Fungal diversity in Namibian Stipagrostis ‘fairy circles’ including the description of new Curvularia species

In the Namib desert a poorly understood phenomenon is found, known as “fairy circles”. These are almost circular, baren patches of land that are surrounded by a margin of flourishing Stipagrostis ciliata (Poaceae). Over the past 50 years, these circles have received considerable attention, however, no consensus has been reached regarding their maintenance or cause. One of the more recent hypotheses, is that they could result from the activity of microbial phytopathogens. In this study, we provide a comprehensive review of literature pertaining to the various hypotheses surrounding their formation and maintenance, their life-cycle, as well as their distribution. In this study, we surveyed the fungal diversity associated with S. ciliata tissues collected from fairy circles located at two sites in the Namib. For each fairy circle, five samples were collected from the almost barren inside, five from the lush margin, and five from the matrix vegetation that occurs between circles. Plant tissues were surface disinfested and plated onto Fusarium Selective Media (FSM), Malt Extract Agar (MEA) and Dichloran-Glycerol (DG18), supplemented with chloramphenicol and streptomycin. A total of 487 strains, representing 54 genera and 114 species were isolated and identified based on DNA sequence data of the beta-tubulin for Penicillium, calmodulin for Aspergillus, glyceraldehyde-3-phosphate dehydrogenase for Bipolaris, Curvularia, Exserohilum and other Pleosporales, internal transcribed spacer rDNA region (ITS) and/or 28S large subunit rDNA (LSU) for morphologically unidentified genera, RNA polymerase II second largest subunit for Didymellaceae, and/or translation elongation factor 1-alpha for Trichoderma and Fusarium. The most prevalent genera identified included Curvularia (n = 73), Fusarium (n = 73), and Monosporascus (n = 41), with Curvularia including the largest number of species (n = 12). Four Curvularia species were considered novel based on comparisons with known species using both multi-locus sequence and morphological comparisons. Descriptions of these species are subsequently provided. Overall, this study indicates the rich fungal diversity present in the Namib desert that could play a role in the biology of the curious phenomenon, which certainly requires more explorations in future.