Urediospores of Uromyces phaseoli var. typica (Pers.) Wint. race 32 Arth. germinated on mineral oil – nitrocellulose membranes and sequentially developed appressoria, vesicles, and infection hyphae. The nuclear behavior during in vitro differentiation of infection structures was studied by use of the Feulgen technique. The two urediospore nuclei divided in the germ tube before the formation of appressorium. This was followed by a second division of the four daughter nuclei in the appressorium, and occasionally by a third division of the eight nuclei in the vesicle and infection hypha. Haustorial mother cells were formed in infection hyphae in vitro and contained from two to five nuclei. In contrast, nuclear division did not occur in germ tubes where growth continued linearly. Infection structures that developed in vitro resembled those produced during infection of the host by urediospores of other species of rust fungi.
