A 33-year Evaluation of Resistance and Pathogenicity in the Apple Scab–crabapples Pathosystem

Abstract

Crabapples (Malus spp.) are popular ornamental trees in the commercial and residential landscape. Over a 33-year period at the Secrest Arboretum, Wooster, OH, 287 accessions of ornamental crabapple were evaluated for their resistance to apple scab caused by the fungus Venturia inaequalis. Of these 287 accessions, 31 had no symptoms of scab for longer than a 10-year period and were identified as resistant to the disease. Of these 31 resistant accessions, 14 eventually displayed symptoms, presumably as a result of infection by one or more newly present races of the pathogen in the trial plot. Notable resistance breakdowns in accessions previously classified as resistant include the development of scab on M. × ‘Prairifire’, M. × ‘Bob White’, M. × ‘Red Jewel’, and M. floribunda. Corresponding to these changes of resistance is the putative development of new V. inaequalis races in North America: Race 5, possessing virulence to the Vm gene in ‘Prairifire’; Race 3 that infects M. × ‘Geneva’ but not M. baccata ‘Dolgo’; and the first identification and report of scab on a M. floribunda population that was reported as resistant even before the first 25 years of the evaluation. The detection of scab on this species suggests the presence of Race 7 in North America for the first time. Five named accessions remained free from scab for the entire 33-year trial: M. sargentii ‘Sargent’, M. baccata ‘Jackii’, M. × ‘Beverly’, M. × ‘Silver Moon’, and M. × ‘White Angel’ and may serve as sources of durable resistance in crabapple and commercial apple breeding in the Midwest.