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Managing apple scab on ornamental trees and shrubs

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Quick facts

  • Apple scab is the most common disease of ornamental crabapples in Minnesota.
  • Infected leaves have olive green to brown leaf spots.
  • Leaves with many leaf spots turn yellow and fall off early.
  • 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 of flowering crabapples.
  • Fungicides can be used to manage apple scab in susceptible varieties. Proper timing of sprays is needed for fungicides to control disease.
Apple scab infections that are olive green with fringed edges on leaves.
Apple scab infections are initially olive green with fringed edges.

How to identify apple scab

  • Leaf spots are round, olive-green in color and up to ½-inch across. They 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.

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How to manage apple scab

For control of apple scab on apple and edible crabapple varieties see pest management for the home apple orchard guide for home apple growers.

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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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