MSU Extension

Young fruit clusters are highly susceptible to all major diseases such as 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. The fungi that cause black rot, Phomopsis and anthracnose infections on the berries first make themselves known by infecting leaves earlier in the season. Numerous lesions on leaves also indicate a high risk of fruit infection, and in the case of black rot and anthracnose, contribute additional inoculum for fruit infections. Therefore, Michigan State University Extension advises careful scouting on a weekly basis and growers are advised to protect flower and fruit clusters from infection using effective fungicides. The risk of infection is especially high if we experience multiple rain events and moderate to high temperatures (70 to 85 degrees Fahrenheit).

In general, aim to protect the clusters of juice grapes from immediate pre-bloom until four to five weeks after bloom. As the berries develop, they become naturally resistant to black rot and anthracnose, so the need for protection diminishes after the susceptible period ends. 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 after bloom. While berries remain susceptible throughout their development to Phomopsis, the risk of infection diminishes after bunch closure because spore supplies become exhausted, especially in rainy years.

Black rot

Frequent rains and temperatures in the high 70s and low 80s are perfect for black rot infection. Black rot is a tricky disease because infections can remain latent (invisible) for weeks, so you won’t know the berries are infected until is it too late to do anything about it. However, one can scout for leaf spots, which will also contribute conidia for fruit infections. Old fruit cluster remnants left hanging in the trellis are risky due to their proximity to current-year fruit. 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, focusing 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 early in the season. One immediate pre-bloom and two post-bloom fungicide sprays should be sufficient. Sterol-inhibitor fungicides (e.g., Rally, Elite) continue to provide outstanding control and have several days of post-infection activity.

There are various “generic” tebuconazole products on the market, e.g., 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 after a recent infection period, use the highest labeled rate because post-infection activity is strongly rate-dependent, particularly when extended “kickback” activity is required. The strobilurin fungicides (Abound, Flint, Sovran, Pristine) are also excellent against black rot, but have limited post-infection activity and are better applied in a preventative mode. Luna Experience and Quadris Top are pre-mix fungicides with very good black rot activity.

Serenade (+Nu-Film P) is currently the best option for organic control of black rot, although bicarbonates also have moderate efficacy.

Phomopsis

Each rainfall event will lead to additional spore dispersal from old, overwintered canes and spurs and can lead to successful infection on the rachis and berries. The optimum temperature for infection is 59 68 F, 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.

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 (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.

Sterol inhibitors are not very effective against Phomopsis, although fungicides containing difenoconazole (Revus Top, Quadris Top, Inspire Super) tend to be a bit more effective. Ziram is a moderately good protectant against Phomopsis and can be a tank-mix partner with a phosphorous acid fungicide. EBDC fungicides and Captan are good protectants, but cannot be applied after bloom has started in juice grapes grown for the National Grape Cooperative. In addition, EBDC’s have a 66-day pre-harvest interval.

Serenade (+Nu-Film P) is the best organic option for control of Phomopsis, other than dormant applications of lime sulfur or copper.

See also

“Protecting young grape clusters from powdery and downy mildew”