Alterations to fungal and viral pathogen infection in coinfected plant cultivars with diverse single-pathogen host resistances

To examine how pathogen coinfection influences the efficacy of single-pathogen host resistances, we investigated the interactions between the fungus (Leptosphaeria maculans) and turnip mosaic virus in canola (Brassica napus). A resistance-breaking (RB) virus strain was inoculated onto the cotyledons of three cultivars. These cultivars featured distinct host resistance profiles: fungal single dominant gene resistance (SDGR) and polygenic resistance (PGR) along with viral temperature sensitive systemic invasion resistance (TSSIR) as well as a cultivar without fungus resistance serving as a control. Subsequently, systemically virus-infected true leaves of varying maturity were inoculated with RB or non-resistance-breaking (non-RB, i.e., wild-type) fungal strains both individually and in combination. This allowed assessment of the impacts of already established virus infection upon fungal disease severity and the reciprocal effects of subsequent fungus infection upon virus concentration (VC). In the youngest leaves, high VC values corresponded with the complete absence of fungal disease, whereas the lowest VC values in the oldest leaves were associated with maximum fungal disease severity. When the RB fungal strain was present alone, it overcame SDGR more effectively when the virus was absent. Conversely, presence of the RB fungal strain in SDGR-carrying plants decreased the VC values, regardless of the presence or absence of the non-RB fungus strain. In plants possessing both fungal PGR and viral TSSIR, regardless of fungal strain, fungal infection only occurred in the lower leaves of older plants which developed consistently low VC values like those in plants inoculated with the virus alone. In the fully fungus-susceptible cultivar, fungal disease levels were consistently lower in virus-infected than virus-mock-inoculated plants. However, in coinfected plants of this cultivar, VC values increased or decreased depending upon whether the fungal RB or non-RB strain was inoculated first. By examining virus-fungus pathogen interactions across a range of host resistance backgrounds, leaf developmental stages, and plant ages, this research provides fundamental insights into the complex dynamics of agricultural plant pathogen coinfections.