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Apple scab of apples and crabapples

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.
Apple scab spots
Corky spot on fruit
Advanced scab

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.

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How to manage apple scab in apples and crabapples

Plant resistant varieties

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.

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Cultural practices

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Fungicides

The type of fungicides that can be used and the timing of the sprays depends on whether you intend to eat fruit from the tree or if the tree is only ornamental.

  • Do not use fungicides:
    • On apple and crabapple varieties that are resistant or immune to apple scab. 
    • To manage apple scab in ornamental trees or fruit trees where fruit quality is not a concern (i.e. fruit will be used for cider).
  • Fungicides can be used:
    • To protect fruit trees from leaf loss and fruit infection, on trees where fruit quality is important (i.e. apples for eating fresh) and the tree has a history of severe apple scab.
    • To improve the health of a tree that has lost the majority of its leaves to apple scab over 3 years in a row.
  • Good sanitation and cultural control practices should always be used in place of or in addition to fungicides.
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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2019

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