Paecilomyces is a form-genus with connexions to various teleomorph genera, belonging to the Clavicipitales and Eurotiales. Paecilomyces section Paecilomyces (the type subgroup of the form-genus which contains the original described species) has links to the Eurotiales. Paecilomyces section Isarioidea Samson on the other hand, belongs to the Clavicipitales, and the following description refers to this group. Several species in this group are important as agents of biological control. See Samson (1974) and Domsch et al. (1980) for comprehensive descriptions, keys to the genus, and further information.

Colonies often brightly coloured. Hyphae hyaline to slightly pigmented. Conidiophores with several verticillate branches, sometimes aggregated into synnemata. Conidiogenous cells in whorls of 2-7 at the apex of branches and often inserted laterally also, flask-shaped, usually rather less elongated than in Paecilomyces section Paecilomyces, producing conidia successively from apical loci. Conidia formed in chains with the oldest one at the apex, smooth or ornamented, hyaline or brightly coloured, rarely septate. Resting-spores present in some species.

Paecilomyces carneus (Duche & Heim) A.H.S. Brown & G. Smith

Colonies growing slowly, matted, sometimes powdery, at first pure white but becoming pale pink with a green reverse. Conidiophores hardly differentiated, with a few usually verticillate branches. Conidiogenous cells in whorls of 2-4, 9-18 x 1.5-2.5µm, flask-shaped with a distinct neck. Conidia 3-4 x 2-2.5µm, ± globose to ellipsoidal, roughened with tiny spines.

Paecilomyces carneus is found on a wide range of substrates, and especially from soil. It is sometimes isolated from insects, though it appears not to be strongly pathogenic. Several metabolites of the antibiotic group cephalosporins are produced by some isolates (Kitano et al., 1974).

Paecilomyces farinosus (Holm) A.H.S. Brown & G. Smith

Colonies growing quite rapidly, felty or powdery, at first white but often eventually becoming yellowish. Conidiophores verticillately or irregularly branched, sometimes aggregating into synnemata. Conidiogenous cells 5-15 x 1.5-2.5µm, the basal part swollen, tapering towards the apex. Conidia 2-3 x l-2µm, ellipsoidal to fusiform, smooth-walled, hyaline. Resting-spores not produced.

Paecilomyces farinosus is a well-known insect pathogen, and there has been interest in its use as an agent of biological control. The species is polyphagous, but isolates are commonly restricted in their host preferences. P. farinosus is also commonly isolated from soil.
Domsch et al. (1980) state the teleomorph of this fungus to be Cordyceps memorabilis CBS., but the anamorph species as currently recognized is morphologically simple but polymorphic, and in practice the name P. farinosus would probably cover the anamorphs of a range of species of the Clavicipitales (Samson, 1974) . Teleomorphs are not as a rule produced in culture.

Paecilomyces fumosoroseus (Wize) A.H.S. Brown & G. Smith

Colonies growing fairly rapidly, deeply floccose, white at first but becoming dull pink, the reverse pale yellow. Conidiophores erect, sometimes aggregated into pale pink synnemata, with often closely-spaced verticillate branches. Conidiogenous cells in whorls of 4-6, 5.5-8 x 1-2µm, the basal part globose to ellipsoidal and with a distinct neck. Conidia 3-4 x 1-2µm, cylindrical to fusiform, hyaline to pale pink. Resting-spores not produced.

Paecilomyces fumosoroseus is an insect parasite, causing economic damage to silkwork production in eastern Asia (Samson, 1974), and has also been investigated for biological control. It has also been isolated from soil, butter and gelatin.

Paecilomyces lilacinus (Thorn) Samson

Colonies growing rapidly, floccose, at first white but becoming purplish or greyish violet, with a light to dark purple reverse. Conidiophores variably developed, often rather thick-walled, yellowish or purple at the base, sometimes aggregated into synnemata, verticillately branched. Conidiogenous cells in whorls of 2-4, 7.5-9 x 2.5-3µm, flask-shaped. Conidia 2.5-3 x 2-2.5µm, ellipsoidal to fusiform, hyaline but purple in mass. Resting-spores not produced.

Paecilomyces lilacinus has been found on insects on numerous occasions, but is most commonly encountered in isolations from soil. It has also been implicated as a human and animal pathogen, is found on various organic substrates including plastic contact lenses, synthetic rubber and polyurethane, and is used in fungus resistance testing. Strains produce the antibiotics leucostatin and lilacin (Arai et al., 1973). P. lilacinus was originally described as Penicillium lilacinum Thorn, but its colonies lack the characteristic green colour of Penicillium species, its Conidiogenous cells have distinct necks, and its Conidiophores are not divaricately branched (Samson, 1974),.

Paecilomyces marquandii (Massee) S. Hughes is very similar to P. lilacinus, but has colonies with a yellow reverse, smooth-walled hyaline conidiophores, and small globose to ellipsoidal resting-spores are often present.

Paecilomyces tenuipes (Peck) Samson

Colonies rather slow-growing, floccose or powdery, at first white but becoming cream or pinkish buff. Conidiophores erect, hyaline, sometimes aggregated into synnemata, with short branches arranged verticillately or irregularly. Conidiogenous cells in whorls of 2-6, 4.5-6.5 x 2.5-3.5µm, the basal part globose and with a ditinct neck. Conidia 3-7.5 x 2-2.5µm, ± cylindrical, often curved, smooth-walled, hyaline, sometimes septate, the septate conidia significantly longer.

Paecilomyces tenuipes is similar to P. farinosus, especially on the insect host rather than in culture, but its colonies lack yellow pigments, the conidiophore branches and conidiogenous cells are shorter and more swollen, and the conidia are larger and cylindrical. Early reports linked P. tenuipes with the teleomorph Cordyceps polyarthra Moller, but these may not be reliable (Samson, 1974).