Influence of crop and stubble management practices on blackleg disease of canola (Brassica napus) in Australia

Abstract

Context: Australian canola (Brassica napus) growers have shifted towards earlier sowing and flowering times in addition to the widespread adoption of stubble conservation practices. Aims: This study determined the consequent impact of these crop and stubble management practices on blackleg crown canker and upper canopy infection caused by the stubble-borne pathogen Leptosphaeria maculans. Methods: Ascospore maturation and release were measured across seasons in commercial canola crops in south-eastern Australia on standing/erect and lying/knocked down stubble. We determined the severity of blackleg crown canker and upper canopy infection in crops with early or late sowing and flowering windows exposed to different stubble treatments. Key results: Ascospore maturation was delayed and ascospore release was 10-fold lower in standing stubble in the first season post-harvest. In the second season when standing stubble was knocked down, total ascospore production was up to 4-fold higher than in the first season. Stubble orientation had no effect on blackleg disease severity. The main driver of infection at vulnerable crop stages was determined by sowing and flowering times such that disease severity was reduced by 35-76% and 98% in early sown and later flowering crops, respectively. Conclusions: Canola grown in short rotations or intensively within a landscape will be exposed to high blackleg inoculum loads from retained stubble. Revision of current blackleg recommendations are required to account for modern crop management practices. Implications: Innovative blackleg control strategies are urgently required to safeguard host genetic resistance and fungicide efficacy in modern farming systems to ensure sustainable intensification of canola production in Australia and globally.