Participants in the 2024 Ohio Plant Diagnostic Workshop held this past Friday in the OSU OARDC Secrest Arboretum in Wooster, OH, observed weeping cherries in various stages of defoliation. Leaves were turning an intense yellow and dropping from the trees.
Until recently, the Wooster region was suffering from droughty conditions; however, the defoliation of the cherry trees was not related to lack of water. The leaf loss is the parting shot of Cherry Leaf Spot caused by the fungus, Blumeriella jaapii. The fungal pathogen was formerly known as Coccomyces hiemalis and the disease is sometimes called “Coccomyces Leaf Spot” in older literature.
Other common names for the disease are Yellow Leaf owing to the chlorosis that appears on heavily infected leaves and Shothole Disease derived from the holes that commonly appear in the leaves. Both illustrate that disease symptom development is a dynamic process with symptoms changing over time.
Holes in leaves are commonly associated with leaf-feeding insects. However, the holes produced by the cherry leaf spot fungus provide a good example of symptom progression with symptoms changing in a somewhat predictable way over time. The dead (necrotic) tissue produced by the cherry leaf spot fungal infections contracts as it dries out causing the necrotic tissue to separate and eventually drop from the leaves to produce holes. This symptom progression isn’t confined to cherry leaf spot.
The cherry defoliation is indeed related to environmental conditions; however, they occurred much earlier in the season. The fungus overwinters in fallen infected leaves. Infectious spores are produced in the spring and ideal environmental conditions that support heavy infection are heavy rainfall and cool temperatures.
Cherry leaf spot can be devastating for orchardists in the Midwest with defoliation of sweet and sour cherries sometimes occurring in mid-summer. Spray programs are required to avoid serious losses in production and to protect tree health.
However, the disease is mostly a curiosity on ornamental and woodland cherries. The leaf spots may develop early in the season, but leaf yellowing and leaf loss don’t occur until late in the season after the trees have produced and stored enough carbohydrates to support leaf production the following season. Indeed, we’re close to the time of the year when we see normal fall leaf drop.
The late-season defoliation means fungicide applications aren’t needed to protect the health of ornamental cherries. Of course, leaf yellowing and heavy defoliation can affect aesthetics. Leaf infections next year can be reduced by raking and destroying leaves shed this season.
On a final note, the images below provide anectodical evidence that some cherries may be less susceptible. The first image was taken last season and shows a grafted weeping cherry with a topknot of stems sprouting from the root stock. The second image was taken a few weeks ago and shows leaves on the sprouts from the root stock that are heavily infected by the cherry leaf spot fungus while the leaves on the scion show little evidence of fungal infections.