Identification of ascospores of phoma stem canker causal pathogens (Leptosphaeria maculans and L. biglobosa) on stems of winter oilseed rape cultivars with different resistances

Phoma stem canker is a damaging disease on oilseed rape (Brassica napus) in the UK, causing yield losses worth £100M p.a., despite use of fungicides costing £20M p.a. (Fitt et al., 2008; Stonard et al., 2010). The disease is caused by two closely related species L. maculans and L. biglobosa, which co-exist on their oilseed rape host (Fitt et al., 2006). Epidemics of phoma stem canker are initiated by ascospores of L. maculans and L. biglobosa released from stem debris from the previous season (Huang et al., 2005). The proportions of ascospores of L. maculans and L. biglobosa produced on stem debris of different cultivars will affect the severity of phoma leaf spotting in autumn and subsequent phoma stem canker in the following summer. Therefore, information on proportions of L. maculans and L. biglobosa ascospores in the autumn is important for effective control of these two pathogens by fungicides and cultivar resistance. This project aims to investigate effects of cultivar resistance on production of ascospores of Leptosphaeria maculans and L. biglobosa for effective control of phoma stem canker on oilseed rape.

Stems of different cultivars with phoma stem canker symptoms were incubated outdoors to induce pseudothecial development. The project will assess pseudothecial densities on those stems of different cultivars, isolate ascospores from these stems and confirm whether they are L. maculans or L. biglobosa. If the ascospores are L. maculans, they will be tested to investigate if they are virulent or avirulent against Rlm7. Results of this project will provide new information about differences between different cultivars in their ability to produce ascospores of L. maculans and L. biglobosa for infection of crops in the next season.