Monogenic resistance in wheat (Triticum aestivumL.) to leaf rust, caused byPuccinia reconditaRob. ex. Desm. f. sp.tritici, has generally not been durable. Durable resistance, as well as an improved expression of resistance, have been associated withLrgene combinations. In this study, the assumption thatLr12andLr13may interact to condition improved resistance to leaf rust, was investigated. Four Thatcher (Tc) x RL6011 F3lines (13/12#x2013;3, 13/12#x2013;9, 13/12-19 and13/12#x2013;40), homozygous for bothLr13andLr12, were selected and their leaf rust resistance compared with the parents (CT263 =TcLr13 and RL6011 =TcLr12), the single gene linesTc/13#x2013;22and TC/12#x2013;16, and Thatcher. In addition to infection type studies in seedlings and adult plants, lines were compared according to macroscopic components of resistance, as well as disease ratings in the field. Flag leaf infection type studies showed thatLr12is effective against most pathotypes ofP. reconditaf. sp.triticioccurring in South Africa. Conversely,Lr13is ineffective against the dominant pathotypes, implying that the gene has no value as a monogenic source of resistance. Based on the fact that several pathotypes are avirulent to these genes, they should be manipulated with relative ease in local breeding programmes directed at utilizing these sources in combination with otherLrgenes. Infection types on the flag leaves of lines carrying bothLr12andLr13often displayed chlorosis and necrosis. These ratings on primary and flag leayes, as well as the quantitative components latent period, uredium density and uredium size, did not indicate clear differences between the digenic lines and the most resistant parent. In the absence of a pathotype virulent to both genes, the combination lines were resistant in the field. Data obtained were not conclusive in suggesting pronounced resistance enhancement due to combiningLr12withLr13.
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