Controlling black rot and Phomopsis from grape clusters

Black rot and Phomopsis are active in grapes. Aim to protect clusters from infection as soon as possible and rely on systemic fungicides for post-infection activity, particularly during rainy periods.

Young grape clusters are highly susceptible to all major diseases such as black rot, Phomopsis and anthracnose, particularly during the first three to four weeks after bloom. The weather has been very conducive to fungal infections due to frequent rains, warm temperatures and high relative humidity. The first black rot infections on fruit have been seen already as cream-colored spots and tiny brown specks on grape rachises indicate Phomopsis infection. 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 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 of 70-85 degrees Fahrenheit. 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 or 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 focusing primarily on protecting the clusters from infection. EBDC sprays applied earlier in the season for Phomopsis will have also controlled 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 very good 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 effective 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.

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 than Elite. 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, EBDCs 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.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.