Ferns are one of the most significant plant groupings that comprise a substantial proportion of the plant flora due to the fact of their great diversity, especially in tropical areas. The biodiversity of fungi associated with ferns and fern-like hosts has also received little attention in studies. Plant samples were collected from diseased and dead plants of ten fern or fern-like species from Chiang Rai in northern Thailand. Forty-one isolates were selected from the obtained isolates for molecular and morphological analysis, with a focus on pathogenic fungal genera and consideration of the diversity in host and geographical location. Twenty-six species belonging to seven genera (Colletotrichum, Curvularia, Diaporthe, Fusarium, Lasiodiplodia, Neopestalotiopsis, and Pestalotiopsis) in six families were identified. Thirty new hosts, eight new geographical hosts, and one new species, Colletotrichum polypodialium, are described. Nepestalotiopsis phangngaensis, N. pandancola, Diaporthe tectonendophytica, D. chiangraiensis, and D. delonicis were isolated for the first time from leaf spots. Additionally, new reservoirs and geographical locations for species previously isolated from leaf spots or whose pathogenicity was established were found. However, more studies are necessary to prove the pathogenicity of the fungi isolated from the leaf spots and to identify the fungi associated with other species of ferns.
