Cherry leaf spot

Cherry leaf spot is a fungal disease which infects cherries and plums. Sweet and sour cherries are susceptible to the disease; however leaf spot is much more 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. 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. This fungus overwinters in dead leaves on the ground. In early spring (about petal fall), fungal fruiting bodies called apothecia develop in these leaves. Spores (ascospores) are produced in the apothecia and are forcibly discharged starting at petal fall. These ascospores are spread by wind or rain to the healthy, new, green leaves and thus serve as the primary inoculum in a new growing season. The ascospores can stick to the leaf and if a film of water and optimal temperature is present, germinate within hours of adhesion. Upon germination, the ascospores can enter the leaf through the stomata (natural openings) on the underside of the leaf. Ascospores cannot enter immature stomata very effectively. These ascospores grow in the intercellular spaces in the leaf. Incubation time will vary depending on the amount of available moisture and temperatures. The small purple lesions can appear within 5 days if there are damp conditions and the temperature remains steady between 60–68 °F (16–20 °C). This incubation period could take as long as 15 days though if lower temperatures and drier conditions occur. Generally leaves become less susceptible to the disease as they grow older during the course of the growing season. The first sign of disease may be on the suckers close to the ground. This is considered to be the primary reproduction cycle of cherry leaf spot. However, infection from this primary cycle is fairly low but is essential for the pathogen to be able to produce secondary spores. Once these lesions from the primary infection have formed, there is the potential for secondary infections. Secondary or summer spores called conidia will form on the underside of the leaf from slightly concaved eruptions called acervuli. These conidia are horn-shaped and highly water-soluble. The spores have a whitish appearance on the underside of the leaf. They are spread via wind or rain when broken open and can cause additional infections. Each of these spores can multiply and cause thousands of additional conidia to be produced in a relatively short amount of time. All of these conidia hold exactly the same genetic information and is simply the fungus just replicating its DNA. At this stage the fungal infection overwhelms the host plant and causes the plant to shed its leaves prematurely. The leaf litter produced then becomes the overwintering site of this fungus, and the cycle will begin the next spring. These secondary infections allow the disease to escalate into an epidemic very quickly. The disease is generally distributed throughout the U.S., Canada, and various parts of the world where humid conditions occur. The cherry leaf spot fungus prefers moderately wet conditions, with temperatures above 60 °F (16 °C). Optimal temperature range for the spread of this fungus is between 60–68 °F (16–20 °C). Serious infection of a tree occurs in years with many rainy periods and cooler summers. Very few ascospores will be ejected from the apothecia if the temperature is less than 46 °F (8 °C). This disease thrives in areas that do not use any type of sanitation practice as a preventative measure to prevent disease spread. When selecting a site for an orchard, use areas that have well drained soils, lots of sunlight, and good air circulation. There are no resistant varieties available on the commercial market yet. However, researchers have found the a wild type gene linked to the resistance. They are currently crossbreeding the wild lines with commercial cultivars and beginning to carry out field trials. No data is available yet. For small or backyard growers, collecting and destroying all leaf debris on the ground is an absolute necessity due to the potency of this disease because the fungus overwinters in this leftover leaf litter. This is its main form of survival. By removing and destroying these leaves, a grower can significantly decrease the amount of primary inoculum available in the spring. It will greatly decrease the apparent infection rate. There has also been a study done on the addition of a straw mulch bedding to the ground after all the leaves have been picked up. The addition of this mulch further reduced the spring infection rate. Leaf litter removal is not very practical for large commercial growers due labor needs and number of trees but if at all possible, a majority of the old leaves should try to be collected.