Common smut of corn

​Ustilago maydis, the cause of common smut of corn, has a complex life as it interacts with corn biology.  The dried galls in corn ears, tassels, or leaves include a powder composed of thousands of spores called teliospores.  These thick-walled spores, with a diploid nucleus are able to withstand many environments for many years on and in the soil.  When they germinate, on a corn plant, mitosis in the diploid nucleus creates short hyphal strands with haploid spores called basidiospores.  These spores germinate to produce short hyphal strands that fuse to form new hyphae with each cell containing the two haploid nuclei. These hyphae can form an appresorium on the leaf from which the fungus penetrates an epidermal cell.  This stage of the fungus is total biotrophic, receiving nutrition only from the living cells.
 
During the leaf stage of infection, the hyphae grow to surrounding cells, again invading and feeding on living cells.  As the nutrition is depleted, the two nuclei in the hyphae cells fuse to form a diploid and finally teliospores.
 
The fungus generally grows towards plant meristems.   These could be in those ear meristems located at each leaf node of the plant, resulting in the appearance of stalk smut galls but actually galls formed in those ear meristems that do not normally produce ears.
 
Infection is common in the silks of ears. Teliospores germinate, produce the haploid basidiospores, and short filaments on the silk.  These filaments fuse to form hyphae with the two haploid nuclei in each cell within the filament as they grow towards the ovule at the end of the silk.  Reaching the ovule, it invades to absorb the nutrients from the host cells, causing the cells to swell as it keeps the cells alive but absorbs the nutrients. It is during this time that the nuclei of U. maydis cells fuse to form diploids. Eventually these diploid cells develop the thick, walls of the teliospores, the tissue dries and teliospores spread in the wind.  These spores may remain dormant for several years until the next opportunity for infection of corn plants.
 
One of the functions of the abscission layer that develops at the attachment of the silk to the ovule after pollination is to stop intrusion of potential pathogens such as Ustilago maydis.  Consequently, infection of ears by this fungus is most frequent in poorly or delayed pollinated ear shoots.  Resistance to this disease includes good timing of pollen production and emergence of silks when the plant is under stress conditions.  Occurrence of the smut at the tips of ears suggests that poor pollination occurred at the end of the silking period.
 
Newly formed smut galls include both host plant material and fungus.  These fresh galls are relished as food in the Mexican culture.  Production of large quantities involves taking advantage of the host and pathogen biology, growing male sterile corn and spraying the silks with the teliospores saved from previous years.