Potential for using host resistance to reduce production of pseudothecia and ascospores of Leptosphaeria maculans, the blackleg pathogen of Brassica napus

Pseudothecial density of the blackleg fungus Leptosphaeria maculans and discharge of ascospores was measured from stubble of a range of Brassica species, including Brassica napus (canola) cultivars, with a range of blackleg resistance. Since ascospores are the primary inoculum, these parameters reflect inoculum potential for blackleg. Stubble from a representative line of each of B. carinata, B. nigra, Sinapis alba and B. napus cv. Surpass 400 (incorporates blackleg resistance from B. rapa ssp. sylvestris) had lower pseudothecial density and discharged fewer ascospores than stubble of other B. napus cultivars (Karoo, Oscar, Emblem, Dunkeld and Columbus). These latter B. napus cultivars and a representative B. juncea line had higher pseudothecial densities and discharged higher numbers of ascospores. If this trait of low blackleg inoculum from stubble could be introgressed into commercial canola cultivars, blackleg disease severity could be substantially reduced, resulting in higher and more stable canola yields. However, the trait of reduced ascospore discharge may not be stable, as demonstrated by the B. rapa ssp. sylvestris-derived resistance already being overcome by the blackleg fungus in Australia.