Variations in relative humidity modulate <Emphasis Type="Italic">Leptosphaeria</Emphasis> spp. pathogenicity and interfere with canola mechanisms of defence

Blackleg (phoma stem canker), caused by Leptosphaeria spp., is an important disease of canola (oilseed rape, Brassica napus). Control strategies rely on the use of resistant cultivars, chemical and disease-reducing cropping practices. In Canada, the pathogen population is represented by L. maculans and L. biglobosa, which are considered to be highly and weakly aggressive, respectively. It is largely admitted that L. biglobosa isolates are not able to cause a significant amount of stem canker and develop on the plant only when it becomes senescent, late in the season. The prevalence of L. maculans over L. biglobosa has been considered to be linked to the low aggressiveness of the latter. However, in this study, we show that L. biglobosa isolates could become highly aggressive in terms of lesion appearance on cotyledons, if the right conditions of temperature and relative humidity (RH) are provided. Percent germination of inoculated pycnidiospores was not affected by the RH regimes tested. This is the first study to show the importance of RH as a factor conditioning the pathogenicity of L. biglobosa isolates on canola cotyledons. Concurrent changes in the host defence mechanisms against L. biglobosa isolates in response to variations in the RH were also investigated. Under high RH, the increase in disease caused by the weakly aggressive isolates coincided with a reduced accumulation of lignin at the early stages of infection.