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Differential Responses of Resistant Soybean Entries to Isolates of Phakopsora pachyrhizi

Soybean rust, caused by the fungus Phakopsora pachyrhizi, was detected in the continental United States in 2004. Several new sources of resistance to P. pachyrhizi have been identified in soybean (Glycine max); however, there is limited information about their resistance when challenged with additional U.S. and international isolates. Resistance of 20 soybean (G. max) entries was compared after inoculation with 10 P. pachyrhizi isolates, representing different geographic and temporal origins. Soybean entries included 2 universal susceptible cultivars, 4 sources of soybean rust resistance genes (Rpp_1--4), and 4 and 10 resistant entries selected from field trials in Paraguay and Vietnam, respectively. Of the known Rpp_1--4 sources of resistance, plant introduction (PI) 459025B (Rpp₄) produced reddish-brown (RB) lesions in response to all of the P. pachyrhizi isolates, while PI 230970 (Rpp₂) produced RB lesions to all isolates except one from Taiwan, in response to which it produced a susceptible tan (TAN) lesion. PI 200492 (Rpp₁) and PI 462312 (Rpp₃) produced TAN lesions in response to most P. pachyrhizi isolates. The resistant entries selected from Paraguay and Vietnam varied considerably in their responses to the 10 P. pachyrhizi isolates, with M 103 the most susceptible and GC 84058-18-4 the most resistant. The reaction patterns on these resistant entries to the P. pachyrhizi isolates were different compared with the four soybean accessions with the Rpp genes, indicating that they contain novel sources of rust resistance. Among the P. pachyrhizi isolates, TW 72-1 from Taiwan and IN 73-1 from India produced the most susceptible and resistant reactions, respectively, on the soybean entries.