THE APPLE SCAB DILEMMA: FROM STRATEGIES TO COPE WITH Venturia inaequalis DAMAGES TO THE RISK OF THE EMERGING PATHOGEN Venturia asperata ON RESISTANT APPLE CULTIVARS

Apple scab, caused by Venturia inaequalis, is the most significant disease affecting apple trees. It can negatively impact both plant growth and fruit quality, both in the orchard and during postharvest storage. Various approaches can be employed to mitigate its effects. The experimental activity conducted during this PhD program, and described in this thesis, explores two highly current issues related to the occurrence of Venturia: (I) the risk posed by novel or emerging pathogens to scab-resistant apple cultivars, and (II) the valorization of scab-affected apples through a circular economy approach to reduce postharvest losses. While resistant apple cultivars offer am environmentally sustainable solution for this disease control, the emergence of new Venturia species represents a potential threat to their long-term effectiveness. For instance, the recent isolation of V. asperata on the resistant apple cultivar “Modì” in Trentino province (Italy) exemplifies how the emergence of new pathogens on resistant genotypes can nullify the advantage of resistance. Therefore, the first objective of this thesis was to characterize and study the epidemiology of this new species in Trentino and South Tyrol, comparing it with isolates of V. inaequalis. Beyond agronomic implications, apple scab significantly impacts the marketability of fresh fruit. V. inaequalis causes black spots on the fruit skin, even in cold storage, making a portion of the harvest unsuitable for fresh consumption. The second objective of this thesis was to minimize these fruit losses (i.e., aesthetically imperfect apples). We aimed to achieve this by integrating these aesthetically imperfect but compositionally sound fruits into the cider production chain. To assess their suitability for industrial valorization, we characterized both the alcoholic fermentation process, and the composition of cider made from scab-affected apples, comparing them to cider from healthy fruits. Furthermore, we characterized the apple skin microbiota using a metabarcoding approach, which provided additional insights into the microbial dynamics of scab-infected fruits. Overall, this thesis contributes to a better understanding of Venturia species epidemiology and explores sustainable solutions to reduce the economic losses caused by postharvest apple scab.