Members of the class Ascomycetes are diverse. They range from unicellular yeasts to powdery mildews, cottony molds, and large complex “cup” fungi. In the latter, the hyphae are tightly packed and form a cup‐shaped structure.

During sexual reproduction, the ascomycetes form a sac called an ascus. The ascus forms where sexually opposite hyphae have fused. Within the sac a number of ascospores form, and each can reproduce the entire fungus.

Within the Ascomycetes class is the yeast Saccharomyces. This organism is used in fermentation processes and baking. The producer of penicillin, Penicillium, is also in this class, as is Aspergillus a producer of citric acid, soy sauce, and vinegar. The chestnut blight and Dutch elm diseases are caused by ascomycetes.