Cherry leaf spot and a lot of rain

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.  

With the continuing onslaught of rain, growers are wondering about the best options for cherry leaf spot control. Because the rain has provided so few windows for fungicide applications, there have been varying amounts of fungicide applied to tart and sweet cherry blocks. As rain is expected to continue for the remainder of the week, growers need to make quick decisions about the chemistry to apply when the rain actually stops for a while and the wind dies down enough to apply a spray. The following scenarios are meant to help growers decide the best product for their orchard.

Scenario 1

Growers that applied a full cover of Bravo on Tuesday or Wednesday last week (May 9-10) and had relatively good coverage can assume they were protected, but they will have to make an educated guess as to “how protected.” There will be no way to know how long the Bravo lasted or if it is still there, but for the most part, growers with a full rate of Bravo can assume some level of comfort. If this is the case for a particular farm, a grower should go back in with another full rate of Bravo as soon as possible. This scenario assumes a good degree of protection from the cherry leaf spot (CLS) pathogen, and Bravo is the choice for continued protection into the future, up to shuck split. The one thing to keep in mind with this program is that the grower should shorten the interval between sprays if the rain continues as the material will remain on the trees for a shorter amount of time.

Scenario 2

Growers that applied only a half cover of Bravo should assume they were not protected and need to go back into the orchard with something other than Bravo, as Bravo is only a protectant and does not have the ability to eradicate. As we have little knowledge of after-infection activity of some of our options (strobilurins and Pristine), we have to assume that these products will provide some eradication potential. First, there are no fungicides that will eradicate a spore that has germinated five to seven days ago. Sterol inhibitors (SI’s) used to provide some back action; however, those days are gone as CLS has developed resistance to these products throughout Michigan. Syllit (dodine), another product with some back action, can provide back action up to 36 hours; however, we do not know the current extent of dodine resistance in the CLS pathogen. These two products are an option if an orchard does not have SI or dodine resistance, but the likelihood of having an orchard without SI resistance is very low. Using SI’s to control CLS is not recommended, and a total control failure is probable with these products. Dodine may be more efficacious against CLS, but we have no recent data to verify this assumption; if dodine is used, the flowable rate is 27 oz per acre.

The remaining products are the strobilurins and the strobilurin/boscalid compound Pristine. One tactic for growers that have applied a half side is to come in with a full rate of Pristine at 14.5 oz/acre, Flint at 4 oz/acre, or the new product Gem at 6 oz/acre (See new product write-up for more information.). The full rate is essential to get the best control now and slow a possible CLS epidemic. If a grower applies one of these chemistries at a higher rate, he or she can follow up that spray with another Bravo spray at shuck split timing, about seven days from now. This strategy does not recommend tank mixing at this time for best efficacy against CLS as well as costs. Bravo is better used as the next spray at the shuck split timing. We should also not be cutting back on the rates of the strobilurins and Pristine if orchards have gone without protection during this wet weather.

Scenario 3

Growers that have had no coverage at all in the last week should follow the recommendations for scenario 2, but bump up the rate for Gem to 8 oz/acre.A final reminder for all growers is that we still need to keep resistance management in mind as we move through this super rainy period. Tank mixes are usually the best bet, so we should be trying to keep full rates of fungicides alone at a minimum, although dire circumstances often require dire measures.

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