Apple Tree Diseases & Pests

Updated July 19, 2017

You slice open an apple from your backyard tree, but a worm slithers out of your lunch, eliminating your appetite. Many diseases and pests infest apple trees and its fruit, such as the apple maggot, which bores into apples. Often, evidence of the diseases or pests appear in smaller than average fruit size, leaf discolouration, and a powdery substance on the branches. A disease may infect your tree at any point in its 15- to 20-year lifespan.


When you browse the produce section, a variety of apples such as gala, McIntosh, and Rome, decorates the shelves. Each apple variety comes from a different species of apple tree. Each tree possesses different adaptations that might make it less susceptible to a particular strain of disease or pest. Location also affects which pests plague your apple tree. For the most part with proper prevention, any apple tree can provide years of abundant, succulent apples. Other common varieties include yellow delicious, red delicious, jonagold, empire, fuji, stayman, and braeburn.


Most diseases of the apple tree start as either a bacteria or fungus spread by the wind, insects, pruning tools or tree-to-tree contact. Fire blight, caused by bacteria, lives in the tree during the winter. Spring sparks the dormant bacteria to begin killing the flowers and newly birthed stems. Fire blight destroys the leaves. Each infected leaf looks like flames scorched it. This appearance accounts for the disease's name. Some varieties such as the liberty, red baron and red delicious possess a natural resistance to fire blight. Others like beacon, fireside, honey gold and wealthy have little resistance to the infection.


Fungi begin many crippling apple tree diseases. Apple scab, a common disease in apple growing states, affects the leaves, fruit and leaf stems. The infection begins with small yellow, circular lesions and progresses to larger brown lesions. The leaves turn yellow and drop prematurely. The lesions also form on the fruit, making those areas tough and inedible. On severely infected trees, the fruit becomes deformed and splits open. Rust diseases, such as cedar-apple rust and hawthorn rust, cause "rust" spots on the leaves. Rust diseases develop more often during a rainy year.


Like any tree, apple trees host several insects. The flatheaded appletree borer attacks an apple tree by boring into the wood. These half-inch beetles emerge from their winter havens in May to lay eggs through the summer. Adults sun themselves on the sunny side of trees. The young borers devour the outer sapwood--the heart of the tree--destroying the bark in the process. To treat flatheaded appletree borers, wrap the tree in several layers of thick newspapers to prevent egg laying on the bark. Apply insecticide throughout the summer. Scale insects, nearly microscopic, cover a large stretch of branches looking like white powdery mildew, which turns red when crushed. Use dormant oil spray over all branch surfaces repeatedly for several years to kill the insects.


No prevention method keeps pests or diseases away from your apple tree indefinitely. Prevention maintains the health of the tree and a healthy tree fights diseases and pest more effectively. Clean up leaf litter, dropped branches, and fallen fruit each fall. Burn or dispose of the materials away from the apple tree. Boost the tree's immunity once a year with about 0.454kg. per diameter inch of fertiliser. Prune less than 25 per cent of the tree's crown each year. Attend to any infestation or disease immediately with the appropriate treatment.

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