Previous Viral Infection Triggers Fungal Resistance Breakdown in Host Plants

Effects of viral and fungal coinfection upon host plant resistance effectiveness were studied using three canola (Brassica napus) cultivars with differing pathogen resistance characteristics, resistance-breaking (RB) and non-resistance-breaking (non-RB) Leptosphaeria maculans fungus strains, and turnip mosaic virus (TuMV) RB strain. These cultivars carried fungal single dominant gene resistance (SDGR), both fungal polygenic resistance and viral temperature-sensitive systemic invasion resistance (PGR + TSSIR), or no fungal resistance. Cotyledon viral RB strain inoculation was followed by two fungal spray inoculations onto systemically virus-infected plant foliage using different non-RB and RB strain combinations inoculated on the same day or 3 days apart. With PGR, fungal non-RB strain alone inoculation onto virus-infected foliage resulted in resistance breakdown from no infection to severe symptoms. For other fungal strain combinations, resistance breakdown was up to 7.4-fold with SDGR and 3.7-fold with PGR. TSSIR breakdown was absent following fungal inoculations onto virus-infected foliage. This study provides a model system that can not only improve understanding of virus–fungus pathogen interactions, but also identify where diminished single-pathogen host resistance effectiveness warrants reconsideration in coinfection scenarios that increase the likelihood of adverse economic impacts and where new cultivars with more effective fungal and viral pathogen host resistance are required.