Virulence and Aggressiveness ofPhakopsora pachyrhiziIsolates Causing Soybean Rust

Abstract

Isolates of P. pachyrhizi (the soybean rust pathogen) from Australia, India, Puerto Rico and Taiwan differed in virulence on an array of soybean genotypes. Three distinct infection types were recognized: TAN, RB and 0. Type TAN, a tan lesion about 0.4 mm2, usually with 2-5 uredia on the abaxial surface of the leaf 2 wk after inoculation, indicated host susceptibility. Type RB, a reddish-brown lesion about 0.4 mm2, usually with 0-2 uredia on the abaxial surface of the leaf 2 wk after inoculation, indicated host resistance associated with hypersensitivity. Type 0, the absence of macroscopically visible evidence of rust, indicated host immunity or near immunity. With increasing time after inoculation, lesion area and number of uredia per lesion increased for both TAN and RB, but the rates of increase were slower for RB. Culture Australia-72-1 induced both TAN and RB on each of 8 accessions; it may be composed of at least 2 physiologic races. Culture Taiwan-72-1 induced TAN on 13 accessions. Culture India-73-1 induced either RB (on 8) or 0 (on 5). On 20 soybean cultivars, culture PR-Comp from Puerto Rico induced RB only, but the other cultures induced TAN. All 4 cultures induced only RB on PI [plant introduction] 230970 and PI 230971. Cultures Australia-72-1, India-73-1, and Taiwan-72-1 induced TAN on ''Wayne'' soybean. The rate of lesion enlargement and the rate of increase in number of uredia per lesion on the lower surface of leaves of ''Wayne'' were about equal for India-73-1 and Taiwan-72-1, but were lower for Australia-72-1, which indicates that Australia-72-1 was less aggressive on ''Wayne'' than the other two under the test conditions used. Culture Taiwan-72-1 consistently produced more uredia per lesion at a given time on the upper surface of leaves of ''Wayne'' than did India-73-1.