Factors Influencing Verticillium Stripe in Canola: Blackleg Interaction, Host Resistance, and pH

Abstract

Verticillium stripe, caused by Verticillium longisporum, is emerging as an important disease of canola (Brassica napus) in Canada. This thesis aimed to enhance understanding of potential interactions between Verticillium stripe and blackleg (Leptosphaeria maculans), assess the influence of pH on the growth of V. longisporum and the severity of Verticillium stripe, and identify sources of resistance to this disease. The impact of V. longisporum/L. maculans interactions on yield was evaluated under both field and greenhouse conditions. Co-inoculation resulted in increased blackleg severity and yield losses. In some cases, Verticillium stripe caused greater yield losses than blackleg. The influence of pH on the growth of V. longisporum was assessed by measuring colony diameter after 14 and 21 days of incubation on potato dextrose agar at varying pH levels (4.7, 5.5, 6.5, 7.4, and 8.6). Colonies of V. longisporum exhibited approximately 16% greater diameter at pH 7.4 and 8.6 compared to pH 5.5. The impact of pH on disease development at the seedling stage was examined using a semi-hydroponic system with half-strength Hoagland’s solution at different pH levels (4.4, 5.4, 6.3, 7.5, and 8.4). Disease severity was most pronounced at pH 7.5 and 8.4. In a follow-up experiment, canola seedlings previously inoculated with the fungus were transplanted into potting mix with pH levels of 5.6, 6.4, 7.2, and 7.8. Verticillium stripe was most severe at pH 7.8, indicating a substantial risk of increased disease and yield losses in neutral to slightly alkaline soils. Finally, 211 Brassica genotypes were screened for reactions to V. longisporum under greenhouse conditions and subjected to a genome-wide association study to identify single nucleotide polymorphism (SNP) markers for resistance. Eleven non-commercial Brassica accessions and nine out of 35 commercial canola cultivars displayed a low normalized area under the disease progress curve, suggesting their potential as sources of resistance against V. longisporum. Additionally, 45 significant SNP markers were identified, with promising hotspots located on chromosomes A03 and A10. Collectively, the results highlight the need for proactive strategies to manage Verticillium stripe in canola.