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Cornell University : Cornell Cooperative Extension
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| Yates Association |
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Apple Tree Diseases |
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Among the most common reasons for disappointing yields and poor fruit quality in backyard apple plantings are four plant diseases: apple scab, rust, fire blight and powdery mildew. Essentially all of the well-known apple varieties are highly susceptible to one or more of these diseases and must be regularly sprayed for good disease control. Failure to follow a satisfactory spray program will usually result in damaged fruit and weakened trees. Apple scab, one of the most common diseases of apple, is easily recognized by olive green spots on the fruit and foliage. Severely infected leaves may be dwarfed, cupped, or curled and may drop prematurely. Fruits infected during the early season may be severely deformed or may drop by early June. The scab organism overwinters in dead apple leaves on the ground. Primary infections occur during rainy periods from the time green tissue appears in the spring through the end of June. These spores are spread by rains and cause new infections all through the summer. Rust diseases of apple (cedar-apple rust, hawthorn rust, and quince rust) are caused by fungi that complete part of their life cycle on the red cedar and part on apple, crabapple, hawthorn, or quince. Cedar-apple and hawthorn rusts produce bright orange-colored spots on the leaves. Both cedar-apple and quince rusts infect the fruit; the cedar-apple rust spots are orange whereas the quince rust spots are sunken and dark green. Infections of these fungi occur during rainy periods in early spring from green tip stage through petal fall. Powdery mildew of apple is caused by a fungus that overwinters in the dormant buds. Leaves emerging from infected buds are covered by a white fungus growth. Secondary spread of the disease to other leaves, buds, and even fruits occurs from the time the buds open until the terminal buds form in late June. Fire blight may cause severe damage to apple trees. The bacteria that cause the disease overwinter at the margin between live bark tissue and that killed during the previous season by fire blight. In the spring, the bacteria become active again and may be disseminated by windblown rain, insects or pruning tools. Fire blight usually attacks the blossom; but succulent shoots are also susceptible and during the late spring and summer direct infection of young shoots is common. However, for those who aren't sure when to spray, those who prefer to minimize chemical usage, and those who are always happy to eliminate another backyard task, there is an alternative: plant disease-resistant apple varieties such as liberty. Cornell
Cooperative Extension Yates Association |
