Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 2001
Publication Date: N/A
Interpretive Summary: Speckled leaf blotch, caused by the fungus Septoria passerinii, is an economically important disease of barley in the northern United States, Canada and Europe. Although S. passerinii has been known to science for more than 120 years, only an asexual spore type has been found. Whether or not S. passerinii has a sexual stage, and its evolutionary relationships to oother fungi, are not known. On the basis of asexual spore shape and growt characteristics, S. passerinii appears similar to the septoria tritici blotch (STB) pathogen of wheat, which has a sexual spore stage in the genus Mycosphaerella. Therefore, S. passerinii also could have a Mycosphaerella sexual stage. To test this hypothesis, a small region of the ribosomal DNA of S. passerinii was sequenced and compared to those of many other fungi. These analyses identified two forms of S. passerinii, one from cultivated barley and the other from foxtail barley. Comparisons of these ribosomal DNA sequences to those of many other fungi revealed that both forms of S. passerinii were closely related to the STB pathogen of wheat and had other close relatives within the genus Mycosphaerella. Therefore, the sexual stage of S. passerinii, if it exists, must be a Mycosphaerella. Furthermore, the isolate from foxtail barley probably represents a new, undescribed species that is separated from the S. passerinii of cultivated barley by host specificity. Knowledge of the evolutionary relationships and probable sexual stage of S. passerinii will help plant pathologists understand the epidemiology of speckled leaf blotch. This information also may help fungicide companies and plant pathologists target disease management strategies, because methods and products that work on one fungus also may be effective against close relatives.
Technical Abstract: Septoria passerinii causes speckled leaf blotch of barley. Although this fungus has been known to science for more than 120 years, no teleomorph has been found and its phylogenetic relationships are not known. On the basis of spore morphology and growth characteristics in culture, S. passerinii appears similar to the wheat pathogen Mycosphaerella graminicola. To test the hypothesis that S. passerinii and M. graminicola are closely related, the internal transcribed spacer (ITS) region of both species was sequenced and compared to those of many other fungi in the Loculoascomycetes. Two ITS sequences were identified among seven isolates of S. passerinii. Six isolates from cultivated barley (Hordeum vulgare) shared the same 569 base-air sequence. One isolate from the wild barley H. jubatum had a 569 base-pair sequence that differed from the first sequence by seven nucleotid The ITS sequences of both forms of S. passerinii differed from that of M. graminicola by 10 nucleotides. These differences were large enough that all three ITS sequences could be distinguished by restriction enzyme digestion of polymerase chain reaction-amplified products. Cluster analyses revealed that both forms of S. passerinii were closely related to M. graminicola and were contained within a large, monophyletic group of Mycosphaerella species. Therefore, the teleomorph of S. passerinii, if it exists, must be a Mycosphaerella. Furthermore, the isolate from H. jubatum probably represents a new, undescribed species that is separated from S. passerinii by host specificity.