Effect of Leaf Rust Resistance Gene Lr34 on Grain Yield and Agronomic Traits of Spring Wheat R. P. Singh* and J. Huerta-Espino ABSTRACT Leaf rust, caused by Puccinia recondita Roberge ex Desmaz. f. sp. tritici, is an important disease of wheat (Triticum aestivum L.) worldwide. The Lr34 gene is known to confer durable resistance. We evaluated the effect of Lr34 on grain yield and other traits in the absence and presence of leaf rust. 'Jupateco 73R' and 'Jupateco 73S' (near-isogenic reselection from the Mexican spring wheat cultivar 'Jupateco 73' for the presence and absence of Lr34, respectively) and 22 random inbred F 6 lines, 11 with and 11 without Lr34 (derived from the cross 'Jupateco 73R/Jupateco 73S'), were planted in replicated field trials during the 1992-1993 and 1993-1994 seasons in northwestern Mexico. The mean grain yield of Jupateco 73R was 5.9% lower (P < 0.05) than that of Jupateco 73S in protected plots in the 1992-1993 experiment. Significant reductions (P < 0.05) were also observed for biomass, kernels per spike and kernels m~ 2 . Significant (P < 0.01) reductions of 5% in mean grain yield and 3.7% in mean kernel weight were again evident in one of the two experiments sown during the 1993-1994 season. Comparison of grain yield in protected and non- protected treatments indicated that though leaf rust could significantly (P < 0.01) reduce grain yield by approximately 15% in the presence ofLr34, the reductions in the absence ofLr34 were substantially higher and ranged between 42.5 to 84% depending on planting date and year. Reductions in all other traits were also significantly higher in the absence of Lr34. We conclude that although the presence of Lr34, which is linked with leaf tip necrosis of adult plants, may carry a slight yield penalty in some disease free environments, its use in leaf rust prone areas could provide substantial protection to grain yield and other traits. L AP RUST is an important disease of wheat worldwide. Cultivation of resistant cultivars is the most econom- ical and environmentally benign way to reduce losses caused by the disease. Several genes are currently known to confer resistance (Mclntosh et al., 1993). Roelfs (1988) suggested that Lrl2 or Lrl3 in combination with R.P. Singh, International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apdo. Postal 6-641, 06600, Mexico, D.F., Mex- ico, and J. Huerta Espino, CIANO, Apdo. Postal 515, Calle Norman E. Borlaug, Km. 12, 85000, Cd. Obregon, Son., Mexico. Received: 3 April 1996. *Corresponding author (E-mail: RSINGH@CIMMYT.MX). Published in Crop Sci. 37:390-395 (1997). Lr34 could have contributed to the durable leaf rust resistance of several wheat cultivars, such as Frontana and Era. Singh (1993) identified several Mexican spring wheat cultivars that carry the Lr34 gene, which confers slow rusting against the Mexican population of P. recon- dita; however, final disease ratings are often unacceptable (Singh and Gupta, 1992) for breeding purposes. The "Lr34 complex", defined as the product of additive inter- action involving Lr34 and two to three additional slow rusting genes, appears to confer highly effective adult- plant resistance worldwide (Singh and Rajaram, 1992). The Lr34 gene is known to be either pleiotropic or closely linked with leaf tip necrosis of adult plants (Dyck, 1991; Singh, 1992). Symptoms of this condition begin to occur at about flowering and include 2 to 3 cm of necrosis at the ends of the leaves extending an additional 2 to 4 cm down the edges of the leaf. Our objectives in the present study were to (i) establish the effect of Lr34, as evidenced by leaf tip necrosis, on grain yield and other agronomic traits, and (ii) determine the contribution of slow rusting conferred by Lr34 on grain yield and other agronomic traits. MATERIALS AND METHODS 1992-1993 Experiment Two wheat (Triticum aestivum L.) genotypes were used in the experiment: Jupateco 73R (CIMMYT accession BW17890), which carries Lr34, and Jupateco 73S (BW17891), which lacks it (Singh, 1992). The two genotypes are reselec- tions from the heterogeneous Mexican cultivar Jupateco 73, which originated by pedigree selection up to the F 6 generation, and therefore, could be considered a near-isogenic pair for Lr34. The experiment was sown on three planting dates, 26 Nov. 1992, and 15 and 30 Dec. 1992, under optimum manage- ment in the Yaqui Valley in Sonora State, Mexico, during the 1992-1993 crop season. The three plantings represented three different environmental conditions for crop growth with respect to day length and temperature. Leaf rust pressure was expected to be higher at earlier crop growth stages in later plantings. The experimental design was a split plot (with four replicates) with factor A (planting dates) being the main plots, and a combination of factor B (rusted and fungicide-protected treat- Published March, 1997