Is there a chance that a scab-resistant apple cultivar will become scab-susceptible?

Yes, and several scab-resistant cultivars already are scab-susceptible at some locations. Most scab-resistant apples trace their origins to a collaboration between Purdue University, Rutgers University, and the University of Illinois. The PRI apple breeding program began in 1926 when crosses made from the crab apple, Malus floribunda 821, were found to show some resistance to apple scab. The PRI group then bred the resistant Vf gene from Malus floribunda 821 into commercial apple cultivars. Decades of subsequent hybridization and selection …

Do scab-resistant apple cultivars still need to be protected against scab infection?

Yes. Fungicide applications are an essential component of effective scab management, even though a cultivar may be resistant to the scab fungus. The fungicide applications contribute to the preservation of the plant’s resistance genes, which are subject to continuous evolutionary pressure from plant pathogens. For scab-resistant cultivars in particular, the most critical period for fungicide applications is during the primary infection cycle in the spring. Spores from over-wintered leaves on the ground are the “offspring” of the previous year’s infections …