultivars. Inspire Super also contains difenoconazole. Luna Experience is a new fungicide for control of powdery mildew, black rot, Phomopsis and anthracnose.
Downy mildew, black rot
For most varieties, foliar infections are the main phase to be concerned about. However, the downy mildew pathogen can also infect clusters. Cultivar Chancellor is the poster child for downy mildew cluster infection. Both the rachis and berries can be destroyed. If active infections are found, use fungicides with post-infection activity at the highest labeled rate. For downy mildew, Ridomil Gold (MZ or Copper) are the strongest fungicides, followed by phosphorous acid fungicides like Phostrol and ProPhyt. When using phosphorous acids, applying a “booster spray” five days after the first spray will enhance the curative effect. Strobilurin fungicides have limited post-infection activity and should preferentially be used in a preventive mode.
Newer fungicides for downy mildew control are Presidio, Revus and Revus Top (don’t apply Revus Top to Concord or Noiret vines due to risk of phytotoxicity), Gavel (contains mancozeb), Forum, Reason, Ranman and Tanos. While some of these new fungicides have post-infection (curative) activity, they are best applied on a preventative basis. They are good for integration into a fungicide resistance management program as many of them represent new and different chemistries.
Black rot lesions have been seen on grape leaves in various locations and range from 1 to 5 mm in size. They can be recognized by the tiny, black pimples (pycnidia) in a ring along the inner edge of the lesion. Temperatures in the high 70s and low 80s are perfect for black rot. At these temperatures, only six to seven hours of wetness are needed for infection, so a nightly dew period may be sufficient 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 the berries are infected until is it too late to do anything about it. However, one can scout for the small, roundish leaf spots – a lot of black rot leaf lesions indicate high disease pressure from ascospore inoculum and will also contribute conidia for fruit infections. Conidia produced in leaf spots are rainsplashed, whereas the old fruit mummies produce airborne ascospores. 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 between bunch closure and veraison. Black rot is relatively easy to control in the period from immediate pre-bloom through early fruit development.
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 one to two 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, Mich., just two post-bloom applications of SI fungicides (Rally, Elite) provided very good control under high black rot pressure.
Sterol inhibitor fungicides (e.g., Rally, 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, like Orius and Tebuzol, that may be more cost-effective. The difenoconazole ingredient in Revus Top and Inspire Super is similar to Rally and Elite when it comes to black rot control. 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 Luna Experience are also excellent against black rot, but provide only limited post-infection activity. Flint, Pristine, Inspire Super and Revus Top should not be used on Concord grapes because of potential phytotoxicity.
Cane and leaf lesions have been showing up in fairly 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 to 68 degrees Fahrenheit, at which time about six to 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. Sterol inhibitors, overall, do not have good efficacy against Phomopsis, although fungicides containing difenoconazole (Revus, Revus Top, Inspire Super) are among the more effective. The best fungicides for control of Phomopsis during and after bloom are Abound and 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 kickback 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. Luna Experience is also quite effective. 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 (these fungicides are suspected carcinogens). EBDC’s have a 66-day pre-harvest interval.
Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.