Rust fungi

Rust fungi are obligate plant pathogens in the Basidiomycota. About 7,000 species of rust fungi are known which infect as many as 8,000 species of plants. They often cause severe disease, especially on cereal crops. Rust fungi are morphologically simple with no fruiting body, but they often have a complex life cycle with multiple spore stages. Many rust fungi have alternate hosts, with some spore types occurring on one host and other spores on the alternate host. One of the best studied rust fungi is Puccinia graminis the species that causes stem rust of wheat and has an alternate host on barberry plants. 

Two types of spores, uredospores (urediniospores) and teliospores, are airborne and frequently seen in our air samples. The uredospore stage is known as the repeating stage, spreading the infection from one plant to another. The uredospores are formed in lesions called uredia (uredinia) that form on the stalk through most of the growing season. At the end of the growing season, the uredia become telia producing two-celled teliospores. The teliospores constitute the overwintering stage providing a source of infection for the next season. The following spring, the teliospores germinate and produce basidiospores, which go on to infect barberry plants. Two spore stages occur on barberry; one of these aeciospore carry the infection back to wheat. 

The images below show wheat stalks with red uredial lesions on the left and black telial lesions on the right