Pyrenomycetes

a group of ascomycetous fungi whose fruiting bodies—perithecia—open at the apex by a pore or crack. There are about 640 genera, embracing 6,000 species (according to other data, up to 10,000 species). The fruiting bodies are formed either directly on the hyphae of the mycelium or inside a special mass of hyphae—the stroma. Sporebearing is ascous or, in many species, conidial (as in imperfect fungi). Many species in the conidial stage parasitize higher plants; after the plant dies, they develop as saprophytes and by spring form asci. Thus the fungus is able to exist under the most diverse ecological conditions.

The majority of Pyrenomycetes are saprophytes and live predominantly on plant substrates. Together with other microorganisms, they play a role in the cycle of matter. Parasitic species are extremely destructive. For example, Claviceps purpurea, whose sclerotia contain alkaloids, are toxic to man and lower animals (ergotism). Epichloë typhina is the causative agent of cattail disease, a fungus infection of grasses. Some species of Nectria, Eutypa, Nummularia, Diatrype, and Valsa cause cancer and necrosis of trees and shrubs. Species of the order Coryneliales are the causative agents of many tropical plant diseases. Ceratocystis ulmi, or Ceratostomella ulmi, causes the dessication of Ulmaceae, Calonectria graminicola causes snow mold of winter plantings, and species of Phyllachora produce leaf spot in cereals. Species of Cordyceps parasitize insect larvae and fungi, and species of Nitschkia live on some fungi. The fungi Neurospora crassa and N. sitophila, widely used in biochemical and genetic research, belong to Pyrenomycetes. Many species of Pyrenomycetes are mycobionts of lichens.