Leaf spot Disease
Cherry leaf spot is caused by the fungus Blumeriella jaapii. The disease reduces flowering and weakens the tree. Cherry leaf spot often defoliates the tree by midsummer. Repeated defoliation makes the tree more susceptible to winter injury and may eventually kill it.
Leaves show small purple to brown spots with definite boundaries in early summer. The leaves turn yellow and fall. In July, the centers of the infected spots frequently fall out, giving a shot-hole appearance.
The fungus overwinters on fallen leaves. In spring following wet weather, spores form and winds blow them to infect leaves. When temperatures are favorable (16°C to 24°C), infection requires a wet period of only a few hours. Leaves are susceptible when fully unfolded — this usually occurs near petal fall. The initial leaf infections form spots, and more spores are produced in the spots. These spores are splashed by rain to other leaves and infect them. Secondary spread and infection by spores continues repeatedly, whenever wet warm weather occurs, until leaves fall in autumn.
Cherry leaf spot is caused by the ascomycete fungus Blumeriella jaapii (formerly known as Coccomyces hiemalis) anamorph Phloeosporella padi (Lib.) Arx. 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 telemorph 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 (15°C) and 68 degrees F (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 degrees Fahrenheit (15°C) . Optimal temperature range for the spread of this fungus is between 60 (15) to 68 degrees Fahrenheit (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 degrees F (7°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.>
Modelling the infection by Blumeriella jaapii
The Ascopsore maturation Model: Under optimal Temperatures Ascospores will get mature within a 2 week period. Optimal temperatures are above 16°C. In least optimum conditions the period of ascospore discharge can take up to 6 weeks. The model is showing the period of possible ascospore discharge. During this period we have to expect primary infections of the disease.
The Ascospore discharge Model: Ascospore discharge is depending on wetting of the apothecia and the following drying up. It is bigger when more apothecia are wetted (heavier rain or more rain) and when the temperature during and after the rain are higher. It will start at temperatures of 16°C.
The Infection model for Blummeriella jaapii
Infection is predicted with the table by Eisensmith and Jones (Table 1), which uses leaf wetness duration and air temperature data. In the model, a wetting period is initiated when the leaf wetness sensor becomes wet. The wetting period ends when the RH drops below 90% and the leaf wetness sensor is dry. However, if the leaf wetness sensor again becomes wet within 8 hrs of the time the RH fell below 90%, it is considered to be the same wetting period with the dry hrs added as well. A wetting period may result in no infection or infection by the cherry leafspot pathogen.
Use of the model:
The cherry leaf spot model can be used as a guide to determine the need for eradicative sprays if a protectant coverage was weak during a rain episode. The model should not be used as a routine guide for eradicative spray applications- a protectant spray program is the most reliable approach for control of cherry leaf spot. The model is useful for comparing predicted leaf spot infection activity between years.
Ascospore and Conidia Infection Model based on:
EISENSMITH, S.P. and A. L. Jones (1981): Infection Model for timing fungicide applications to control cherry leaf spot. Plant Dis. 65
Eisensmith, S. P. and A. L Jones. 1981. A model for detecting infection periods of Coccomyces hiemalis on sour cherry. Phytopathology.71:728-732.Jones, A. L. and T. B. Sutton. 1996. Diseases of tree fruits in the east. Northcentral Regional Bulletin 45, Michigan State University
Shane, B. (2011): About Enviro-weather's Cherry Leaf Spot Report, http://enviroweather.msu.edu/aboutThisModel.php?mod=f_cls