Two decades ago, a virulent race of stem rust known as Ug99 was identified in Uganda. The race went on to cause major epidemics in Kenya in 2002 and 2004. It continues to evolve and emerge into new races. Ug99 and its variants have since spread across East African highlands to South Africa, and to Yemen and Iran, threatening regional food security. The Kenyan varieties in this subset exhibit resistance to Ug99.