Sexual stage development of Uromyces appendiculatus and its potential use for disease resistance screening of Phaseolus vulgaris.

In June 1989. pycnia and aecia of the bean rust pathogen were observed in eastern Colorado on volunteer plants of pinto bean cvs. UI 114 and UI 126 that grew from seed that remained in the field after harvest the previous season. Harvested aeciospores were viable and produced typical reddish-brown uredinia on unifoliolate leaves of UI 114 seedlings in the greenhouse. Evidence of bean rust overwintering has been confirmed in 10 years from 1989 to 2002 in eastern Colorado and the surrounding region. Overwintering conditions were reproduced at Fort Collins, CO in fall 1992 and spring 1993. Debris treatments had significantly higher disease incidence on stems. total number of lesions on stems, disease incidence on leaves, and total number of lesions on leaves of plants of pinto cvs. UI 114 and Olathe than plants without debris. Bean leaves of both cultivars had significantly higher disease incidence than stems. There also was an increased incidence of aecial infection for UI 114 seedlings that germinated through leaf debris with rust compared with stem debris under greenhouse conditions with abundant moisture. Bean leaf debris smaller than 0.36 mm in diameter resulted in significantly more aecial lesions on UI 114 than larger leaf debris and stem debris. Additional greenhouse experiments demonstrated that diverse collections of naturally occurring, overwintered, rust-infested bean debris from eastern Colorado produced different levels of pycnial and aecial infection on pinto evs. UI 114, Olathe. and Chase. Chase. currently resistant to the uredinial stage of the prevalent bean rust races in Colorado and surrounding states, had a low incidence of aecial lesions on seedling stems when exposed to pycnial and aecial stages in the greenhouse. These observations indicate that selection of bean rust resistance genes should rely on multiple sources of resistant germ plasm to counter the potential increase in new races that could derive from sexual recombination in bean-production regions.