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IPM Scouting in Woody Landscape Plants

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Apple scab
IPM scouting in woody landscape plants > apple scab
Cause: Venturia inequalis (fungus)

Hosts: Crabapples and apple vary widely in their resistance to scab. Related (but different) species of the fungus that cause apple scab produce scab on hawthorn, mountain ash and pyracantha.

Symptoms: Symptoms appear as velvety, olive green spots on leaves varying in size from a pinhead to 1/2 inch. As lesions enlarge, they run together and form larger areas. Leaves turn yellow and drop off in mid summer. Severely affected trees can be nearly defoliated by September. Fruits may also develop rough, corky, brown spots.

How it’s spread: The fungus produces two types of spores. Sexual spores are produced on fallen leaves infected the previous year, to infect new leaves by wind and splashing rain. Subsequent infections spread on the tree through asexual spores (called conidia) via wind and rainsplash. The disease is more severe in years with wet spring weather.

Management: Rake up and remove infected leaves from underneath trees. Avoid overhead watering. Apple scab is never fatal but disfigures the tree and reduces vigor. Plant resistant cultivars where possible; see listing below. Registered fungicides can be applied preventatively at 2-week intervals beginning at budbreak until after petal fall.

A partial listing of crabapples evaluated by Michigan State University reported as having no scab to slight scab ( with no defoliation) include the following (listed alphabetically): Malus baccata ‘Jackii’, ‘Beverly, ‘Bob White’, ‘David’, M. floribunda, ‘Harvest Gold’, ‘Liset’, ‘Mary Potter’, ‘Ormiston Roy’, ‘Prairifire’, ‘Professor Sprenger’, ‘Red Jewel’, ‘Red Splendor’, M. sargentii, ‘Sentinel’, ‘Silver Moon’, ‘Sugar ‘Tyme’, ‘White Angel’, and ‘Winter Gold’.
Apple scab Resistant crabapples
Symptoms appear as velvety, olive green spots on leaves varying in size from a pinhead to 1/2 inch. As lesions enlarge, they run together and form larger areas. Leaves turn yellow and drop off in mid summer. Resistant crabapples may develop spots on their leaves, but retain their leaves, so the scab is less noticeable.

Additional information

  • MSU Extension Bulletin E-2177, “Crabapples- A Selection Guide”
  • MSU Diagnostic Services for assistance in pest identification.
  • MSU Landscape Alert newsletters for information about the current growing season, pest concerns and plant advice.
This information was developed from A Pocket IPM Scouting Guide for Woody Landscape Plants by Diane Brown-Rytlewski. Purchase this in a pocket-sized guide for reference in the orchard from MSU Extension (publication E-2839).
The MSU IPM Program maintains this site as an access point to pest management information at MSU. The IPM Program is administered within the Department of Entomology, fueled by research from the Michigan Agricultural Experiment Station, delivered to citizens through MSU Extension, and proud to be a part of Project GREEEN.
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Updated 10/11/07