Teliospores are the initial inoculum. Infested seed is the most likely mode of long distance transport. Once in an area, soil-borne teliospores are the principal inoculum source. Teliospores germinate at the soil surface in response to moisture. Some research suggests that as little as 2 mm of soil coverage can prevent successful teliospore germination.
Spores germinate throughout the growing season.
Buried teliospores will not germinate but can serve as a new source of inoculum when they are brought to the surface by redistribution of soil due to wind, irrigation, cultivation or rain. Each germinating teliospore produces other spores called primary sporidia. Primary sporidia are wind-borne and land on plant leaves. Primary sporidia can grow on any plant surface as long as a food source is available. Aphid honeydew is a good food source.