Environmental and management determinants of blackleg crown canker disease (Leptosphaeria maculans) of canola (Brassica napus)

Blackleg disease (Leptosphaeria maculans) impacts canola (Brassica napus) productivity worldwide, but the factors driving severity are not well defined. This study characterised blackleg disease progression from leaf lesion appearance to crown canker development. Leaf lesion severity at four crop growth stages and crown canker severity were empirically associated in crops planted at different times. The biotrophic growth rate of L. maculans in planta was determined at temperatures between 12 and 28 °C. Data from 24 Australian field experiments on spring B. napus cultivars were analysed to relate infection timing relative to crop growth stage and site conditions to crown canker severity at maturity. Despite variations in lesions across seasons and planting times, no significant relationship was found between lesion severity or incidence and crown canker severity. Crown canker severity increased significantly when younger plants were infected. The average in planta hyphal growth rate of L. maculans was 0.13 mm/°C-day. The analyses of biotrophic hyphal growth alongside the timing of infection were used to identify the onset of crown canker development. Simulations showed that thermal time and plant available water accounted for 61% and 36% of the variance in crown canker severity for moderately susceptible and moderately resistant genotypes, respectively. Blackleg crown canker severity is a nuanced interplay between time of infection relative to crop growth stage, genetic resistance, temperature, and moisture. Disease-crop models incorporating these factors could be used to explore how, for example, sowing time, level of host genetic resistance or extreme weather conditions affect disease infection and crop yield.