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Resistance

Virulence and Aggressiveness of Phakopsora pachyrhizi Isolates Causing Soybean Rust. K. R. Bromfield, Research plant pathologist, Plant Disease Research Laboratory, AR-SEA, U.S. Department of Agriculture, P.O. Box 1209, Frederick, MD 21701; J. S. Melching(2), and C. H. Kingsolver(3). (2)(3)Research plant pathologists, Plant Disease Research Laboratory, AR-SEA, U.S. Department of Agriculture, P.O. Box 1209, Frederick, MD 21701. Phytopathology 70:17-21. Accepted for publication 6 July 1979. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1980. DOI: 10.1094/Phyto-70-17.

Isolates of Phakopsora pachyrhizi (the soybean rust pathogen) from Australia, India, Puerto Rico, and Taiwan differed in virulence on an array of soybean (Glycine max) genotypes. Three distinct infection types were recognized: TAN, RB, and 0. Type TAN, a tan lesion about 0.4 mm2, usually with 25 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 02 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 eight accessions; it may be composed of at least two physiologic races. Culture Taiwan-72-1 induced TAN on 13 accessions. Culture India-73-1 induced either RB (on eight) or 0 (on five). On 20 soybean cultivars, culture PR-Comp from Puerto Rico induced RB only, but the other cultures induced TAN. All four cultures induced only RB on PI 230970 and PI 230971. Cultures Australia-72-1, India-73-1, and Taiwan-72-1 induced TAN on soybean cultivar Wayne. 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.

Additional keywords: disease resistance, pathotypes, physiologic specialization.