While cherry leaf spot is a serious fungal disease for sour cherry, the disease also occurs in sweet cherry and certain plums. The disease mainly affects the foliage, but damage may also be found on the fruit, petioles, and stems.
For region-specific information, please contact your local Giving Grove partner. If you would like more information like this sent straight to your inbox, consider subscribing to The Serving, The Giving Grove’s quarterly newsletter.
Appearance:
Fruit bud formation can be severely impacted for the following two seasons.
On sour cherry leaves:
Variously colored spots develop on the upper surface.
The spot or lesion rapidly enlarges, becoming brown or purple, and dies from the center outward. Infected spots are irregular or round and occur over the entire leaf surface.
Spot development precedes yellowing and leaf dropping. The area adjacent to the spot may remain green while the rest of the leaf turns yellow (the "green island" effect).
On sweet cherry leaves:
Spots are often larger and nearly circular.
Cream-colored fungal spore masses (acervulii) appear on the lower leaf surface.
On fruit stems, infections sometimes girdle the stem to cause a fruit drop.
While infections do occur on the fruit, they are less common than on foliage.
Treatment:
The disease overwinters on fallen leaves. Rake up and destroy fallen leaves under cherry trees. Rake up fallen fruit.
Use a thick layer of mulch to cover the soil. Mulch will reduce weeds and prevent the disease pathogen from splashing back up onto the leaves.
Prune plants to improve air circulation.
Spraying with a baking soda solution (a tablespoon of baking soda, 2 1/2 tablespoons of vegetable oil, a teaspoon of liquid soap, to one gallon of water), or neem oil. Baking soda may burn some plant leaves. Spray only a few and then check for a reaction before applying applications every two weeks.
For more severe cases, apply sulfur sprays or copper-based fungicides weekly at first sign of disease to prevent its spread. These organic fungicides will not kill leaf spot, but prevent the spores from germinating.
Resources: “The Holistic Orchard”, Planet Natural Resource Center, University of Minnesota Extension, Ohio State University Extension, Pacific Northwest Extension, and Patrick L. Byers, Horticulture Specialist.