Flessa, Fabienne (2022). Contributions towards a better understanding of sooty mould mycobiomes. (Dissertation, Universität Bayreuth, Fakultät für Biologie, Chemie und Geowissenschaften)

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Sooty mould communities are darkly pigmented planar mycelial mats consisting of several fun-gal species that colonise surfaces of a variety of host plants without penetrating the tissue. Most of the research on sooty mould (SM) biomes has been conducted in the tropics and subtropics. However, the comparison with communities from the temperate and alpine regions has been lacking. Therefore, in this PhD project, SM mycobiomes from temperate and alpine regions were compared with those from a tropical region. They are mainly formed by the Dothideomycetes. In subtropical and tropical regions there is also a high proportion of Sordari-omycetes. At the order level, the Capnodiales predominate, with Pleosporales, Dothideales and Tremellales also regularly occurring with somewhat lower relative abundance. At family and genus level, the spectrum is much more diverse. Diversity among sites varies significantly when sites also differ in terms of their geographic location, climatic conditions and, above all, in their host plant species. The SM mycobiomes of the investigated habitats are composed of regularly occurring, characteristic species or OTUs as well as of associated, facultative or sporadically occurring taxa. A close correlation between honeydew and SM incidence has been postulated in the literature. Since SM mycobiomes also occur on plants without this nutritional source, the influence of different nutrition sources (leaching products from plant tissues, plant secretions from glands, and honeydew) on diversity was investigated in this PhD project for the first time. If only leaching substances are available as a nutritional source, ubiquitous fungi predominate, whereas plant secretion products lead to more specific fungi communities on leaves. Composition in SM mycobiomes from two tropical sites differs significantly between host plants with the presence of sap-feeding insect (SFI) and those without, although the groups overlap considerably. On leaves of evergreen plants with ubiquitous dominant fungi within a site, they do not differ significantly between current and previous year's leaves when the nutrient source does not change. However, they differ significantly when the nutritional source changes between young and old perennial leaves during leaf senescence. Contrary to the previously postulated host independence, a significant correlation was found between the taxonomic affiliation at genus, family and order level of the host plant and the composition of the SM community. Succession within an SM mycobiome with increasing leaf age had not been studied before this project. A study on host plants with annual and perennial leaves exposed to the same conditions during the growing season, but with only the perennial group overwintered in a green-house, allowed investigation of succession stages. The predominant fungi in the initial colonisation communities differed in both groups. SM biofilms on host plants with ubiquitous fungi, separated from the spore pool of plants in the field during winter by hibernation in the glass-house, differed significantly in the composition of SM biofilms on young perennial leaves from the approximately equally old leaves of deciduous plants in the field. The spore pool in winter or at the beginning of leaf sprouting has an important influence on the initial colonisation of fresh leaves. It was also observed on an alpine plant species in its natural habitat that the diversity differs significantly between leaves of different ages. The relative amount of darkly pigmented fungi in the core SM mycobiome is considerably higher in colder sites than in warmer locations. This result supports for the first time the theory of thermal melanism in SM mycobiomes. Moreover, epiphytic SM fungi are darkly pigmented in a higher proportion than endophytic fungi of the same host plant (Rhododendron ferrugineum). Among the fungi of the SM mycobiome, there are those that are facultatively pigmented, and others whose pigmentation is not variable on different culture media, having either unpigmented or permanently pigmented hyphae or spores. In this project, the diversity of the SM mycobiome was compared with the endophytic mycobiome of one host plant for the first time. Both mycobiomes differ in terms of composition and how they are affected by various factors such as leaf age and geographic location. While the SM biofilms are more affected by leaf senescence and differ significantly between current year’s and those of the previous year, the endophytic fungi are markedly shaped by the altitudinal and the geographical region. The key factors leading to infestation of plants with SM biofilms in the temperate region were identified for the first time in this project, and a predictive model was developed. Prolonged infestation with SFIs (≥ 4 observation dates), horizontal leaf position and sunken leaf veins have the strongest effect and lead to a 3.7-fold higher risk of SM occurrence.

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