Quick facts

• Apple scab is the most common disease of apple and crabapple trees in Minnesota.
• Scab is caused by a fungus that infects both leaves and fruit.
• Scabby fruit are often unfit for eating.
• Infected leaves have olive green to brown spots.
• Leaves with many leaf spots turn yellow and fall off early.
• Leaf loss weakens the tree when it occurs many years in a row.
• Planting disease resistant varieties is the best way to manage scab.
• Fungicides can be used to manage apple scab. Proper timing of sprays is needed for fungicides to control disease.

How to identify apple scab

• Leaf spots are round, olive-green in color and up to ½-inch across. 
• Spots are velvet-like with fringed borders.
• As they age, leaf spots turn dark brown to black, get bigger and grow together.
• Leaf spots often form along the leaf veins.
• Leaves with many leaf spots turn yellow and drop by mid-summer.
• Infected fruit have olive-green spots that turn brown and corky with time.
• Fruit that are infected when very young become deformed and cracked as the fruit grows.

How does apple scab survive and spread?

Apple scab is caused by the fungus Venturia inaequalis. It infects crabapples and apples (Malus spp.), mountain ash (Sorbus spp.), pear (Pyrus communis) and Cotoneaster (Cotoneaster spp.).

The apple scab fungus has several host-specific strains that can cause disease on one type of plant but not any other. For example, the strain of V. inaequalis that infects mountain ash will only infect other mountain ash trees and will not infect crabapple trees. Apple and crabapple trees are infected by the same strain of the apple scab fungus because the trees are in the same genus.

Apple scab is caused by the fungus Venturia inaequalis.

• The apple scab fungus overwinters on fallen diseased leaves.
• In spring, these fungi shoot spores into the air.
• Spores are carried by wind to newly developing leaves, flowers, fruit or green twigs.
• Spores need several hours of moisture on the plant surface in order 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 leaves remain wet long enough.
• Warm, rainy weather in the spring and summer creates ideal conditions for apple scab.
• Leaves with many leaf spots turn yellow and fall off early. This weakens the tree.
• Several years of early leaf loss can result in decreased growth, reduced bloom and increased susceptibility to winter injury.
• Many ornamental crabapple trees are susceptible to apple scab, so the disease can be spread to your fruit trees from nearby flowering crabs.

Fungicides to protect fruit trees from apple scab:

Apples and edible crabapple trees that become severely infected with apple scab have poor quality fruit and reduced health as a result of leaf loss. Fungicides can be used to protect healthy trees from apple scab, but will not cure an infected tree.

Thanks and regards,

Jessey

Managing Editor

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