Initial spread of aeciospores is limited to a few feet. In July and August, spores from the apple leaves (aeciospores) are produced. When the sexual outcrossing occurs the dikaryotic aeciospores and urediniospores are initiated. The heaviest production of aeciospores occurs during March and April. The chains of aeciospores are surrounded by a bell-like enclosure of fungal cells. These are actually aecia containing aeciospores for dispersal, making the broom a sign of disease. This fertilization allows for aecia to form and in turn produce aeciospores. Aeciospores from "R. cathartica" are the primary inoculum for infecting barley. The aeciospores are found in chains of 7-8 spores and are sessile. Aeciospores are one of several different types of spores formed by Rusts.