Understanding symptomless growth of Leptosphaeria maculans in Brassica napus (oilseed rape) to manage phoma stem canker

摘要

Two winter oilseed rape cultivars differing in quantitative resistance to L. maculans (Darmor resistant, Eurol susceptible)were used to investigate relationships between symptomless growth of L. maculans in autumn (October/November) and severity of stem canker at harvest (July). In field experiments at Rothamsted, Darmor had at least as many phoma leaf lesions as Eurol at the initial leaf infection stage in autumn. However, at harvest in summer Darmor had less severe cankers than Eurol in both the 2003/04 and 2004/05 growing seasons. To visualise symptomless growth of L. maculans from leaf lesions to stems, leaves of Darmor and Eurol were inoculated with a GFP-expressing isolate in controlled conditions. No significant difference between Darmor and Eurol was observed in growth rate ofL. maculans in petioles, and stem cankers developed on both cultivars. However,L. maculans was often confined to the stem cortex of Darmor but penetrated to the stem pith of Eurol. These results indicate that understanding symptomless growth of L. maculans along leaf petioles and within stem tissues is crucial for understanding quantitative resistance to L. maculans.