Comparative epidemiology of Leptosphaeria maculans and L. biglobosa on oilseed rape in the UK

Phoma stem canker is a major disease problem on winter oilseed rape (Brassica napus) in the UK. The disease is caused by two closely related species, Leptosphaeria maculans and L. biglobosa. Previous epidemiology work has focused on L. maculans and there is little information on L. biglobosa. Because L. maculans is often associated with stem base canker while L. biglobosa is often associated with upper stem lesions, L. maculans is considered more damaging than L. biglobosa. Both L. maculans and L. biglobosa are present in the UK. To control the disease effectively, it is necessary to understand the epidemiology of both pathogens. This presentation reports studies on these two related species at different stages of their life cycles. Results show that there are differences between these two species in their biology and epidemiology, which enable them co-exist on the oilseed rape crop. There are differences between L. maculans and L. biglobosa in pseudothecial maturation. Pseudothecia of L. biglobosa mature more slowly than those of L. maculans at 5-10°C; pseudothecia of L. maculans are mainly produced on the surface of stem bases, pseudothecia of L. biglobosa are mainly produced under epidermis of upper stem. The differences in pseudothecial maturation between L. biglobosa and L. maculans result in differences between them in timing of ascospore release and thus in the timing of the resulting phoma leaf spot lesions in autumn. Ultimately, there are differences in timing and severity of stem cankers the following summer. The differences in biology and epidemiology between these two species at key stages of their life cycles, such as initial leaf infection, asexual production, subsequent stem canker development, survival and sexual production on crop debris will be discussed.