Expression of Wheat Leaf Rust Resistance Gene Lr34 in Seedlings and Adult Plants. R. P. Singh, International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico. A. K. Gupta, Department of Genetics, Punjab Agricultural University, Ludhiana, India. Plant Dis. 76:489-491. Accepted for publication 22 November 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0489.

The expression of wheat leaf rust resistance genes Lr34, LrT3, and Lr12 was studied in seedlings of cv. Thatcher and the near-isogenic Thatcher genotypes RL6058-Lr34, line 920-Lr34, line 922-LrT3, and RL6011-Lr12 inoculated at 8 and 13 days old. Several Mexican pathotypes of Puccinia recondita f. sp. tritici were used for inoculation. Incubation was carried out at four temperature regimes (three greenhouse and one growth chamber). Gene Lr34 conferred resistance to all pathotypes, although the seedling infection types (ITs) varied among tests and ranged from slightly chlorotic flecks to 3+. Low ITs were best recognized in the older first leaves, which indicated that leaf growth stage affected the expression of resistance. The IT was influenced by the genetic background, temperature, and other environmental factors (perhaps light). The pathotype-specific response of LrT3 could be seen only at lower temperatures, whereas Lr12 was ineffective to all pathotypes at all temperature regimes. Gene Lr34 conferred highly effective adult plant resistance at the Ciudad Obregon location, but it was only partially effective at the El Batan location with the same pathotypes. Genes LrT3 and Lr12 did not confer any adult plant resistance in the field.