Mycelium or uredinia on volunteer wheat, are the most important source of inoculum in tropical and subtropical climates. Windblown urediniospores are usually from earlier maturing wheat from the south in the northern hemisphere, or from the north in the southern hemisphere. Urediniospores and aeciospore germinate when in contact with free water. Infection by penetration through the stoma. Penetration requires at least a low light intensity.
Stem rust can survive as teliospores during winter when aeciospores are a major source of inoculum.
It generally survives as mycelium or uredinia on volunteer wheat during the non-wheat growing season. Uredinospore can be spread by wind into disease-free areas. Sporulating uredinia are active in tropical and some subtropical areas throughout the winter. Occasional dormant mycelium may survive beneath the snow pack in more northern temperate regions. Urediniospores and aeciospores are wind borne.