any fungus of the class Ascomycetes (or subdivision Ascomycota) in which the spores are formed inside an ascus.

A fungus whose spores are borne in an ascus (a sack-like structure), e.g. Penicillium, yeast.

constitute the largest class of fungi characterized by the production of sexual spores in structures call asci. This includes plant pathogens, saprobes and decomposers. With a few exceptions, most Ascomycetes do not grow in buildings and are seldom agents of wood rot. Ascomycetes are the perfect stages of molds like Aspergillus and Penicillium. At high levels, Ascomycete spores may cause allergies. Since most Ascomycetes are plant pathogens, ascospores are common during the growing season of plants and are rare during winter, such as those of the Ascomycetes genera: Daldinia, Hypoxylon, Paraphaeospaeria, Phaeosphaeria and Leptosphaeria.

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