Population genetics and epidemiological effects on Venturia inaequalis from mixed cultivar apple orchards

The apple industry in the UK produces half a million tonnes of fruit a year; its most economically important disease is apple scab caused by the fungal pathogen Venturia inaequalis. Mixing cultivars with differing resistance to the pathogen, in the same orchard, has been shown to reduce the levels of apple scab. In this study we investigated the population genetics and epidemiology of apple scab in mixed cultivar orchards. Using molecular techniques, populations of V. inaequalis differed on the hosts present in an orchard, this difference remained over time. This indicates a “super race” of the pathogen that can infect all of the cultivars present has not emerged and become dominant, therefore making the concept of mixed cultivar orchards more feasible. The lack of emergence of a super race might be, in part, due to asexual conidia spores forming a proportion of primary inoculum as opposed to being purely from sexually produced ascospores. Conidia accounted for 20-50% of the primary inoculum in the orchard studied. The importance of conidia is likely to be heavily dependent on local conditions. For a reduction in levels of apple scab in a mixed cultivar orchard it is important that the cultivars present have differing resistance. Comparing scab populations on a number of desert and cider cultivars showed that populations on cider cultivars are most different. An assembled V. inaequalis genome is presented and was used to align isolates from different cultivars within the same orchard. Looking at differences between these isolates has indicated that there is a lack of mating occurring between isolates from the different cultivars present within the same orchard. This indicates that mating is most likely initiated between isolates on the same leaf. The findings of this thesis could contribute to apple orchard practises, regarding apple scab control, in the short, medium and long term.