Rhagoletis pomonella These worms, also known as railroad worms or apple fruit flies, are the larvae of flies that resemble the common housefly. Apple maggots infest plums, cherries, and pears as well as apples. Adult flies emerge from pupae between late June and the beginning of September. They lay eggs in the fruit through holes they puncture in the skin. The maggots that emerge from the eggs make brown trails through the flesh as they feed. Infested apples usually drop to the ground. Mature maggots emerge from the apple and burrow in the soil to pupate. They remain in the soil through the winter and emerge as adult flies the following June.

This plant disease is caused by a fungus (Gymnosporangium juniperi-virginianae) that affects both apples and certain species of juniper and red cedar. This fungus cannot spread from apple to apple, or from juniper to juniper, but must alternate between the two. In the spring, spores from brown and orange galls on juniper or cedar are blown up to 3 miles to apple trees. During mild, wet weather, the spores germinate and infect the leaves and fruit, causing spotting and, eventually, premature leaf and apple drop. During the summer, spores are produced in the small cups on the undersides of leaves. These spores are blown back to junipers and cedars, causing new infections and starting the cycle again.

Cydia pomonella This worm, the larva of a 1/2-inch-wide gray-brown moth, is one of the most serious apple pests in the United States. The moths appear in the spring when the apple trees are blooming, usually flying at twilight. They lay their eggs on the leaves, twigs, and developing fruit. When the eggs hatch, the larvae tunnel into the fruit. They feed for several weeks, then emerge from the fruit, often leaving a mass of dark excrement on the skin and inside the fruit. After pupating in sheltered locations on or around the tree, another generation of moths emerges in midsummer. Apples may be damaged by worms continuously throughout the summer. In the fall, mature larvae spin cocoons in protected places such as under loose bark or in tree crevices. They spend the winter in these cocoons, emerging as moths in the spring. They may also overwinter on other plants under the trees.

This disease is caused by a bacterium (Erwinia amylovora) that commonly affects apples, pears, and several ornamental plants. The bacteria overwinter in cankers on branches and twigs. Just before the tree blooms, bacteria ooze out of the cankers and are carried to the blossoms by splashing rain and insects. Honeybees continue spreading the bacteria to healthy blossoms. Summer storms can also spread bacteria, leading to sudden and severe fire blight outbreaks. Bacterial decay causes cankers to develop on twigs and branches, often resulting in conspicuous dieback.

This plant disease is caused by a fungus (Podosphaera leucotricha) that thrives in both humid and dry weather. The fungus spends the winter in leaf and flower buds. In the spring, spores are blown to the emerging young leaves, which are very susceptible to infection. The fungus saps plant nutrients, causing distortion and often death of the tender foliage. The fruit yield may be greatly reduced, because infected blossoms do not set fruit. Young, developing apples that are attacked become dwarfed and turn russet color. The fruit can be eaten if peeled, however. Powdery mildew is favored by warm spring days and cool nights, reduced light, and lack of rainfall.

This plant disease is caused by a fungus (Venturia inaequalis). It is one of the most serious diseases of apples in areas where spring weather is mild (60° to 70°F) and wet. The fungus spends the winter in infected leaf debris on the ground. Beginning at bud-break, spores are ejected into the air when the leaf debris becomes wet. Air currents carry them to emerging leaves. If a film of water is present on the leaf, the spores germinate and infect the leaf. The infected tissues produce more spores, which in turn infect other leaf and fruit surfaces. If the fruit stays wet for 2 or 3 days at a time in late summer or early fall, spores can infect the fruit, but symptoms do not develop until the fruit has been stored, sometimes for several months.