Michigan apples advice from MSU

Apple scab

Apple scab fungus overwinters in infected leaves on the orchard floor. When the leaves become wet, spores are discharged and disperse into surrounding trees. Infection occurs on foliage, blossoms, petioles, and fruit during periods of sufficient wetting at given temperatures (see Adapted Mills Table).

Primary infections usually develop first on spur leaves and on the blossom end (calyx) of fruit. Conidia are produced abundantly in these velvety-brown to olive lesions and serve as sources of secondary infection spread by wind and rain.

Are conditions right for apple scab?
Forecast models for apple scab are available at Enviro-weather. Select a weather station from the map that is closest to your location. Then click on "fruit" for a list of weather resources and models for fruit production.


Apple scab initial symptoms.	Primary scab infection on spur leaves of McIntosh apple.	Primary scab infection on the blossom end of McIntosh apple.

Monitoring: Scout spur leaves and fruit in late May for primary scab symptoms. These primary infections will provide the inocu-lum for secondary scab infections on terminal leaves and fruit.	Apple scab	Sporulating scab lesions on McIntosh apple leaves.


Late-season scab infection on a Hampshire apple.	Spur leaves of McIntosh apple (red arrows) with primary scab infection; (currently) uninfected terminal leaves (blue arrows).

Adapted Mills Table
Approx. wetting period required for primary apple scab infection at various
air temperatures and time required for conidia to develop.

Additional information

For more monitoring information and evaluation of available pesticides:
Michigan Fruit Management Guide
A Practical Guide to Scouting Apple Orchards – a DVD showing how to scout apple orchards.
MSU Diagnostic Services for assistance in pest identification.
MSU Fruit Crop Advisory Team Alert newsletters for current pest/crop conditions.

This information was developed from A Pocket Guide for IPM Scouting in Michigan Apples by David Epstein, Larry J. Gut and George W. Sundin. Purchase this in a pocket-sized guide for reference in the orchard from MSU Extension (publication E-2720).