Colonies moderately rapidly growing. Conidiophores fairly well differentiated from the usually hyaline vegetative hyphae, erect or prostrate, hyaline or pigmented, verticillately branched over most of their length, with successive whorls of conidiogenous cells. Conidiogenous cells slender, gradually tapering, producing conidia successively from single apical conidiogenous loci. Conidia small, hyaline, usually aseptate, in most species aggregating into spherical slimy heads but formed in chains in a few taxa.
Verticillium is clearly an unnatural form-genus, with links both to the Clavicipitales and to the Hypocreales. Many of the species of interest to the industrial mycologist have links with the Clavicipitales, so the form-genus is treated here. However, the type species V. tenerum Nees has links with the Hypocreales, and a few other species of Nectria have since been reported with Verticillium anamorphs, but the anamorph has such a simple structure that it is likely that convergent evolution has taken place.
The latest taxonomic work on Verticillium is the study by Jun et al. (1991), which compared isolates of V. lecanii from insect and fungal hosts. Further information may be found in Domsch et al. (1980), Ellis (1971), Evans & Samson (1986), Gams (1971, 1988) , and Gams & van Zaayen (1982). Verticillium species are widespread in soil, and are pathogens of other fungi, insects and plants. Recently, they have become important in biological control programmes.
Verticillium albo-atrum Reinke & Berthold
Colonies white at first, then turning black in patches. Conidiophores usually only with one order of branching, hyaline though sometimes with a darkened base. Conidiogenous cells in whorls of 2-4, narrow, gradually tapering. Conidia 3.5-10.5 x 2-5µm, ellipsoidal to cylindrical, hyaline, occasionally septate. This species is a serious parasite, causing wilt diseases of many plants. Verticillium dahliae Kleb. is also a serious pathogen, which has dark multicellular resting structures sometimes referred to as microsclerotia, hyaline conidiophores, and slightly smaller conidia.
Verticillium fungicola (Preuss) Hassebrauk
Colonies white to cream, powdery to velvety. Conidiophores erect, with a single order of branching. Conidiogenous cells in whorls of 3-9, narrow, gradually tapering. Conidia 3-8.5 x 1-2.5µm, ellipsoidal to cylindrical, sometimes curved in older colonies, aseptate. Resting spores not produced.
This species, also referred to in some circles as Verticillium malthousei Ware, causes the diseases of commercial mushroom beds known as “dry bubble” and “brown spot”. It can cause serious losses.
Verticillium psalliotae Treschow is similar to V. fungicola, but has less-branched conidiophores which are usually prostrate, purple pigments at the edge of the colonies, Conidiogenous cells usually in groups of 2-3, and conidia which are crescent-shaped in young cultures and cylindrical to ellipsoidal in older colonies. V. lamellicola (F.E.V. Sm.) W. Gams is similar to V. psalliotae, but has conidia which are not curved. It also causes diseases of mushroom beds.
Verticillium lecanii (Zimmermann) Viegas
Col0nies white to pale yellow, cottony. Conidiophores not well-developed, erect or prostrate, with a single order of branching. Conidiogenous cells in whorls of 3-5, narrow, gradually tapering. Conidia variable in size (dependent on the host), ranging between 2.3-10µm in length and 1-2.6µm in width, cylindrical to ellipsoidal.
Interest in Verticillium lecanii as an agent of biological control is considerable. The fungus is pathogenic on a number of economically important insects, and strains are also parasitic on rust and other fungi. Jun et al. identified a number of taxonomic groups within the species aggregate largely based on host preference. The fungus also produces antiviral agents (Bucknall et al., 1973).
Verticillium tenerum Nees
Colonies growing rapidly, velvety, brick-red to orange-brown. Conidiophores erect, stiff, strongly pigmented, often branched several times. Conidogenous cells 12-23 x 2-4µm, flask-shaped. Conidia 3.5-5 x 2-2.5µm, pale reddish brown in mass. Resting structures not produced. This fungus is better-known under the name Verticillium lateritium (Ehrenb.) Rabenh, and the synonym Acrostalagmus cinnabarinus Corda may also be familiar to some. It has been reported to be the anamorph of Nectria inventa Pethybridge, but that fungus is poorly-known, and is certainly not a typical Nectria. The evidence of connexion is based solely on a single early report (Pethybridge, 1919), and the teleomorph has not recently been found. V. tenerum is a common and cosmopolitan soil fungus. It has also been found on paper, cloth, cotton fibres in conveyor belts, and rotting vegetables, and been isolated from milk, cheese, giner, wheat and salami (Domsch et al., 1980; Pitt & Hocking, 1985a).