Metarhizium Sorokīn, Veg. Parasitenk. Mensch Tieren: 268. 1879.

Synonyms: Chamaeleomyces Sigler, J. Clin. Microbiol. 48: 3186. 2010. Metacordyceps G.H. Sung et al., Stud. Mycol. 57: 27. 2007. Nomuraea Maubl., Bull. Soc. Mycol. France 19: 296. 1903.

Type species: Metarhizium anisopliae (Metschn.) Sorokīn, Plant Paras. Man Anim.: 268. 1883.

The concept of the genus Metarhizium was revised to exclude species not belonging in the core Metarhizium clade in Fig. 1 of Mongkolsamrit et al. (2020). It is not so simple to recognise if a specimen belongs to Metarhizium or not, but there are characteristics that one could look for to facilitate classification. The sexual morphs have predominantly cylindrical or clavate stromata, either solitary or multiple and irregularly branched in shades of pale yellow, green to greenish brown or dark purple. Hosts are mostly insect larvae or nymphs buried in the ground ranging from 2–5 cm deep (e.g. M. chaiyaphumense, M. prachinense, M. takense) to 30 cm (e.g. M. kalasinense), rarely on adults buried in the ground (e.g. M. phuwiangense). Rhizoids that connect the host to the stromata emerging from the ground are sometimes covered with green conidia. Perithecial orientation varies from oblique to ordinal arrangement. The majority of the ascospores are filiform and whole with septation while in a few species they dissociate into part-spores, such as M. campsosterni (Zhang et al. 2004) and M. phuwiangense. Different kinds of conidiogenesis can be observed in Metarhizium. All forms are phialidic and the most predominant phialide morphology is the production of cylindrical phialides with short necks as is understood of Metarhizium in the last century. Most importantly, most of the species identified morphologically as Metarhizium possess these characters having a candelabrum-like arrangement of cylindrical phialides forming a compact hymenium. These types of conidiogenesis belong to the M. anisopliae, M. flavoviride species complexes, pathogens of Coleoptera in the Coleoptera clade, and those occurring on small hoppers comprising M. album, M. brasiliense, M. candelabrum, M. cercopidae, M. ellipsoideum and M. huainamdangense. Nomuraea-like and paecilomyces-like phialides are found interspersed in the basal branches of the Metarhizium clade. Species on cicada produce nomuraea-like phialides with two differing classes of conidial shapes, while nomuraea-like species belonging to M. dendromatilis, M. ovaspora, M. prachinense, M. rileyi and M. samlanense form only one kind of conidia. Metarhizium granulomatis, M. phuwiangense, M. reniforme, and M. viride produce paecilomyces-like phialides. All species in Metarhizium form mononematous conidiophores except for M. dendromatilis that is synnematous. We recommend the use of SDAY/4 for studying the micro-morphologies and colony colour in Metarhizium. Growth on both PDA and SDAY/4 are fast as well as the sporulation, but in SDAY/4 sporulation is less compared to PDA making it is easier to observe the shapes of the phialides for a better diagnosis. The use of PDA and OA is suitable only in two species in Metarhizium, M. purpureonigrum and M. purpureum, where using SDAY/4 does not result in sporulation. OA is suitable for slow-growing species in Metarhizium.