Cherry leaf spot is a fungal disease which infects cherries and plums. Sweet, sour, and ornamental cherries are susceptible to the disease, being most prevalent in sour cherries. The variety of sour cherries that is the most susceptible are the English morello cherries. This is considered a serious disease in the Midwest, New England states, and Canada. It has also been estimated to infect 80 percent of orchards in the Eastern states. It must be controlled yearly to avoid a significant loss of the crop. If not controlled properly, the disease can dramatically reduce yields by nearly 100 percent. The disease is also known as yellow leaf or shothole disease to cherry growers due to the characteristic yellowing leaves and shot holes present in the leaves upon severe infection.
Host and symptoms
Cherry leaf spot is caused by the ascomycete fungus Blumeriella jaapii (formerly known as Coccomyces hiemalis; anamorph Phloeosporella padi). This fungus overwinters in dead leaves on the ground as its main form of survival. In the spring, apothecia develop on these leaves. Ascospores are produced in these fungal fruiting bodies and are forcibly ejected during rainy periods for about 6–8 weeks, starting at petal fall. This is the teleomorph or sexual stage and results in new genetic recombination and variation for the disease.
This pathogen mainly affects and is most noticeable on the leaves of cherry trees. The most obvious symptom of this disease is the yellowing of leaves before they drop in the early spring. Infected leaves are filled with numerous black lesions that cover an obvious portion of the leaf. These lesions first appear as small purple spots which turn red to brown and then black. Most spots are circular but can amalgamate to form large, irregular dead patches. These spots tend to enlarge and separate from healthy tissue. Lastly, they drop out of the leaf giving a "shot-hole" appearance. This shot hole effect is more common on sour than on sweet cherries. In more severe and advanced cases, lesions are found on the fruit, petioles, and stems. Leaves with lesions present typically fall off early in the growing season and it is common for seriously affected trees to be defoliated by midsummer. Leaves severely infected by the disease turn very yellow before falling off.
If the disease is not controlled and allowed to persist for several years there could be various adverse effects to the overall cherry production of a tree primarily due to the cycle of early season and repeated defoliation. These effects include dwarfed and unevenly ripened fruit which has poor taste, trees more susceptible to winter injury, death of fruit spurs, small and weak fruit buds, decreased fruit size and set, and eventual death of the tree. If the disease has significantly infected the leaves enough in early spring it is possible that the fruit will fail to mature in that year. This fruit will typically remain light colored, soft, and very low in soluble solids and sugar.
Disease cycle
Sexual reproduction
This fungus overwinters in dead leaves on the ground. In early spring (at the time of petal fall), fungal fruiting bodies called apothecia develop in these leaves. Ascospores produced in the apothecia are forcibly discharged into the air starting at petal fall. They can start new infections in new, green leaves and serve as the primary inoculum in each new growing season. The ascospores can stick to the leaf and, given a film of water and optimal temperature, germinate within hours of adhesion. Upon germination, hyphae can grow into the leaf via stomata (natural openings) on the underside of the leaf. Hyphae cannot invade immature stomata very effectively. After invasion, the hyphae grow in the intercellular spaces in the leaf, killing and feeding on leaf tissues. Incubation times vary with available moisture and temperature. The small purple lesions can appear within 5 days if there are damp conditions and the temperature remains steady between , or up to 15 days in lower temperatures and drier conditions. Leaves become less susceptible to infection as they age. The first sign of disease may be on the suckers close to the ground. Sexual reproduction is associated with the primary infection cycle of cherry leaf spot. Infection levels in this primary cycle are low, but essential for the success of the pathogen. Because the ascospores are produced through sexual reproduction, this part of the life cycle can also introduce genetic variation that may be important, for example, in the evolution of fungicide resistance.