Chapter 17: Loculoascomycetes

The fruiting bodies, called pseudothecia, structurally resemble perithecia but develop through a unique pathway involving locule formation prior to nuclear fusion. Members occupy remarkably diverse ecological niches as saprotrophs, endophytic colonizers, plant and animal pathogens, allergen sources, and industrial organisms. The chapter concentrates on two principal orders: Pleosporales and Dothideales, both of significant agricultural and medical importance. Within Pleosporales, pseudoparaphyses and characteristic pseudothecial architecture define relationships among genera. Leptosphaeria species range from weak decomposers to serious pathogens, exemplified by L. maculans causing blackleg disease in oilseed rape with its Phoma lingam anamorph establishing persistent endophytic infections and stem cankers. Phaeosphaeria nodorum attacks cereal crops, while Ascochyta species parasitize legumes and Pleospora herbarum initiates leaf blights through its Stemphylium stage. Alternaria represents ubiquitous saprotrophs with pathogenic capacity against diverse crops including tomato, brassicas, and potatoes, while simultaneously functioning as mycotoxin producers and allergens. Cochliobolus encompasses devastating cereal pathogens whose Bipolaris and Curvularia anamorphs synthesize host-specific toxins determining pathogenicity. Pyrenophora species cause economically significant diseases via proteinaceous phytotoxins, and Venturia inaequalis produces apple scab through atypical subcuticular infection mechanisms. The Dothideales order contains Mycosphaerella, one of the largest fungal genera exceeding 2000 species with variable anamorphic states including Septoria, Cercospora, and Cladosporium. Mycosphaerella graminicola causes wheat leaf blotch while Cercospora beticola produces cercosporin, a light-activated toxin affecting sugar beets. Cladosporium species dominate atmospheric spore concentrations, contaminate food substrates, trigger asthma through allergenic compounds, and include C. fulvum, controlled through gene-for-gene resistance mechanisms. Aureobasidium pullulans functions as a pleomorphic black yeast inhabiting plant surfaces, offering biocontrol potential and producing industrially relevant polysaccharides. This chapter demonstrates the ecological versatility of Loculoascomycetes across decomposition, phytopathogens, allergenesis, and host-pathogen coevolution paradigms.