Near the end of the 19th century, Brefeld worked out many of the details of the disease cycle of common smut which coincides with the life cycle of U. maydis. The fungus overwinters as diploid teliospores in crop debris or soil. Christensen estimated that a single smut gall of medium size may contain more than 200 billion spores (Figure 23). Teliospores can remain viable for several years. Teliospores can be disseminated directly by wind or splashing rain, or they can germinate and undergo meiosis to form haploid sporidia which also can be disseminated by wind or splashing rain. Sporidia bud in a yeast-like manner, mate and form dikaryotic infection hyphae. Infection hyphae of compatible mating types (i.e., different alleles at the a and b loci) penetrate and infect the host. All infection is local. Any above-ground plant part can be infected, particularly young actively growing meristematic tissues. The fungus also may grow down stigmas (ear silks) and into developing ovaries (kernels), resulting in ear galls. Each gall on an infected ear is the result of infection of an individual ovary. Disfigured tissue which develops into galls may be noticeable within days after infection. Galls form within a week after infection and enlarge for up to 3 weeks after infection. About 1 to 2 weeks after infection, hyphae in galls begin to gelatinize, and hyphal cells become round and form cell walls of teliospores. As galls mature and dehydrate, the periderm ruptures releasing teliospores.