MSU Extension

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.

Cherry leaf spot is the most important fungal disease of tart cherry in Michigan. The leaf spot fungus Blumeriella jaapii infects leaves with symptoms first appearing on upper leaf surfaces as small purple spots. As spots accumulate on leaves, the leaves turn yellow and fall. The amount of lesions required causing leaf yellowing and drop is variable. Sweet cherries can tolerate quite a few lesions before leaf drop occurs, however, Montmorency tart cherries will drop with only a few lesions, signifying the importance of proper leaf spot management.

Conditions in Michigan during 2009 have been favorable for leaf spot spore discharge. Ascospore discharge is highest over a wide temperature range (60 to 85*F) and lowest at 41 to 46*F. The optimum conditions for lesion development are temperatures of 60-68*F with rainfall or fog. After lesions appear on upper leaf surfaces, examination of the underside of leaves reveals a proliferation of white spore masses. These spores are dispersed by rain and wind within trees and to adjacent trees; such secondary cycles can continue repeatedly under favorable conditions through autumn.

Preharvest defoliation can result in a crop that does not mature adequately to be marketable, plus cause serious tree damage. Even late summer (August, early September) defoliation reduces the ability of trees to store photosynthate in roots leading to an overall loss of vigor and leaving trees more susceptible to killing by winter injury. Early-defoliated trees also typically exhibit reduced flower bud formation and often set less fruit the following season.

There are five major classes of fungicides registered for leaf spot control that could be used as cover sprays (Table 1). Of the five classes listed only the SI’s represent a bad choice for management because resistance to SI fungicides in the cherry leaf spot fungus occurs universally throughout Michigan orchards. Thus, SI fungicides should never be used alone for leaf spot control. SI resistance is quantitative meaning that leaf spot populations exhibit a wide range of susceptibilities to these fungicides. However, use of SI's will continually shift orchard populations such that they contain more and more highly resistant individuals. These shifts result in significant control failures, and we have observed such control failures in several orchards over the last few years.

After shuck split, the use of the strobilurin Gem (3.0 to 3.8 oz / A) or the strobilurin/boscalid Pristine (10.5 to 14.7 oz / A) at the first cover timing is an excellent choice because both of these materials are also excellent powdery mildew materials. Remember that both Gem and Pristine are excellent fungicides, but both are at risk long-term for the development of fungicide resistance. Growers need to think about long-term protection of these materials such that they remain effective over a significant number of years. Overuse of these fungicides now could compromise this effectiveness. We suggest that both Gem and Pristine are not used more than two times per season.

Gem is also available as one component of the fungicide Adament. Adament is a mixture of two fungicides – Gem and the SI Elite. For leaf spot control, the Elite component of Adament would be essentially ineffective. Thus, Adament would be most effective when used at a timing where brown rot control is also needed. It is also criticial to use a rate of Adament that ensures that the Gem component is present at a rate sufficient for leaf spot control. Adament at the 6 oz/A rate is equivalent to 3 fl oz Gem + 3.3 oz Elite. Adament at the 8 oz rate is equivalent to 4 fl oz Gem + 4.4 oz Elite.

One fungicide which is definitely worth a look is Syllit (dodine). Syllit FL at 27 fl oz per acre is an excellent leaf spot material. In a survey conducted in 2007, we did not isolate strains of the leaf spot fungus that were resistant to Syllit. However, experience tells us that Syllit is a fungicide that is resistance prone. Thus, Syllit should be tank-mixed with Captan as a resistance management strategy to prevent any buildup of dodine resistance. Also, Syllit is not as effective in controlling powdery mildew or brown rot. Thus, the second and third cover timings represent the best timings for a Syllit + Captan application.

Copper has proven to remain a highly effective fungicide for cherry leaf spot control. Results from several experiments consistently show that treatments utilizing one, two, or three cover sprays of copper sulfate (1.2 lbs metallic Cu per acre) provided excellent cherry leaf spot control equivalent to or better than standard programs using conventional fungicides such as strobilurins. Successful efficacy of copper compounds aids both conventional growers, as copper extends the life of traditional fungicides, and organic growers because copper is the only viable option for disease control in tart cherry. The only detriment to copper use is the potential for phytotoxicity effects to tart cherry trees. When copper compounds are applied to tart cherry trees in advance of hot, dry weather, the trees can exhibit phytotoxicity symptoms such as bronzing on the undersides of leaves, large yellow and brown blotches on the upper surface of a few leaves, or blackening of veins on the undersides of leaves. In severe cases, copper phytotoxicity can also cause leaf defoliation. Thus, the second and third cover timings are good for copper use if temperatures are not projected to remain above 80*F for several days.

Table 1. Class of chemistry and fungicides registered for cherry leaf spot control.

Dr. Sundin's work is funded in part by MSU's AgBioResearch.