Importance of Leptosphaeria biglobosa as a cause of phoma stem canker on winter oilseed rape in the UK
Phoma stem canker is a major disease of oilseed rape in the UK, leading to annual yield losses worth more than £100M. The disease is caused by two closely related species, Leptosphaeria maculans and L. biglobosa. L. maculans is generally considered more damaging, causing stem base canker; L. biglobosa is generally less damaging, causing upper stem lesions. Therefore, previous work has mainly focused on L. maculans and there has been little work on L. biglobosa. This work investigated the contribution of L. biglobosa to stem canker epidemics by assessing the amounts of DNA of L. maculans and L. biglobosa in upper stem lesions or stem base cankers on winter oilseed rape cultivars with different types of resistance against L. mac ulans. Diseased upper stem and stem base samples were collected from nine oilseed rape cultivars in a 2011/2012 field experiment at Rothamsted. The presence of L. maculans or L. biglobosa in each stem sample was detected by speciesspecific PCR. The abundance of L. maculans or L. biglobosa in each stem sample was measured by quantification of L. maculans DNA and L. biglobosa DNA using quantitative PCR (qPCR). The amounts of L. biglobosa DNA were greater than those of L. maculans DNA in both upper stem and stem base samples. These results suggest that the severe upper stem lesions and stem base cankers in the 2011/2012 cropping season were mainly caused by L. biglobosa, suggesting that L. biglobosa can sometimes cause considerable yield loss in the UK. There were differences between cultivars in the amounts of L. maculans DNA and L. biglobosa DNA, with the susceptible cultivar Drakkar having more L. maculans DNA than L. biglobosa DNA while resistant cultivars had less L. maculans DNA than L. biglobosa DNA. These results suggest that L. biglobosa can be an important cause of phoma stem canker on oilseed rape in the UK.