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 (CLS) 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 to cause leaf yellowing and drop is variable. Sweet cherries can tolerate quite a few lesions before leaf drop occurs, but Montmorency tart cherries will drop with only a few lesions, signifying the importance of proper leaf spot management. Balaton® trees can tolerate more lesions that Montmorency, but they, too, drop their leaves more readily than sweet cherries.

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 typically exhibit reduced flower bud formation and often set less fruit the following season.

Ascospores (primary inoculum) of the leaf spot fungus are released from leaves on the orchard floor by rainfall from early bloom to about six weeks after petal fall. Infection takes place through natural openings (stomata) located on leaf undersides. Once leaves are unfolded, they are susceptible to the CLS pathogen and remain so throughout the season.

Management of cherry leaf spot should be initiated around petal fall or sooner if susceptible leaf tissue is present. The use of chlorothalonil (Bravo) is not allowed after shuck split (except post-harvest). As resistance development to this product is unlikely, Bravo should be used pre-shuck split, as both a protectant and a resistance management tool.

What alternatives are available for leaf spot control and should growers be considering the future potential for fungicide resistance development?

There are six major classes of fungicides registered for leaf spot control that could be used as cover sprays. Each of these classes of fungicide, except copper compounds and Captan, has resistance concerns as resistance in leaf spot or other fungal pathogens has been demonstrated. Therefore, growers must be aware of fungicide resistance potential and be thinking about maintaining fungicide chemistries now and in the future. Resistance to SI fungicides is widespread in the leaf spot pathogen in Michigan. Thus, growers should be transitioning away from this fungicide class for leaf spot control.

Growers should also consider timing and compounds to control the other important tart cherry fungal diseases. For example, the first cover spray timing after shuck split is a good timing for Pristine or a strobilurin, because these fungicides are also effective in powdery mildew control. Currently, the SI's remain an important tool for brown rot control, particularly if we experience warm and humid or wet weather three to four weeks before harvest. Any use of SI's in Michigan should be in a tank mix with 3 to 4 lbs. of Captan per acre.

The strobulirin/boscalid (Pristine) and strobilurin (Flint or Gem) fungicides are our best tools for multiple disease control. However, both are single-site fungicides with the potential for resistance development. Do not apply these fungicides more than twice consecutively or more than four times per season. Our recommendation is that these fungicides not be used more than twice per season. The risk of resistance is high with these fungicides and loss of a class of fungicide means fewer tools for future disease control.

We envision that copper compounds will become more and more important for leaf spot control as other fungicides are lost due to resistance. Copper is highly effective in leaf spot control with the only downside being the potential for phytotoxicity. We are currently recommending a rate of 1.2 lbs per acre of metallic copper with hydrated lime added at 6-9 lbs per acre to reduce phytotoxicity. Do not apply copper compounds prior to periods of warm, dry conditions (temps at or above 80°F). Scout orchards for phytotoxicity symptoms on leaves (bronzing on leaf undersides, conspicuous large yellow blotches on a few leaves, and/or blackening of leaf veins) prior to the next copper application. Do not apply a second copper application if trees are exhibiting phytotoxicity symptoms.

Cherry leaf spot can be successfully managed using currently available fungicides. However, it is uncertain if companies are developing additional classes of fungicides that would ultimately be registered for leaf spot control. It is incumbent upon everyone to maximize the lifespan of these current fungicides through effective resistance management.

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