Abstract
Nineteen isolates of Melampsora medusae (Thum.), collected from natural stands of Populus deltoides (Bartr.) along the lower Mississipi River Valley, were tested for the occurrence of physiologic races by inoculation of nine poplar clones. Eight distinct races were identified based upon differential responses on these clones. However, most of the isolates also differed in their aggressiveness (latent period and uredial number per leaf disk), and a significant isolate × cultivar interaction was observed for both traits. Within a specific geographic location, more than one race was present. Isolates from the northernmost sampling location (37°N latitude) appeared more aggressive than those from southern latitudes (34–36°N latitude). Thus wild pathosystems may be composed of variable pathogen populations differing in virulence and the apparent stability usually observed in such systems may be due to the genetic diversity of the host population in interaction with many epidemiological and ecological factors.
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