Answer: I can't tell from your description whether your cherry tree has developed Cherry Leaf Spot (Blumeriella jaapii), or Brown Rot. Cherry leaf spot is a fungus that usually attacks the leaves but may affect twigs and stems. Leaf Spot occurs in many parts of North America and the world where humid conditions occur. Symptoms include small dark coloured spots on the leaves and the leaves may begin to yellow while areas around the spots may remain green. Some leaves may drop prematurely. During wet weather white patches may appear on the undersides of leaves at the centre of the dark spots. These white patches contain fungal spores. The disease overwinters when infected leaves fall to the ground. Wind and rain splash the spores up onto the tree's new leaves and buds in the spring. To manage Leaf Spot, it is recommended to spray with a fungicide. Rake and destroy any fallen leaves to help reduce the amount of disease the following year.
The fungus that causes Brown Rot disease can infect fruit, blossoms and small branches. Symptoms include blight, cankers and fruit rot. Cherry fruits that are affected rot very quickly, they shrivel and become covered in a brownish-gray substance. Powdery tufts of brown gray spores are visible on the outside of infected flower shucks, and on infected fruit or twig surfaces especially under wet weather conditions. The dried infected fruits are called mummies as a result of their appearance. Wind and rain spread the spores from infected mummies and twigs to uninfected parts of the tree. Mummified fruit and cankers should be pruned during the dormant season and either burned or buried deeply in the soil. Remove and destroy all dropped and rotted fruit from the ground beneath the tree. Blossom blight is much more serious on sweet cherry than on sour cherry. Sweet cherries (not sour) are also very susceptible to infection the first few weeks after fruit set, and a petal fall spray is recommended if the weather is warm and wet. Superior brown rot fungicides should be used on sweet cherries three weeks before the fruits are ripe.
Crabapples are susceptible to scab, caused by the fungus Venturia inaequalis, is the most common and one of the most devastating diseases of crabapple in Connecticut. Defoliation of trees as a result of this disease is not only unsightly, but also reduces the vigor of the tree, making it more susceptible to environmental stress and other opportunistic pests. Scab is usually most severe in wet weather. The fungus causes circular, olive-black spots on the leaves, fruit, and young fruit stems. As the spots develop, the leaves may turn yellow and drop prematurely. Symptoms typically first appear on the leaves in May or early June. Heavy infections can result in defoliation by June.
The fungus overwinters on dead, fallen leaves and produces spores (primary) in the spring that can infect sepals, young leaves, and young fruit during periods of rain. Infection from these primary spores can take place anytime after growth begins until mid to late June if suitable weather conditions exist. During the summer, a different spore (secondary) is produced by the fungus that is capable of inciting more new infections when splashed onto leaves and fruits by rain.
Fire blight, caused by the bacterium Erwinia amylovora, is another devastating disease of crabapple. Fortunately, this disease is only an occasional problem and, when it does occur, it is often isolated to specific geographical locations. However, when infection occurs, the disease can develop quite rapidly and can destroy individual trees in a single season. The bacteria survive the winter in old cankers on crabapples and other plant hosts and in healthy buds. As the weather becomes favorable for growth in the spring, the bacteria begin to multiply rapidly and can be seen oozing out of tissues. This creamy, bacterial ooze is attractive to insects and they pick it up and carry it to open flower buds where infection occurs. The bacteria are also carried by wind and rain to open blossoms. Infected tissues are characterized by their blackened, "burned" appearance, hence the name "fire blight."
The most effective method for control of this disease is to select and plant crabapple varieties that are resistant to fire blight. These include: Adams, Callaway, David, Dolgo, Harvest Gold, Indian Summer, Jewelberry, Liset, Profusion, Red Baron, Selkirk, and Sentinel. Sanitation is also a very important aspect of control. Any cankered or infected branches or twigs should be cut back to healthy wood during the dormant season. All pruning cuts should be made at least 8-12" below visible symptoms. All tools should be disinfested with 10% household bleach (1 part bleach: 9 parts water) or 70% alcohol. Prunings should be removed from the vicinity of the tree. The effects of this disease can also be minimized by maintaining overall tree health by following proper cultural practices that avoid excessive vigor. It is especially important to avoid heavy applications of nitrogen in the spring.
Best wishes with your trees!
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