Apple scab: prevention and resistant cultivars

Apple scab is the most economically significant disease of apples and crabapples in the eastern US and Europe. In susceptible cultivars grown without a fungicide program, scab can reduce the commercial fruit crop to zero in a wet year. For home gardeners, the practical choice between apple scab.

Apple scab is the most economically significant disease of apples and crabapples in the eastern US and Europe. In susceptible cultivars grown without a fungicide program, scab can reduce the commercial fruit crop to zero in a wet year. For home gardeners, the practical choice between apple scab management — which requires precise, season-long fungicide applications — and planting resistant cultivars has a clear answer for most situations.

I don't grow apples at my Long Island property, so this guide draws on Cornell Cooperative Extension (which has done the majority of foundational research on this disease), Penn State Extension, and Clemson HGIC publications.

The pathogen

Apple scab is caused by Venturia inaequalis. Per Cornell Cooperative Extension, the fungus overwinters in infected fallen leaves on the ground beneath the tree. In spring, mature ascospores develop in the overwintered leaves and are released in a period that begins at bud swell and extends through 2–3 weeks after petal fall — the "primary infection season."

The same fungus infects ornamental crabapples (Malus spp.) and causes similar damage. Per Clemson HGIC, a different but related species, V. pyrina, causes scab on pear.