Neoscytalidium dimidiatum

In 1933, British mycologist Dr. Rolland Marshall Nattrass described an arthroconidial asexual fungus that he named H. toruloidea that was responsible for causing die-back disease of plum, apricot and apple trees in Egypt.[3] At the time, he recognized that single spore cultures of the fungus yielded two "forms" in culture – a mycelial form resembling members of the genus Torula that produced fragmenting chains of arthroconidia, and a pycnidial form characterized by the production of greenish, ellipsoidal spores that oozed from tiny sacs.[7] The name H. toruloidea applied to the latter pycnidial form.[7] Others likened the Torula form to Torula dimidiata described by Otto Penzig in 1882. Despite that the fungus was known by this name for over 50 years by one or the other of these names, increased scrutiny of the species and its close relatives using molecular genetic methods spawned significant controversy in its taxonomy and naming.[3][5]

In 1989 Sutton and Dyko created the genus Nattrassia to accommodate H. toruloidea and applied the name Scytalidium dimidiatum to the mycelial synanamorph.[3] They also included in the new genus Nattrassia a fungus described by father-son mycologists Paul and Hans Sydow as Dothiorella mangiferae, which became Nattrassia mangiferae, thought to be very closely related to and perhaps indistinguishable from Nattrass's original pycnidial form. Farr and coworkers recognized that both states were asexual forms affiliated with the genus Fusicoccum, an anamorph of the plant pathogenic ascomycete genus, Botryosphaeria.[3] They proposed the transfer of Scytalidium dimidiatum to the genus Fusicoccum as F. dimidiatum.[3] A reappraisal of the family Botryosphaeriaceae by Crous and coworkers in 2006 concluded that the genus Fusicoccum was polyphyletic, and they created a new genus, Neoscytalidium to accommodate Nattrass's fungus. Separately they erected the genus Neofusicoccum to accommodate Nattrassia mangiferae.[3][5] Crous and colleagues concluded it inappropriate to collapse the entirety of Scytalidium with Fusicoccum because they demonstrated N. dimidiatum to be phylogenetically distinct from Neofusicoccum mangiferae; thus, they interpreted N. dimidiatum to be the correct name for Nattrass's fungus.[3]

UAMH 10614 Neoscytalidium dimidiatum on CER agar incubated at 35 C for 29 days.

This filamentous fungus[3] produces sinuous and irregular hyphae and is characterized by rapidly growing colonies that are deeply tufted with dense, darkly coloured, ropy aerial mycelium.[5][8][9] Cultures are rapidly growing, initially light in colour and becoming dark brown and then black with age.[8] Both arthroconidia and pycnidia may be produced in the same culture.[5] Cultures of the fungus derived from human skin tend to be black in colouration.[5]

Neoscytalidium dimidiatum is mainly found in tropical to subtropical environments,[10] such as in South America, Southeast Asia, India and Africa.[3][5] In addition to these regions, this fungus is endemic to parts of west and central Africa, the Caribbean and Asian,[5] but more cases are being seen in temperate countries, possibly as a consequence of immigration from tropical regions.[11] The fungus occurs in nature in soil and on decaying wood.[10]