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Submitted to: Mycological Research
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/30/2002 Publication Date: N/A Citation: N/A Interpretive Summary: The asexual fungus Rhynchosporium secalis causes an economically important, foliar disease of barley, rye, and other grasses known as leaf blotch or scald. The genus Rhynchosporium has a very simple structure with asexual spores produced directly from fungal tissue without stalks or other structures. This lack of morphological features makes it extremely difficult to classify the genus Rhynchosporium. Because no sexual stage has been associated with R. secalis, essentially nothing is known about its evolutionary relationships. To identify other fungi that might be related to R. secalis, two types of ribosomal DNA were sequenced and compared to those in databases. One type was not useful for elucidating the evolutionary relationships of R. secalis. However, analyses of internal transcribed spacer (ITS) sequences revealed very close relationships among R. secalis and fungi that are pathogens of wheat and mustard (Brassica spp.) crops. These relationships were unexpected, but indicate that the sexual stage of R. secalis, if it exists, is predicted to be a small cup- shaped structure produced directly on dead, infected host tissue from 1-10 months after harvest. The ITS analysis also indicated that higher-level classifications of these fungi need to be revised. This information will be of great interest to barley pathologists worldwide who have long wondered what the sexual stage of R. secalis might be like and to which other species it might be related. Mycologists and plant pathologists can use this information to guide the search for the sexual stage. Knowledge of the sexual stage and of which other fungi are closely related can be used by plant pathologists to develop more effective disease management strategies for this economically important pathogen. Technical Abstract: Rhynchosporium secalis causes an economically important, foliar disease of barley, rye, and other grasses known as leaf blotch or scald. The genus Rhynchosporium produces conidia from vegetative hyphae directly, without conidiophores or other structures. This lack of morphological features makes it extremely difficult to classify the genus Rhynchosporium. Because eno teleomorph has been associated with R. secalis, essentially nothing is known about its phylogenetic relationships. To identify other fungi that might be related to R. secalis, the 18S ribosomal RNA gene and the internal transcribed spacer (ITS) region (ITS1, 5.8S rRNA gene, and ITS2) were sequenced and compared to those in databases. Among 31 18S sequences downloaded from GenBank, the closest relatives to R. secalis were two species of Graphium (Hyphomycetes) and two other accessions that were not identified to genus or species. Therefore, 18S sequences were not useful for elucidating the phylogenetic relationships of R. secalis. However, analyses of 76 ITS sequences revealed very close relationships among R. secalis and species of the discomycete genera Tapesia and Pyrenopeziza, as well as several anamorphic fungi including soybean and Adzuki-bean isolates of Phialophora gregata. These species all clustered together with 100% bootstrap support. On the basis of these results, the teleomorph of R. secalis, if it exists, is predicted to be a small apothecium produced directly on dead, infected host tissue. The ITS analysis also indicated that higher-level classifications within the Discomycetes need to be revised, and that Tapesia and Pyrenopeziza probably do not belong in the family Dermateaceae. |
