Interactions of the bean rust and cowpea rust fungi with species of the Phaseolus – Vigna plant complex. I. Fungal growth and development

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Abstract
Growth and development of two closely related rust fungi, Uromyces appendiculatus and U. vignae (uredospore infections), were compared in several American and African species of the Phaseolus – Vigna plant complex and in Lablab purpureus. In plants inoculated with U. appendiculatus the frequency of formation of both primary and secondary haustoria generally decreased with evolutionary distance from the American host species. Uromyces appendiculatus formed primary haustoria at a high frequency of infection sites in most American and many African species, and also sustained a limited haustorium – plant cell metabolic relationship (as determined by the amount of secondary fungal growth) in one group of African species. Uromyces vignae generally formed very few haustoria in both American and African species not known as hosts of the fungus; however, growth of U. vignae colonies occasionally continued for some time at a few infection sites in nonhost species without sporulation. The implications of these results are discussed with respect to the coevolution of the two fungi with their hosts.