Fungicide resistance management considerations for cherry leaf spot control

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 often with yellow halos around the spots on the tops of the leaves. As spots accumulate on foliage, 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. Balatons® can have more lesions than Montmorency before leaf drop, but Balatons® are just as susceptible to leaf spot as Montmorency.

Preharvest defoliation can result in a crop that does not mature adequately in order for the fruit to be marketable. Additional yearly defoliationcan 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 winter injury and/or mortality. 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 and one mixture registered for leaf spot control that could be used as cover sprays (Table 1). Of the five classes, only the SI’s represent a poor choice for management due to resistance to SI fungicides in the cherry leaf spot fungus that 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. However, growers should 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 the effectiveness of these products against leaf spot in the future. We suggest that both Gem and Pristine are not used more than two times per season.

In addition and more important than limiting use of Gem and Pristine long-term preservation of these fungicides will require more active resistance management strategies. Strobilurin resistance has emerged in the apple scab pathogen Venturia inaequalis and is completely widespread in Michigan currently. This emergence happened quite fast with the first resistant strains probably developing in 2006, only seven years after the registration of the strobilurins. Because of the incidence in apple scab, we think that resistance management strategies for Gem and Pristine for cherry leaf spot control need to include the incorporation of the broad spectrum fungicide Captan. Similar to tank mixing fungicides with an EBDC for apple scab control, tank mixes with Captan should be utilized for leaf spot control.

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 timing where brown rot control is also needed. It is also critical 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. Adament should also be tank-mixed with Captan for resistance management.

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. Although the current Syllit label indicates an incompatibility with Captan, these fungicides can be tank-mixed and a revised label indicating this is planned for 2011.

Copper has proven to remain a highly effective fungicide for cherry leafspot control. Results from several years of experiments consistently show that cherry leafspot treatments utilizing one, two, or three cover sprays of copper sulfate (1.2 lbs metallic Cu per acre) provided excellent cherry leafspot 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 80F for several days. However, we have been trying to quantify the phytotoxicity of copper use for many years, and we have not been successful in showing the potential impacts of multiple copper sprays in hot conditions.

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

Sterol-inhibitors (SI’s) Elite, Indar, Nova, Rubigana
Strobilurins Gem, Pristineb
Mixture (strobilurin + SI) Adament
Guanidine Syllit (dodine)
Heterocyclic Captanc
Inorganic metal ion Copper

a Because of widespread resistance in Michigan, SI’s should never be used alone and are not recommended for leaf spot control.
b Pristine is a mixture of a strobilurin and boscalid, another fungicide (separate chemistry).
c Captan should only be used in combination with other fungicides because the rate (2.5 lbs/A Captan 80WDG) allowed on cherries is too low for effective disease control.

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