Cherry leaf spot (Blumeriella jaapii) 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.Gianessi, L., Williams, & Ashley. (2011, May). Crop Life Foundation. Retrieved from Fungicides Prevent Defoliation of Cherry Trees in Eastern States: http://croplifefoundation.files.wordpress.com/2012/07/14-cherries.pdf 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.Hickey, K.D. 1991. Fungicide Benefits Assessment Fruit and Nut Crops – East. USDA National Agricultural Pesticide Impact Assessment Program. January. 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.Farmer, B. (n.d.). UNL Extension: Controlling Cherry Leaf Spot. Retrieved from University of Nebraska-Lincoln:
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.Agrios, G. N. (1997). Plant Pathology (4 ed.). San Diego, California, US: Academic Press.
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.Ellis, M. A. (2008). Cherry Leaf Spot. Columbus, OH: The Ohio State University, Department of Plant Pathology. Retrieved from http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3021_08.pdf 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.Ogawa, J.M., et al., eds. 1995. Compendium of Stone Fruit Diseases. APS Press.
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.Cherry Leaf Spot. (2013). Retrieved from Penn State Extension: http://extension.psu.edu/plants/gardening/fphg/stone/diseases/cherry-leaf-spot Hyphae cannot invade immature stomata very effectively.Travis, J., Rytter, J., & Biggs, A. (n.d.). Cherry Leaf Spot. Kearneysville, WV: West Virginia University, Fruit Pathology. Retrieved from http://www.caf.wvu.edu/kearneysville/disease_descriptions/omchlfsp.html 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.
Asexual reproduction Once these lesions from the primary infection have formed, there is the potential for secondary infections. Secondary or summer spores called conidia form in whitish patches on the undersides of leaves in tiny, concave fruiting bodies called acervuli. These conidia are horn-shaped and have hydrophilic outer cell walls that make them easily miscible in water. When spread during rain or by wind they can cause new leaf infections.Keil, Harry L. Cherry Leaf-Spot and its control. [Leaflet] s.l. : United States Department of Agriculture, 1961. https://archive.org/stream/cherryleafspotit489keil_0#page/n9/mode/2up. Each conidium can multiply to form many additional conidia relatively quickly. When conditions are favorable, additional infections caused by conidia may overwhelm plant defenses, leading the plant to shed badly infected leaves. These secondary infections allow the disease to escalate into an epidemic very quickly.Erin Lizotte, George Sundin. Michigan State University Extension. Cherry leaf spot management and resistance considerations for 2011. [Online] MSU, February 15, 2011. [Cited: December 3, 2013.] http://msue.anr.msu.edu/news/cherry_leaf_spot_management_and_resistance_considerations_for_2011. The conidia form through an asexual process, so all the conidia formed in one leaf spot are genetically identical. The fungus overwinters in diseased leaves on the ground, producing sexual spores in the Spring that infect the emerging leaves.
Environment 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 . Optimal temperature range for the spread of this fungus is between .Sundin, G. (2013, June 7). Fungicide considerations for cherry leaf spot control at first cover. Retrieved December 3, 2013, from Michigan State University Extension: http://msue.anr.msu.edu/news/fungicide_considerations_for_cherry_leaf_spot_control_at_first_cover Serious infection of a tree occurs in years with many rainy periods and cooler summers. Very few ascospores are ejected from the apothecia if the temperature is less than . 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, much sunlight, and good air circulation.
Disease management Resistance 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.Carlson, Eric. Research may help control leaf spot. Leelanau Enterprise. [Online] February 7, 2013. [Cited: December 3, 2013.] http://www.leelanaunews.com/news/2013-02-07/Life_in_Leelanau/Research_may_help_control_leaf_spot.html .
Small or backyard growers 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.Holb, I. J. 2013. Effect of sanitation treatments on leaf litter density and leaf spot incidence in integrated and organic sour cherry orchards. Plant Dis. 97:891-896. 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.
When planting, growers should select locations which have a large amount of direct sunlight such as a south facing slope. Proper pruning should also be completed to increase the amount of sunlight penetration and air circulation through the canopy. Any practice that increases the faster drying of leaves will help reduce the risk of infection. Growers may also consider making an after harvest fungicide application using a combination of Benomyl (50% WP) and Captan (50% WP) at rates of ¼ Tablespoon and 2 Tablespoons respectively per gallon of water. This will help reduce the rate at which pathogens may develop resistance to Benomyl products. Prior to shuck split, the recommended fungicide for cherry leaf spot is chlorothalonil (Bravo and generics). This fungicide is a multi-site protectant and is excellent for leaf spot control and is not at risk for fungicide resistance development. At least two applications of chlorothalonil should be made before shuck split with the goal to minimize the potential of infection at this early stage.
Commercial growers For commercial growers, the disease is primarily controlled by use of fungicide sprays. Fungicides are much more effective when applied early in the season when the inoculum load is low as cherry leaf spot is a prolific, unrelenting, tireless disease.
Fungicide applications should begin at petal fall or shortly after leaves have unfolded. These sprays should continue on a schedule of every 7–10 days until harvest. Upon harvest, one or two postharvest applications should be administered, beginning 2–3 weeks after harvest. It is suggested that spraying alternate sides of trees on a 7-day program is more effective than spraying both sides of trees on a 10-day schedule.