Classes of Ascomycetes

Several classes of ascomycetes exist. The class Hemiascomycetes includes the yeast, these the unicellular or mycelial, but all lack ascogenous hyphae and fruits. Most yeasts are saprobic, commonly occurring on plant parts, in soil, and in other locations with adequate moisture and organic material. A small group is parasitic on the leaves, twigs, and branches of vascular plants, causing leaf curl and witches brom (tufts of branchlets resulting from repeated branching).

Another fungi class of ascomycetes, Plectomycetes, includes several economically important fungi that form their asci in small, closed, fruiting structure (cleistothecia). The powdery mildews, so named from the powdery appearance of infected leaves, are all obligate parasites of higher plants and are largely host specific. The fungus grown on the surface as a white, cottony mycelial mat and produces many simple conidiophores and ellipsoidal spores (conidia). The surface cells of the host are invaded by special extensions called haustoria. The cleistothecia become brown or black at maturity, bear a number of characteristically shaped external appendages, hook, shaped or spearlike, for example, and over winter on the fallen leaves.

A second major group of plectomycetes includes the commercially utilized genera Aspergillus and Penicillium, as well as important pathogens of plants and humans. Sexual reproduction is relatively rare among species of Aspergillus and Penicillium. Aspergillus produces chains of pigmented, asexual conidia on the surface of an inflated region of a branch, called a conidiophores. Conidium formation is similar in Penicillium, but the conidiophores is brached to form a brushlike structure (penicillius) instead of having an inflated vesicle. The conidia are connected in chains on the conidiophores but are readily dispersed by air currents. The green, black, yellow, and gray colors of the colonies of these common microfungi are the result of the color of the huge number of pigmented conidia produced on the surface.