Apple scab is a fungal disease that affects both edible and ornamental apple tree varieties. The fungus typically develops in late summer and causes the fruit and leaves to develop green spots that eventually turn black and cause fruit loss. The spots also have a distinctive fuzzy appearance. Once apple scab develops on a tree, it is impossible to get rid of it in the same growing season. Treatment measures typically take one full growing season to completely get rid of the apple scab fungus.
Rake up all of the leaves underneath the apple tree to prevent the apple scab fungus from spreading. Although the majority of raking will occur in the fall, you should rake leaves up regularly throughout the year. In addition, collect any diseased fruit that falls from the tree as well.
Place all of the collected leaves and fruit into a trash bag and dispose of it in the trash. Do not place it in a compost area since the fungus will grow.
Wait until the early spring of the next growing season,when trees are just emerging from dormancy. Place a fungicide containing lime sulphur, or sulphur into a garden sprayer and mix it with water as directed by the packaging instructions.
Watch the tree until you notice pink buds appear. Spray the limbs and trunk of the tree with the fungicide until they are saturated.
Wait until the buds open into flower blossoms, which typically occurs within two to three weeks of bud development. Reapply the fungicide as soon as the flowers open.
Reapply the fungicide two more times during the early growing season, once when the petals fall off of the apple tree and again two weeks after that.
Cornell University notes you can also use a fungicide containing copper soap during the pink-bud stage, but you should not use it for later sprayings.
Cornell also says that sulphur can be harmful to some types of apple, so ensure the varieties you are growing are compatible with sulphur treatment.