Pathogenicity of Leptosphaeria biglobosa to oilseed rape (Brassica napus) cultivars differing in resistance against Leptosphaeria maculans

Field and controlled environment experiments were done to examine the pathogenicity of L. biglobosa on cultivars with good resistance (carrying the Rlm7 gene) or moderate resistance (carrying the Rlm4 gene) against L. maculans, causal agents of phoma stem canker on oilseed rape. Leaves with phoma leaf spots were sampled from established winter oilseed rape field trials at three representative sites in the UK. The proportions of lesions caused by L. maculans and L. biglobosa were scored on cultivars carrying the Rlm7 gene (Excel and Roxet) or the Rlm4 gene (Adriana and Bilbao) and the susceptible cultivar Drakkar. The proportion of L. biglobosa leaf spotting was greater on the cultivars that carrying R genes than on Drakkar. Leaf spotting caused by L. biglobosa was greater in the south than the north of England for all the cultivars. The aggressiveness of L. maculans and L. biglobosa isolates obtained from Excel (LmExc, LbExc) and Drakkar (LmDr, LbDr) was tested on Drakkar, Excel and Roxet. LbExc was more aggressive than LbDr on the three cultivars. There was no difference between the three cultivars in aggressiveness of LbExc and LbDr. LmDr was more aggressive on Roxet than on Excel. There was no difference between Excel and Roxet in aggressiveness of LmExc. LmExc was less aggressive than LmDr on Drakkar. These results indicate that breeding for resistance against L. maculans affects the susceptibility of the cultivars to L. biglobosa.