Apple Scab on Apple and Crabapple Trees
Apple scab is caused by the fungus Venturia inaequalis. It infects crabapples and apple (Malus spp.), as well as, mountain ash (Sorbus spp.), pear (Pyrus communis) and Cotoneaster (Cotoneaster spp.). This is the most common fungus on apple and crabapple trees and infects both leaves and fruit.
Properly identifying the fungus helps determine what control options are available. The fungus will present itself as round, olive-green leave sports that can be up to 1/2-inch in size. The spots are velvet-like with fringed borders. As they age, the leaf spots turn dark brown to black and they get bigger and grow together. The leaf spots form along the leaf veins. By mid-summer, leaves with many leaf spots will turn yellow and fall from the tree. This is the time when homeowners notice something may be wrong with their tree. Apples and crabapples that are infected have olive-green spots that turn brown and corky with time. If the fruit is infected early, they may appear deformed and cracked as the fruit grows.
The apple scab fungus overwinters on fallen diseased leaves. In the spring, these fungi shoot spores into the air. The spores need only a few hours of moisture on the plant surface to start new infections. These infections grow into spots that can produce more spores within 9 to 17 days. Spores are spread by wind, splashing rain or irrigation throughout the tree canopy or to neighboring trees, starting new infections. The infection cycle can repeat many times throughout the growing season whenever the leaves remain wet long enough. Ideal conditions for apple scab are warm and rainy weather in the spring and summer.
Planting disease-resistant varieties is the best way to prevent apple scab. Many varieties of apple and crabapple trees are resistant or completely immune to apple scab. This information does not help a lot if you already have apple or crabapple trees and they are infected. If trees have the fungus present, clean up leaves in the fall. Removing fallen leaves will help get rid of places where the fungus can survive the winter to re-infect trees the next year. The leaves can be burned, buried, or composted. Instead of raking, the leaves can be chopped with a mulching lawn mower. Fall lawn fertilizer applications will help breakdown leaves that have been chopped. In mulched areas, urea can be applied to chopped leaf litter to help with decomposition.
The apple scab fungus needs moisture on the leaves to begin a new infection. Good air movement through the tree can help dry leaves quickly. A well-pruned tree with an open canopy can help reduce the severity of apple scab in a tree. Prune trees so that the branches are spaced far enough apart to let air move through the branches and dry the leaves quickly. Remove upright suckers and water sprouts that have formed along the trunk of within the canopy. Keep airflow in mind when planting new trees. Do not overcrowd plants. Use plant spacing for the mature size of the tree.
Fungicides are available and should be used at the right time. While fungicides will not cure leaf spots on ornamental crabapple from apple scab, they will protect healthy leaves from becoming infected. Good sanitation and cultural practices should always be in place before any chemical application occurs. The timing of fungicide applications must be such to protect the new leaves as they emerge in the spring. Spray should start when the first green leaf tips appear in the spring. Sprays should be repeated until most of the petals have fallen off the tree. The fungicide label will recommend the spray interval. Most labels offer a range of days to wait before spraying again. Use the shorter spray interval listed on the label when the weather calls for frequent rain or where scab has been a problem in the previous year. Expect to apply two well-timed fungicides in the spring. It may take several years to get apple scab under control.
Small trees can be treated by a homeowner if the fungicide label if followed. A certified arborist should be contacted for fungicide applications to large trees. For chemical recommendations, call your local N.C. Cooperative Extension office.