Reclassification
Stem
canker of oilseed rape is caused by an ascomycetous fungus, whose
anamorph is Phoma lingam. As already shown in detail, huge variability
was found within the anamorph. All phenotypic and genetic gathered data
suggest that Phoma lingam is a species complex.
The classical
definition of a species tells that indivuduals of a species may produce
fertile offspring. A- and B-type NA-1 isolates fullfil these
requirements in a more or less validated manner. Those isolates can be
successfully mated on petri-dishes in a intraspecific manner.
Interspecific crosses never have been shown neither by our or other
groups who established the method of in vitro crossing. This observation
may also, beside the numerous pheno-and genotypic traits, aid to the
conclusion that A-and B-NA1 type isolates belong to different species.
The
differentiation of A- and B-type isolates was consequently suggested by
Shoemaker and Brun (2001), who described morphological differences of
the sexual fruiting bodies (pseudothecia) of these two types. The
authors described a pronounced neck under the ostiolum of pseudothecia
of B-type isolates. Similar results were seen in our group after
studying in-vitro crosses on tooth picks. We attributed these properties
for a long time to artificial conditions. Nevertheless, both
observations clearly describe morphologic differences of the
teleomorphs. Although there are no morphologic traits to dinstinguish
the A and B-type anamorphs, we clearly support the taxonomic
differentiation of these types. Shoemaker and Brun suggested the nomen
novuum Leptosphaeria biglobosa for B-NA1 isolates. The perfect stage of
A-type isolates will still be Leptosphaeria maculans.
Further
differences between A- und B-typ NA-1 isolates became obvious during
studies performed in the context of environmental requirements of these
fungi (Huang et al 2001). It was shown, that ascospores of A-type
isolates preferently germinate from intersitial cells and showed a
pronounced branching of the mycelium. In contrast ascospores of B-type
NA-1 isolates preferently germinanted from terminal cells and showed a
craned and poorly branched mycelium.
Selected publications:
- Huang YK , Toscano-Underwood C , Fitt BDL , Todd AD , West JS , Koopmann B, Balesdent MH , 2001. Effects of temperature on germination and hyphal growth from ascospores of A-group and B-group Leptosphaeria maculans (phoma stem canker of oilseed rape). Annals of Applied Biology 139, 193–207.
- Shoemaker RA , H Brun (2001). The teleomorph of the weakly aggressive segregate of Leptosphaeria maculans. Canadian Journal of Botany 79 pp 412-419.