Black spot is a plant disease that, as the name implies, causes black spots on the plants it infects. One plant prone to black spot disease is the apple tree, though black spot in apple trees is widely known as apple scab. Like other plants stricken with black spot disease, once your apple trees become infected, you must work to eradicate the spots as quickly as possible. If you fail to treat black spot disease in apple trees effectively, infected trees will fail to produce and spread the disease to other nearby apple trees.
Remove leaves that fall from the apple tree as soon as possible. Since exposure to both sunlight and water release the spores that spread black spot disease from the fallen leaves, you must maintain a regular raking schedule, especially in autumn when leaves fall continuously.
Put all fallen leaves from the apple trees into trash bags to smother and dispose of the leaves and spores. Do not use leaves from an apple tree with black spot disease in any other type of application, including compost.
Pull off any apples that do not fall from the tree naturally after the final fall harvest. Fruit that remains to rot on the tree may also contain and spread the spores that lead to black spot. Pick up all fruit from the ground as well and dispose of the fruit in trash bags as you did the leaves.
Spray the apple tree with lime sulphur if you want to keep your apples organic. According to the National Sustainable Agriculture Information Service, lime sulphur is considered an organic fungicide, and, according to Green Harvest, lime sulphur is effective in the treatment of black spot on apple trees. Apply lime sulphur at the start of the growing season to help prevent black spot.
Use a stronger chemical fungicide labelled for black spot if the lime sulphur doesn't work to eliminate the disease. Be aware that if you do use a chemical fungicide other than lime sulphur, your apple tree becomes non-organic.
- Hortnet; Improved Management of Apple Black Spot; David Maktelow and Robert Beresford; September 1993
- Go Organic Gardening: Black Spot Fungus
- Green Harvest; Organic Fungal Control; Frances Michaels
- National Sustainable Agriculture Information Service; Organic and Low-Spray Peach Production; Steve Diver and Tracy Mumma; 2003