Switchgrass cv. Summer infected with switchgrass rust. Switchgrass cv. Summer plants were inoculated by spraying P. emaculata urediniospores at a concentration of 105 spores per milliliter under controlled conditions. Two weeks after inoculation, dark brown rust pustules containing dikaryotic urediniospores appeared on leaf (a) and stem (b) of switchgrass plants. (c) A switchgrass plant in a field severely infested with rust under natural conditions.