Chapter 11: Plectomycetes

The defining features of these fungi include globose, thin-walled asci that passively release small unicellular ascospores, along with anamorphic states that frequently produce arthroconidia or phialidic conidia. Plectomycetes occupy diverse ecological roles, functioning primarily as soil saprotrophs capable of degrading complex organic substrates including cellulose, starch, keratin, and proteins. Many species synthesize bioactive secondary metabolites such as antibiotics and mycotoxins, while others represent significant pathogenic threats to human and animal health. The Ascosphaerales order includes sugar-tolerant species like Eremascus and the medically important Ascosphaera apis, which causes chalkbrood disease in honeybee colonies through the formation of chalk-like larval cadavers; disease progression correlates with cool hive temperatures and is managed through behavioral fever responses and hygienic larval removal. The Onygenales order holds particular significance in medical mycology and includes several families with important representatives. The Onygenaceae contain dimorphic pathogens such as Histoplasma capsulatum, Blastomyces dermatitidis, Paracoccidioides brasiliensis, and Coccidioides immitis, which exist as soil-dwelling hyphae but transform into yeast or spherule forms within host tissues while evading macrophage-mediated immunity. The Arthrodermataceae encompass dermatophytes including Trichophyton, Microsporum, and Epidermophyton species that cause cutaneous infections transmitted through animals, soil, and communal environments. The Eurotiales order demonstrates immense practical significance through ubiquitous genera Aspergillus and Penicillium. These fungi serve as industrial fermentation organisms producing citric acid, enzymes, and antibiotics including penicillin, while also contributing to specialty food production such as fermented soy products and aged cheeses. However, certain species produce dangerous mycotoxins including aflatoxins, ochratoxin A, and patulin, while pathogenic members like Aspergillus fumigatus cause serious respiratory infections. The chapter illustrates how Plectomycetes range from beneficial industrial organisms to destructive pathogens and spoilage agents.