Cladosporium conidia

Cladosporium is a common fungal genus of asexual ascomycetes occurring both indoors and outdoors. Cladosporium species have a worldwide distribution, and on a yearly basis Cladosporium conidia are the most abundant outdoor spore type in temperate climates. The fungus normally exists as a saprophyte or weak plant pathogen, and the spores are known to be allergenic. 

Cladosporium conidia are lightly-pigmented and form in chains, which may branch. Individual conidia typically have two attachment scars, but may have three if there is a branch point. Spores may be unicellular or multicellular with one, two, or rarely three septa. Depending on the species, the size and shape are varied, and the conidial wall may be smooth or ornamented. Occasionally, large clumps of Cladosporium conidia are seen in air samples.