The Alternaria species are imperfect filamentous fungi belonging to the class Deuteromycetes. Some perfect forms (sexual) are known and belong to the class Ascomycetess (Clathrospora, Leptosphaeria, Lewi, Pleospora). 44 species are known, but certainly close to one hundred species exist.
Their velvety colonies are usually gray to black. The dark and septate conidiophores produce several chains of straight or branched conidia at the ends. Initially ovoid, the multicellular conidia, which are divided by characteristic transversal, longitudinal or oblique septa, take the form of a club with a well-differentiated apical rostrum upon aging.
The Alternarias are cosmopolitan. These ubiquitous fungi are phytopathogenic in a number of plant species, causing crop diseases which are classified under the group "Alternariose". They can be found on different substrates such as senescent plants, vegetables, soil, food and various organic materials. The Alternarias are also known to be potent allergens, triggering seasonal reactions during the summer months. However, not all species are pathogenic or undesirable: some of them are used as biological agents to control invasive plants.