The Ascomycota, or sac fungi, is a diverse group and accounts for approximately 75% of all described fungi. It includes most of the fungi that combine with algae to form lichens - the "lichenized ascomycetes". By utilizing air, water and light, the alga produces sugars for itself and the fungus partner, the fungus in return providing shelter for the alga in an often otherwise hostile environment such as an exposed rock surface. Some species associate with the roots of flowering plants to form 'mycorrhiza', in much the same way as many toadstools. Most of these fungi do not produce substantial mushrooms; even fewer produce mushrooms worth eating. But the Ascomycota does contain some of the most highly valued mushrooms: morels and truffles as well as the more unusual mushrooms such as the candlesnuff fungus.
Among the Ascomycota are other famous fungi: Saccharomyces cerevisiae, the yeast of commerce and foundation of the baking and brewing industries (not to mention molecular developmental biology), Penicillium chrysogenum, producer of penicillin. There are also some infamous Ascomycota, a few of the worst being: Aspergillus flavus, producer of aflatoxin, the fungal contaminant of nuts and stored grain that is both a toxin and the most potent known natural carcinogen, Candida albicans, cause of thrush, diaper rash and vaginitis and Cryphonectria parasitica, responsible for the demise of 4 billion chestnut trees in the eastern USA. Asexual Ascomycota, such as Penicillium or Candida species, used to be classified separately in the Deuteromycota because sexual characters were necessary for Ascomycota classification. However, the comparison of nucleic acid sequence, as well as nonsexual phenotypic characters, have permitted the integration of asexual fungi into the Ascomycota.
The taxonomy of fungi is variable depending on the author. Phyla, classes and subclasses are often synonymous in different hierarchical systems.