Effects of host resistance on maturation of pseudothecia of Leptosphaeria maculans and L. biglobosa (cause of Phoma stem canker) in Brassica napus (oilseed rape)

Abstract

The stem canker epidemics are initiated in autumn by air-borne ascospores produced in pseudothecia on infected crop debris from previous crops. This study aimed to investigate the effects of cultivar resistance and environmental conditions (precipitation and temperature) on maturation of pseudothecia on stem debris. In autumn 2011 (2011/2012 growing season), there were no differences in pseudothecia maturation due to dry weather in September 2011 with increases rainfall in December 2011 (average of 0.37mm/h), more pseudothecia matured and more ascospores released was observed. In the third week of January 2012 after the major released of ascospores, the Phoma leaf spots were observed in 6 February 2012. However in 2012 autumn (2012/2013 growing season), when there was more rainfall from September to December (average of 0.28mm/h), there were differences between different cultivars in pseudothecial maturation with the maturation on the most susceptible cv Drakkar faster than on the other cultivars. The first ascospores released in early October 2012 and Phoma leaf spots were observed in 24 October 2012. The early appearance of phoma leaf spotting and high severity of leaf spots in 2012 autumn suggests that there might be severe Phoma stem canker in 2013 summer. Results of this study suggest that weather factors (temperature and rainfall) and host resistance affect the maturation of pseudothecia and the timing of the first major ascospore release. The role of host resistance in pseudothecial maturation requires further investigation.