Studies on Puccinia recondita f. sp. tritici with special emphasis on adult plant resistance in wheat.

Abstract

Leaf rust (Puccinia recondita f.sp. tritici) of wheat (Triticum aestivum) was widespread in South Africa during 1983, 1984 and 1985 and often reached epidemic levels, especially on autumn-sown spring wheat in the Cape Province. Nine physiologic races were identified during the study period. The most common race was avirulent to the leaf rust differential genes Lr3a, 3bg, 3ka, 11, 16, 20 and 30 and virulent to Lr1, 2a, 2b, 10, 14a, 15, 17, 24. Resistance genes Lr9, Lr19, Lr21 and Lr26 were effective to all isolates tested. Evaluation of wheat genotypes for components of resistance, viz. infection type, latent period, number of uredinia and uredinium size, revealed three phenotypic reaction classes. The first group exhibited negligible resistance, the second was susceptible or moderately susceptible as seedlings but resistant as adult plants while the third group was resistant at all growth stages tested. Adult plant resistance was expressed by hypersensitive or non-hypersensitive reactions and the combination of components conditioning resistance varied. Adult plant resistance conferred by gene Lr22a was characterized by a long latent period, small uredinia, reduced sporulation and an absence of a differential interaction between components of resistance and different races of Puccinia recondita f.sp. tritici. Numbers of uredinia on flag leaves of RL6044 (Lr22a) were equal to those of a susceptible check, Line E. Lr22a was inherited as a partially recessive gene in crosses with Zaragoza and SST33. Assessment of latent period, number of uredinia and infection type in F4 and FS families homozygous for Lr22a and derived from crosses between RL6044 and Zaragoza or SST33, revealed significantly different levels of resistance between families. Differences were attributed to other genes modifying the expression of Lr22a. Adult plant resistance of Era, Glenlea, RL6044 and sinton was expressed prior to the fifth-leaf stage. Latent period increased and number of uredinia decreased as each wheat matured. While the latent period of the flag, flag-l and flag-2 leaf was similar within Era, Glenlea and RL6044, differences between these genotypes occurred. The latent period of flag leaves of Sinton was shorter than that of the two lower leaves. Significantly fewer uredinia developed on the flag-2 leaf of Glenlea. A reduction in temperature from 21 C to 15 C significantly increased latent period in Era, Glenlea and RL6044, and also restricted uredinium size on flag leaves of RL6044. The adult plant resistance of Glenlea crossed with Line E was conferred by two partially recessive genes. Additionally, F2 to FS progenies of this cross eXhibited high levels of hypersensitive seedling resistance at 29 - 31 C to certain isolates. The latter resistance was not conferred by Lr1 or by the LrT2 gene for mature plant resistance in Glenlea. The high-temperature expression of resistance could be due to a second gene for adult plant resistance or to a previously undetected seedling gene.