Urediospores of U. viciae-fabae (broad bean rust) germinated well in the range 5–26 °C, with fastest germination at 20 °C. Exposure to 30 °C gave poor germination and damaged the spores. Infection of Vicia faba leaves depended on a moisture film. At 20 °C some infection occurred with only 4 h leaf wetness, but longer wet periods up to 24 h gave increased infection. At lower temperatures, the infection process was slower and final pustule numbers were smaller. Spore germination was delayed by daylight and by all artificial light sources that contained far-red (700–800 nm) wavelengths. The delay was increased at higher light intensities. When spores were subjected to alternating periods of light and darkness, it was found that 40 min of darkness was sufficient for the irreversible induction of germination at 20 °C.