The epidemiology of the blackleg pathogen, Leptosphaeria maculans, impacts fungicide resistance management strategies.

With the continual increase in detection of fungicide resistance in plant pathogens, there is an ever-growing need for management strategies to reduce the chance of resistance evolving in the field, and if it does occur, provide strategies to ensure fungicides can still be used effectively. However, the development of effective management strategies requires a thorough understanding of the epidemiology of the pathogen and how this might impact on selection of fungicide-resistant strains within a population. The blackleg pathogen, Leptosphaeria maculans, can infect canola (Brassica napus) throughout the growing season with yield loss associated with crown cankering or infection of the upper stems/branches. The disease cycle consists of infection of the plant early in the growing season, such as at the cotyledon up to the 4- to 6-leaf stage, to induce cankering on cultivars that do not carry effective plant resistance (or R) genes. Infections later in the season lead to infections of the upper stems/branches. Hence, fungicides can be applied up to four times during the season to control disease symptoms. However, it is currently unknown whether fungicide applications late in the growth of the plant will lead to selection of resistance in the population, as the pathogen would require enough time to colonise the plant and undergo sexual reproduction. Here we show that infections as late as the 30% bloom stage do indeed have sufficient time for the pathogen to grow down and colonise the base of the stem which provides the substrate to persist for the following season and undergo sexual reproduction, i.e., the next generation of inoculum. This has implications for fungicide management strategies as it means any fungicide application throughout the growing season can lead to selection of resistance.