Fungi play vital roles in ecosystems as endophytes, pathogens and saprobes. The current estimate of fungal diversity is highly uncertain, ranging from 1.5 to 12 million, but only around 150,000 species have been named and classified to date. Since the introduction of DNA based methods for species identification, the number of newly described taxa has increased from approximately 1000 to around 2000 yearly. This demonstrates the importance of DNA based methods to identify and distinguish species, especially cryptic species. Many novel species from recent studies have been found in historically understudied regions and habitats, but these still represent only a small percentage of the estimated species. In this paper, we examine 16 genera from the top 40 most speciose genera as listed in Species Fungorum as case studies to examine the diversity of taxa in each genus. The genera treated herein are Cercospora, Diaporthe, Meliola, Passalora, Phyllachora, Phyllosticta, Pseudocercospora, Ramularia (ascomycetes) and Cortinarius, Entoloma, Inocybe, Marasmius, Psathyrella, Puccinia, Russula, Uromyces (basidiomycetes). We critically evaluate the number of species in these genera and correlate these numbers with the number of entries in GenBank. We introduce 18 new species Apiospora multiloculata, Candolleomyces thailandensis, Cortinarius acutoproximus, Cortinarius melleoalbus, Cortinarius pacificus, Cortinarius parvoacetosus, Diaporthe guizhouensis, Entoloma pseudosubcorvinum, Inocybe meirensongia, Marasmius albulus, Marasmius obscuroaurantiacus, Meliola camporesii, Phyllachora siamensis, Phyllosticta doitungensis, Picipes yuxiensis, Pseudocercospora vignae, Puccinia maureanui and Russula inornata. We also introduce a new record of Candolleomyces cladii-marisci and Inocybe iringolkavensis. We discuss the genera Colletotrichum and Pleurotus that are speciose, but do not occur in the top 40. We hypothesize whether there might be more species in these genera and discuss why these genera have some of the largest number of species.

Pluteus is a species-rich genus of saprotrophic agaric in the family Pluteaceae and is widely distributed in tropical, subtropical, and temperate areas throughout the world. Some species in this genus are threatened species according to the International Union for Conservation of Nature (IUCN) red list. During investigations of agarics in northern Thailand, four Pluteus taxa were collected. Morphological characteristics and phylogenic analyses were investigated. Two new species, namely P. chandrasikuliae and P. saisamorniae, were introduced. Pluteus chandrasikuliae is characterized by its relatively large basidiomata, an applanate, dark brown scaly pileus with a cutis pileipellis, two types of hymenial cystidia viz. irregular, as well as diverticulate cells and lageniform cells. In accordance with the phylogenetic results, this new species belongs to the Pluteus sect. Celluloderma. Moreover, P. saisamorniae is distinguished by a plano-convex with a broad umbo, dark brown minute squamules pileus, light brown lamellar edges, greyish orange stipe covered with brown granules, subglobose to broadly ellipsoid basidiospores, an abundance of thin- to thick-walled cheilocystidia and pleurocystidia, a trichohymeniderm pileipellis, and brown caulocystidia in clusters. Pluteus saisamorniae is a member of Pluteus sect. Hispidoderma. Additionally, P. losulus and P. septocystidiatus were discovered in Thailand for the first time and they belong to Pluteus sect. Pluteus. Comprehensive descriptions along with illustrations, photographs, phylogenetic trees showing their positions, and a comparison with phenetically similar taxa are provided.

Coral fungi in Phu Hin Rong Kla National Park, Thailand, were surveyed from June to September of 2011 and 2012. A novel species, Phaeoclavulina pseudozippelii is described together with illustrations, photographs, and a comparison with phenetically similar taxa. Its taxonomic position was confirmed through phylogenetic analyses of the large subunit and internal transcribed spacer (ITS1-5.8S-ITS2) ribosomal DNA sequences.

Two new species of Marasmius secton Sicci are described based on field work conducted in Phu Hin Rong Kla National Park, Thailand. Marasmius thailandicus is characterized by small, thin yellowish white basidiomes, ellipsoid basidiospores and dimorphic caulocystidia. Marasmius rongklaensis is distinguished by a conical, velutinous pileus, ellipsoid basidiospores, dimorphic cheilocytidia and pileipellis a hymeniderm with pileosetae. Comprehensive description along with illustrations, photographs, and a comparison with phenetically similar taxa are provided. The taxonomic position has been confirmed by phylogenetic analyses of the internal transcribed spacer (ITS1-5.8S-ITS2) sequences of the ribosomal DNA.

The genus Collybiopsis is known to be found in tropical and temperate regions throughout the world. Currently, 66 species have been validated and assigned to this genus. In this study, we have updated a brief review of the genus Collybiopsis through an overview of species diversity, geographic distribution, and recently published literature. Prior to this study, only four Collybiopsis species have been reported from Thailand. During surveys of macrofungi in northern and southern Thailand from 2018 to 2022, nine Collybiopsis specimens were collected. These nine specimens were identified as belonging to five species, namely C. gibbosa, C. indocta, C. melanopus, C. menehune, and C. nonnulla based on morphological characteristics and molecular data of the internal transcribed spacer (ITS) and large subunit (nrLSU) of nuclear ribosomal DNA sequences. The present study offers the first record of these five Collybiopsis species in Thailand. Detailed morphological descriptions, photographs, illustrations, comparisons with related taxa, and a phylogenetic tree to show the placement of these five species from Thai specimens are provided. Moreover, a new key to the species of Collybiopsis found in Thailand is presented.