Several diseases and pests affect apple trees, leading to the production of small, distorted or damaged fruit. Monitoring apples trees for signs of pests and disease and following good cultural practices can alleviate many problems. In some instances, application of chemical treatments may be necessary.
Fire blight first appears during wet, spring weather after an apple tree has begun to flower. Young twigs and blooms shrivel and turn brown or black and a sticky liquid may appear on the branches. Remove branches and stems 12 to 18 inches below any affected areas. Use sharp, sterilized pruning tools and clean the tools between each cut with rubbing alcohol or a solution of one part bleach and nine parts water.
The fungal disease cedar-apple rust causes raised, orange or yellow spots to form on the leaves of apple trees. Infected trees produce small, malformed fruit that often drops prematurely. This disease begins on a host cedar tree. Brown galls form on infected cedars in the summer. After spring rains, the galls produce orange spores that travel by wind to infect apple trees. If possible, remove cedars from the area. If removal is not possible, monitor nearby cedars and prune out and dispose of any galls that appear. In severe cases, apply fungicides to the tree.
The fungal disease apple scab appears in the spring. Small spots form on the leaves and may eventually merge, causing the whole leaf to turn brown and fall from the tree. Black spots appear on the fruit. Clean up and remove fallen leaves to prevent development of this disease in the spring. Treat severe infections with fungicides by following the directions on the product's label.
Codling moths lay eggs directly on the fruit of the apple tree. Once hatched, the moths tunnel into the fruit, causing red circles or raised brown spots to appear on the skin. Two or three generations of codling moths may appear in a single year. Apply insecticides only after petals have fallen from the tree to avoid killing honeybees or other pollinators. Follow a regular treatment schedule throughout the growing season. Purchase traps containing pheromones to capture male moths and use these in conjunction with insecticidal treatments. For a natural remedy, use a mixture of one part molasses to 10 parts water to trap both sexes. Clean up and discard of any damaged fruit.
Woolly Apple Aphids
Woolly apple aphids attack twigs, roots and areas near pruning wounds. If left untreated, these aphids may cause the development of cankers that can eventually kill parts of the tree. Apply insecticidal soaps to treat stems affected by aphids.
Apple maggots lay eggs on the fruit of apple trees. When the eggs hatch, the maggots tunnel through the fruit, producing dark trails. The fruit often spoils when infected with apple maggots. Purchase sticky red sphere traps from a garden center to trap apple maggots. Hang the traps in early summer before females lay their eggs to prevent outbreaks. Also consider applying pesticides labeled to treat apple maggots.