Ustilago maydis, the Causative Agent of Corn Smut Disease

Abstract

Smut diseases are caused by Basidiomycetes of the order Ustilaginales. Their occurrence is worldwide and cereals and grasses serve as most common hosts. Among the estimated 1100 species of smut fungi the most prevalent causing significant losses in yield are of the genus Ustilago, causing loose smut of oats (U. avenae),barley (U. nuda) and wheat (U. tritici), covered smut of barley and oats (U. hordei), causing corn (U. maydis) and sugarcane smut (U. scitaminea), respectively; of the genus Tilletia inducing covered smut or bunt of wheat (T. caries and T. foetida), dwarf bunt of wheat (T. contraversa); and of the genus Sporisorium causing smut diseases in sorghum as well as head smut in sorghum and corn (see Agrios, 1988). In general, the host range of most smut fungi is rather narrow and only closely related plant species are infected. Many smut fungi develop within the grain kernels and replace them with masses of dark teliospores resembling smut or soot, giving the name to this disease. As a result considerable losses in yield are encountered and healthy seed is contaminated with spores during harvest. While some smuts infect germinating seedlings and grow internally without causing symptoms until flowering occurs others are able to infect all aerial parts of the plant and cause local disease symptoms around the site of infection. The most prominent disease symptoms are inflorescences which are completely smutted and in which all individual kernels are replaced by masses of dark teliospores. Initially the developing spores are surrounded by a membrane that breaks when teliospore development is completed and sets free the massive amounts of spores. In other instances, like infections with U. maydis,spore development takes place within plant tumors that are induced by the fungus and appear to provide the ideal environment for fungal proliferation and teliospore production. Teliospores developing within the infected tissue are invariably diploid. Upon germination meiosis takes place and the haploid form, the so-called sporidium is generated. These haploid forms can be propagated on artificial media in the laboratory, however, this form is unable to cause disease when applied to host plants in pure culture. Prerequisite for generating the infectious stage is the mating of two compatible, haploid sporidia and the generation of the dikaryon. In contrast to the haploid form this stage cannot be propagated outside the host plant. Once in contact with the plant characteristic infection structures are produced that allow penetration and subsequent proliferation within the infected plant. This leads to the characteristic disease symptoms already described and culminates with the formation of teliospores which represent the resting stage able to survive harsh environmental conditions like drought and winter.