Durable disease resistance in woody ornamentals: the breeders’ challenge

Consumers' demand and legislation force growers to develop sustainable strategies to control pests and diseases. One of the measures growers can take is the choice to produce species and cultivars with improved pest and disease resistance or tolerance. To support a shift toward growing resistant cultivars, breeding programs should focus more on biotic stress resistance. Yet, resistance breeding in woody ornamentals is difficult due to the high diversity of plant genera and the numerous pests and diseases. In addition, long generation cycles, complex genetics, polyploidy, and the relatively low economic value of individual crops or cultivars make breeding for pest and disease resistance challenging. Breeding programs often aim for selection of improved resistance of one plant genotype towards different pests and/or diseases or towards different pathotypes of one disease. However, some alternative routes to obtain more tolerant cultivars can be explored. In research of edible crops an increased interest is seen in breeding strategies focusing on the loss-of-function of so-called susceptibility genes. Also more fundamental knowledge on inducible plant defense mechanisms might open new horizons for plant breeding in the future. Here we illustrate different approaches using examples from our resistance breeding in ornamentals for two fungal pathogens: box blight in Buxus and powdery mildew in rose, and for one pest organism: broad mite in Rhododendron.