ABSTRACT
The pathogens responsible for rust and smut diseases belong to two of the three groups (subphyla) of Basidiomycota (Figure 14.1). The majority of species in these subphyla, Pucciniomycotina (rusts) and Ustilaginomycotina (smuts), are plant parasites. In nature, rust and smut fungi are obligate biotrophs, requiring living plant hosts for nutrition and to complete their life cycles. Rust and smut fungi produce basidiospores and basidia similar to other Basidiomycota, but their basidia emerge from thick-walled teliospores (Figures 14.2 and 14.3) rather than being formed on or in fruiting bodies such as mushrooms and puffballs. In rust and smut fungi, the parasitic phase of the life cycle is filamentous (mycelial) and dikaryotic, with paired nuclei of compatible mating types (sometimes designated “n + n”). Dikaryotic teliospores are formed from this stage, and nuclear fusion (karyogamy) occurs in the teliospore, resulting in a brief diploid phase prior to germination to produce a basidium. Meiosis occurs in the basidium, and haploid
basidiospores are formed. Some smut fungi also have a haploid saprotrophic phase in which they grow in a yeastlike manner and can be cultured on artificial media. Rust and smut fungi cause important plant diseases, but rust fungi, in general, are more important economically worldwide than are smuts.
