This fungus has several spore stages in its life cycle, all of which are microscopic and may be observed on Canada thistle throughout the growing season. Systemically diseased Canada thistle shoots emerge in spring bearing fragrant, orange-colored pustules and pycniospores on the undersides of leaves. These eventually give rise to powdery reddish-brown aecia that produce aeciospores, darker brown uredia that produce primary urediniospores, and very dark brown telia that produce primary teliospores. The primary urediniospores and teliospores are wind-borne and cause local infections of neighboring Canada thistle shoots and leaves in late spring and early summer. These local infections result in the formation of uredinia that produce secondary urediniospores and also telia that produce secondary teliospores throughout summer. As plants with localized infections senesce in autumn, the leaves bearing urediniospores and teliospores are shed throughout the local area. When conditions are right, shed primary or secondary teliospores germinate in contact with living Canada thistle tissue, producing basidiospores that infect Canada thistle tissue and establish the rust as mycelium in the rootstock. Infection may be present as mycelium with no aboveground spores. When mycelium infects the adventitious root buds, new Canada thistle shoots are morphologically discernible as being systemically diseased, beginning the spore cycle again.