IMPROVING DISEASE MANAGEMENT ON HONEYCRISP APPLES IN MARITIME CANADA

Abstract

For decades, growers have relied on chemical eradicative fungicides as an efficient means to control fungal diseases in apple fruit. However, increasing concerns over fungicide resistance, along with the environmental and human-health risk associated with these products have led to increased restrictions on their use. To maintain effective disease management, chemical fungicide must therefore be complemented by up-to-date information on local pathogen populations as well as eco-friendly alternatives, such as the use of plant defence elicitors (PDEs) or manipulation of the host microbiome. This thesis aimed to (i) characterize the Colletotrichum species responsible for apple bitter rot in Nova Scotia, (ii) describe the microbiome of ‘Honeycrisp’ apple fruit, and (iii) evaluate the use of salicylic acid (SA) as a PDE in an integrated disease management program. In the first study, multi-locus sequencing revealed three distinct species within the Colletotrichum acutatum species complex (CASC) affecting apple fruit in Nova Scotia: C. fioriniae, C. salicis and C. nymphaeae, and compared their relative virulence and fungicide sensitivities. The second and third studies investigated the fungal and bacterial microbial communities associated with ‘Honeycrisp’ fruit in Maritime Canada, respectively. Abiotic and biotic factors such as the environmental conditions of the growing season, management system (organic vs conventional), geographical location and the specific tissue-microhabitat were found to significantly influence the structure and diversity of apple fruit microbial communities. These findings highlight major challenges for disease control through microbiome manipulation. In the fourth study, the impact of non-eradicative treatments such as PDEs was compared to that of eradicative chemical treatments on the fungal communities of apple fruit. Chemical treatments were shown to significantly disrupt microbial network complexity, potentially leading to reduced network stability and increased vulnerability to infection. Finally, in the fifth study, the efficacy of an integrated spray program including SA was assessed. The integration of SA into a conventional fungicides program allowed a 27% reduction in fungicide use without compromising disease control on ‘Honeycrisp’ leaves during the growing season or on apple fruit at harvest and post-storage.