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41

A detached leaf method to evaluate soybean for resistance to rust

Presenter: R. Bandyopadhyay

All authors and affiliations: M. Twizeyimana (1), R. BANDYOPADHYAY (1), P. Ojiambo (1), C. Paul (2), and G. L. Hartman (2,3). (1) International Institute for Tropical Agriculture, PMB 5320, Ibadan, Nigeria; (2) National Soybean Research Center, 1101 W. Peabody Drive, Urbana, IL 61801; and (3) USDA-ARS

The potential of soybean rust to cause severe yield losses makes this a most serious soybean disease worldwide. The causal agent, Phakopsora pachyrhizi, is an obligate parasite and a highly variable pathogen. Host plant resistance may offer a potential practical solution; although rapid screening methods to evaluate host resistance and pathogen variability need to be developed to hasten progress in developing rust-resistant cultivars. The objective of our research was to develop a detached leaf method to evaluate soybeans for resistance to rust. We optimized the following components of the method: concentration of hormones to retard senescence of detached leaves, leaf age, and spore concentrations for inoculation. Pieces of detached leaves from 3- to 4-week-old plants of 14 lines were placed on 1% agar amended with 10 mg of kinetin per liter and inoculated with 1 10(^5) spores per ml. Leaves remained green for nearly 30 days at 20C and were evaluated for days to lesion appearance, days to pustule formation, days to pustule eruption, lesion number, lesion length, lesion type, number of pustules, and spores per pustule. Parallel experiments for same genotypes were carried out in the field and the greenhouse. Pearson correlation coefficients were highly significant between detached leaf and greenhouse for various disease evaluation parameters (r = 0.86 to 0.99; P < 0.01) and field screening for plant reaction (r = 1.0) and sporulation (r = 0.82; P < 0.01) across genotypes. The detached leaf assay is a rapid method to screen soybean for rust resistance and maintain pure cultures of P. pachyrhizi.

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