Population genetics of apple scab
Venturia inaequalis causes annual epidemics of apple scab worldwide. High standards of fruit quality require very high levels of control, particularly in dessert cultivars. Despite enormous research effort, this disease is still managed mainly through scheduled applications of fungicides. In the UK, up to 15 applications of fungicides per growing season are used to manage scab and powdery mildew. Scab control in cider and juicing apples is less stringent than on dessert apples but is still important.
Given the huge effort already focused on host resistance breeding, epidemiology and management of apple scab, it is highly unlikely that current knowledge will lead to any significant changes in scab management in the short-term. Although several sources of resistance to scab have been identified, only one resistance gene from Malus floribunda has so far been incorporated into commercial apple cultivars, but this has already been overcome in several locations. Thus breeding for durable resistance to scab remains an elusive long-term goal. The current scab management strategy is reliant on elimination of overwintering leaf litter and scheduled application of fungicides during the growing seasons but does not exploit the potential in reducing scab through differential responses to scab among popular ‘susceptible’ cultivars. An alternative strategy to monoculture is the use of mixtures of cultivars that differ in their resistance genetic background to diseases. Mixture of apple cultivars with different resistance genes can significantly reduce development of apple scab.
To adopt mixtures in commercial production, we need to answer the following two important questions: (1) how quickly the scab fungus will evolve to acquire virulence genes against the range of resistance genes existing in the mixture, and (2) which current cultivars can be used as mixture components to reduce scab development. We have been studying population variability in mixed orchards to provide answers to these two questions.