Inter- and Intrapathogen Interactions Emanating from Coinfection with Different Fungal and Viral Strains in Canola Cultivars with Differing Host Resistances

Few recent investigations examine coinfection interactions between fungal and viral plant pathogens. Here, we investigated coinfections between <i>Leptosphaeria maculans</i> and turnip mosaic virus (TuMV) in canola (<i>Brassica napus</i>). Different combinations of <i>L. maculans</i> isolate P11 and resistance-breaking isolates <i>L. maculans</i> UWA192 and TuMV 12.1 were inoculated to three cultivars with differing pathogen resistances/susceptibilities. They were inoculated first to entire or half cotyledons 10 to 12 days after emergence and second to opposite entire or half cotyledons on the same day (day 0) or 3 or 7 days afterward. The parameters measured were <i>L. maculans</i> cotyledon disease index (%CDI), and TuMV systemically infected leaf symptom intensity (SI) and virus concentration (VC). Except when both day 0 inoculations were with isolate UWA192, %CDI values were supressed strongly or only weakly when isolates P11 and/or UWA192 were inoculated to plants with <i>L. maculans</i> single-gene resistance (SGR) or polygenic resistance, respectively. However, except when isolate P11 was inoculated first and UWA192 second, these values declined after inoculation day 0 when SGR was absent. TuMV infection suppressed %CDI values, although this decrease was usually smaller following day 0 half cotyledon inoculations. When TuMV temperature-sensitive systemic invasion resistance was present and both inoculations were with TuMV, SI and VC values diminished greatly. However, the extent of this decrease was reduced when second inoculations were with <i>L. maculans</i>. SI and VC values were also smaller when SGR was present and second inoculations were with <i>L. maculans</i>. When <i>L. maculans</i> resistance was lacking, SI and VC values were smaller when second inoculations to entire cotyledons were with <i>L. maculans</i> rather than TuMV. This also occurred after second half cotyledon inoculations with isolate P11 but not isolate UWA192. Therefore, diverse intra- or interpathogen interactions developed depending upon host resistance, isolate combination, cotyledon inoculation approach, and second inoculation timing.