Pathogen Fungi

Mycosphaerella is a genus of ascomycota. With more than 10,000 species, it is the largest genus of plant pathogen fungi.
The following introduction about the fungal genus Mycosphaerella is copied (with permission) from the dissertation of W. Quaedvlieg (named: Re-evaluating Mycosphaerella and allied genera).
Species belonging to the fungal genus Mycosphaerella (1884) (Capnodiales, Dothideomycetes) have evolved as endophytes, saprotrophs and symbionts, but mostly Mycosphaerella species are foliicolous plant pathogens which are the cause of significant economical losses in both temperate and tropical crops worldwide. The generic concept of Mycosphaerella is based on the type species of the genus, M. punctiformis, which was introduced 130 years ago in order to describe small loculoascomycetes with few distinct morphological traits. Species belonging to Mycosphaerella were characterised as having pseudothecial ascomata that can be immersed or superficial, embedded in host tissue or erumpent, having ostiolar periphyses, but lacking interascal tissue at maturity. Ascospores are hyaline, but in some cases slightly pigmented and predominantly 1-septate, although taxa with 3-septate ascospores have been recorded. This description appears to be quite distinctive, but is in fact very broad and actually lead to 120 years of confusion in which the generic name Mycosphaerella was being used as a dumping ground for small loculoascomycetes with few distinct morphological traits. In the 19th and 20th centuries, thousands of species and infrasprecific taxa were described in the genus Sphaerella, only to have the entire genus and about 1000 additional species redescribed into the genus Mycosphaerella at the end of the 20th century.
The identification of Mycosphaerella species by morphological means is extremely difficult as these taxa produce very small fruiting structures with highly conserved morphologies, tending to grow and sporulate poorly in culture and for over 120 years, identification was based on morphology alone. These identification difficulties are amplified by the fact that up to six different species can inhabit the same lesion as either a primary or secondary pathogen, making even host-specific species difficult to identify. The introduction of affordable sequencing technology during the first decade of the 21st century allowed for much more accurate species delimitation and phylogenetic elucidation, leading to the conclusion that the broad taxonomic description of the genus Mycosphaerella and a lack of clear morphological features led to many Mycosphaerella and mycosphaerella-like species being misidentified. Because the classic taxonomic description of Mycosphaerella is broad and includes so many mycosphaerella-like species, the traditional generic concept of Mycosphaerella will hereafter be referred to as Mycosphaerella sensu lato (s. lat.) in order to avoid confusion.