• Yongju Huang - Rothamsted Research, Harpenden and University of Hertfordshire, Hatfield

• Bruce Fitt and Avice Hall - University of Hertfordshire, Hatfield

Comparative biology and epidemiology of A-group and B-group Leptosphaeria maculans on winter oilseed rape Stem canker (blackleg), caused by Leptosphaeria maculans, is a serious disease of oilseed rape in the UK, which causes losses of about £40 M per season. Populations of L. maculans can be divided into A-group and B-group. Previous epidemiology work has mainly been on the A-group, with little information about the B-group and has mainly used conidia of L. maculans, rather than the ascospores, which play the major role in the epidemiology of the disease. The aim of this study was to compare the biology and epidemiology of A-group and B-group L. maculans.

Ascospores of both A-group and B-group Leptosphaeria maculans germinated at temperatures from 5 to 20ºC on distilled water agar or oilseed rape leaves. Both percentage of A-group/B-group ascospores that had germinated after 24 h of incubation and germ tube length increased with increasing temperature from 5 to 20ºC. Under the same conditions, B-group ascospores produced longer, thinner germ tubes, and fewer (3.1) germ tubes per ascospore than A-group ascospores (3.8). Germ tubes originated predominantly from interstitial cells of A-group ascospores and terminal cells of B-group ascospores. After 24 h of incubation, the B-group hyphae grew in almost straight lines, whilst the A-group hyphae grew tortuously.

Hyphae from ascospores of both A-group and B-group L. maculans penetrated leaf tissues predominantly through stomata, at temperatures from 5 to 20ºC, and appressorium-like structures were formed by A-group but not B-group hyphae. The percentage of germinated ascospores that penetrated stomata increased with increasing temperature from 5 to 20ºC and was greater for A-group than for B-group L. maculans after 40 h of incubation.

Mature pseudothecia of both A-group and B-group L. maculans developed on oilseed rape debris at 5-20°C, but maturation was faster at 15-20°C than at 5-10°C. At 5-10°C, A-group pseudothecia matured faster than B-group pseudothecia but there was no difference between the two groups at 15-20°C. A-group pseudothecia were mainly produced on the surface of stem bases and B-group pseudothecia were mainly produced under the epidermis of upper stems. The two groups cannot be reliably differentiated by the size of ascospores or pseudothecia.

Seasonal differences in the timing of the first maturation of ascospores were related to differences in summer rainfall. The first phoma leaf spotting appeared in untreated plots of winter oilseed rape 8-15 days after the first release of ascospores. Seasonal differences in stem canker severity were related to differences in the timing of the first phoma leaf spotting and severity of leaf spots in autumn/winter.

Ascospores were produced on stem base debris buried in sand for 2, 4, 6, 8, 10 or 12 months after subsequent incubation on the surface of the ground for 2 months, but not on buried upper stem debris. Only A-group L. maculans was isolated from ascospores produced on the buried debris; no B-group was isolated. Both A-group and B-group L. maculans survived longer on unburied debris than on buried debris. A-group ascospores survived longer than B-group ascospores under dry conditions in darkness at 5-20ºC.

Proportions of A-group leaf lesions (70%) throughout autumn/spring and A-group isolates obtained from cortex (71%) and pith (91%) of the crown (stem base) before harvest were greater than those of B-group. However, isolates obtained from cortex of upper stems were predominantly B-group (61%). Ascospores produced from pseudothecia on stem base debris were mainly A-group (95%), while ascospores produced from pseudothecia on upper stem debris were mainly B-group (74%).