Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/21/1999
Publication Date: N/A
Interpretive Summary: Septoria passerinii is the causal agent of speckled leaf blotch of barley. This disease occurs in many barley production areas and can be severe when conditions are favorable to disease development. The fungus was first described in 1879 and has been known since then only in an asexual state; a sexual stage is assumed, but has not been identified. In culture, S. passerinii looks very similar to the septoria tritici blotch pathogen of wheat, Mycosphaerella graminicola. Both fungi have similar asexual spores and colony types on agar media. The purpose of this research was to test the hypothesis that S. passerinii and M. graminicola are closely related. To do this, nucleotide sequences of the internal transcribed spacer (ITS) regions of the ribosomal DNA were obtained for both species and for a large number of other fungi. Comparisons of the ITS regions of S. passerinii and M. graminicola showed that they differed by only 10 out of 571 nucleotides and, therefore, these species must be very closely related. Further comparisons with the ITS sequences of a large number of other fungi showed that S. passerinii and M. graminicola grouped together with other species of Mycosphaerella. This answers a question first asked 120 years ago: the unknown sexual stage of S. passerinii must be Mycosphaerella. This information will be useful to mycologists to better understand the taxonomy of the asexual form genus Septoria, and will help understand how new species of plant pathogens evolve in nature. The information also may be useful to plant pathologists in controlling the disease, because control measures designed for the wheat pathogen M. graminicola also may be effective against its close relative on barley.
Technical Abstract: Septoria passerinii is known only from its anamorphic Septoria state; no teleomorph has been identified. In culture, S. passerinii looks very similar to Mycosphaerella graminicola from wheat. Comparisons of the nucleotide sequences of the internal transcribed spacer (ITS) regions of both species and of many other fungi in the Dothideales revealed that the ITS regions of S. passerinii and M. graminicola differed by only 10 bases. Therefore, these species are very closely related. Phylogenetic analysis showed that S. passerinii and M. graminicola grouped together within a large cluster of Mycosphaerella species. Thus, S. passerinii almost certainly has a Mycosphaerella teleomorph.