Introduction

Fungi living predominantly or exclusively as yeasts are encountered in three classes of Basidiomycota, namely the Heterobasidiomycetes (Chapter 21), Urediniomycetes (Chapter 22) and Ustilaginomycetes (Chapter 23). We shall discuss basidiomycete yeasts together in the present chapter because these organisms, although taxonomically diverse, are unified by many biological features.

Ecology

Little is known about the ecology of basidiomycete yeasts. They occur in marine and freshwater habitats, the soil and the plant rhizosphere, and especially on above-ground plant surfaces such as tree bark, leaves, flowers and fruits. A certain degree of specificity of yeast species relative to plant species or organs of a given plant host has been observed (Phaff, 1990). Basidiomycete yeasts may be found in all climatic zones from the arctic to the tropics. They generally exist as saprotrophic phylloplane organisms. When nutrients become available, there may be a steep increase in the population density of these yeasts. Many yeasts isolated from soil have their origin in vegetation which becomes incorporated into humus after leaf fall. Basidiomycete yeasts are not noted as plant pathogens, but some species can infect animals. Cryptococcus neoformans is one of the most serious fungal pathogens of humans (pp. 661–665). Members of another group (Malassezia spp.) live commensally on the skin of humans and other mammals, causing superficial dermatomycoses under suitable conditions (p. 671).