Interactions of the bean rust and cowpea rust fungi with species of the Phaseolus–Vigna plant complex. II. Histological responses to infection in heat-treated and untreated leaves

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Abstract
A comparison of the histological responses of species within the Phaseolus-Vigna plant complex to single isolates of the bean [uromyces appendiculatus var. appendiculatus] and cowpea rust fungi, [u. vignae] revealed that no particular response was restricted to any plant taxonomic group, although species differed in the proportion of infection sites at which a particular response was exhibited. Related species did not always show similar frequencies of responses and sometimes there were differences between different genotypes within a nonhost species. In host and nonhost species, preinoculation heat treatment commonly inhibited prehaustorial denfenses and delayed the death of the invaded cell. Growing fungal colonies subsequently developed in many species, even those considered nonhosts, particularly if they exhibited a high frequency of prehaustorial defenses in untreated leaves. It is argued that a lack of heat-induced colony formation is a sign of parasite-specific resistance, which most likely evolved only in originally susceptible plants. Consequently, the data suggest that the bean rust fungus has had a long association with American species of the complex and that extant nonhost species may have evolved from susceptible ancestors. In contrast, the cowpea rust fungus appears to have had little evolutionary contact with these American species and may not be as closely related to the bean rust fungus as originally thought.

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