Symptoms are somewhat similar to common rust, however, with
southern rust the pustules(uredia) are cinnamon brown, circular
to oval and densely scattered on the upper leaf surface.
The chocolate brown to black stage (telia) is circular to
elongate and frequently appear as circles around the uredial pustules. Southern rust pustules (containing the spores) remain covered by the epidermis for a longer time than do those of common rust.
Southern rust is caused by the fungus Puccinia polysora
Development of the disease is favored during periods of high temperature (80 degrees F,
27 degrees C) and high relative humidity. Southern rust does not occur as often from
year to year as does common rust. On susceptible hybrids, the disease can develop very rapidly during warm, humid conditions.