Aspergillus Fumagatus Causes Witt Disease

In addition to their roles in the decay of plant and animal residues and in food spoilage, these fungi are of great significance to humans in other ways. Aspergillus fumagatus, a common inhabitant of heated compost, can cause respiratory disease in humans, and a number of related species may produce aflatoxin, a tumor-inducing alkaloid, in poorly stored, noldy grain. Species of both Penicillium and Aspergilllus are used extensively in commercial fermentations. This class also includes other species that causes diseases in humans, animals, and plants; for example, the fungus Ceraqtocystis ulmi is responsible for Dutch elm disease, other species cause a Witt diseases in oaks, and still other reduce the quality of lumber.

All fungi in the class Pyrenomycetes produce asci and ascospores as an organized hymenial layer in a fruiting body called a perithecium. The perithecium is a small, flask shaped structure with a thin wall that surrounds a basal tuff of asci; the opening at the top is called an ostiole. The ascospores are typical discharged violently from the tips of the asci as they sequentially protrude through the ostiole. This class includes many sprobes that grow on dung or cellulosic materials such as tree stumps and logs. Other species cause diseases of higher plants. They include Claviceps purpurea, which causes the disease Ergot of rye.

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.