SOYBEAN RUST DEVELOPMENT AND THE QUANTITATIVE RELATIONSHIP BETWEEN RUST SEVERITY AND SOYBEAN YIELD

Phakopsora pachyrhizi was inoculated on two soybean (Glycine max) genotypes at three different reproductive growth stages (GS) in four trials. Leaf rust was more severe on Taita Kaohsiung No. 5 (TK 5), a commercial cultivar, than on SRE-B15-A (B15 A), a genotype selected for tolerance to leaf rust. At GS R6, the percentage of leaf area infected ranged from 14 to 95% for TK 5 and from 0 to 34% for B15 A. Values for area under disease progress curve (AUDPC) were significantly greater for TK 5 than B15 A. Yields in fungicide-protected plots ranged from 2,312 to 3,546 kg/ha and were not significantly different between the genotypes. Average yields of plants inoculated at GS R1 were reduced by 62 and 22% and seed weights by 35 and 14% for TK 5 and B15 A, respectively, compared with fungicide-protected plots. Regressions of yield percentage of fungicide-protected plants on disease severity assessments at GS R6, AUDPC, and area under the green leaf area curve were significant for both genotypes.