Peer-review: Yes - Open Access: Yes|http://science.thomsonreuters.com/cgi-bin/jrnlst/jlresults.cgi?PC=MASTER&ISSN=0014-2336

Two spring barley composites, one based on eight West-European two-rowed cultivars (A) and the other, the predominantly six-rowed composite XXI, based on several thousands of barley cultivars (B), formed the starting point of a recurrent selection procedure. The aim was to study how effective a repeated process of mild selection against susceptibility followed by recombination of the remaining material was in accumulating partial resistance in four host-pathogen situations.|i) Only partial resistance is present and the pathogen population is defined (a given race).|ii) Partial and major gene resistance both occur and the pathogen population is defined.|iii) Only partial resistance is present and the pathogen population is not defined; a mixture of races that varies over the years.|iv) Partial and major gene resistance are both present and the pathogen population is not defined.|The variation in partial resistance to barley leaf rust was large in both populations. Population A carried no effective major resistance genes, population B possibly a few at low frequency. The variation in partial resistance to powdery mildew was moderate in A and possibly large in B. As far as effective major resistance genes is concerned A did not carry any, although some recombinations of defeated genes might have been partially effective, while B seemed to carry many. During the selection procedure the populations were always exposed to race 1-2-1 of barley leaf rust and to the mixture of powdery mildew races that was naturally present. The selection procedure consisted of three cycles of recurrent selection. In the initial heterogeneous populations (S0) single plant selection was applied followed by line selection the next year. In both populations 12 lines were selected that were intercrossed in all directions within the two populations. Again single plant (S3) and line selection were exercised and 12 lines selected. The 12 A-lines were intercrossed in all directions with the 12 B-lines and the recombined population again exposed to single plant (S6) and line selection (S7). The selection in this population was done within two-rowed entries (A*) and within six-rowed entries (B*).|The selection pressure was mild. In each selection stage about 30% plants or lines most affected by barley leaf rust and some 30% plants or lines most affected by powdery mildew were removed. Among the remaining plants or lines (ca. 45%) a selection for other useful agronomic characteristics was applied. The response to selection was measured in four evaluation trials. Both single plant and line selection contributed to the progress in resistance in both populations to both pathogens.|The gain in partial resistance to barley leaf rust was the same in the two populations. The average amount of sporulating leaf tissue in the S7 was about twenty times less than that in the S0. The best S7 lines showed a sixty fold decrease compared with the S0. Corrected for the levelling effect of interplot interference, very strong with barley leaf rust, these gains become 300-and 900-fold respectively. This is comparable with a gain from a very susceptible cultivar to one which is resistant enough to prevent any significant damage in Western-Europe, even in barley leaf rust conducive years. The gain in resistance to powdery mildew from S0 to S7 was far less, being only fourfold and, after correcting for the interplot interference effects in the order of ten- to thirtyfold. The A population contributed more to this gain than the B population, despite its smaller genetic variation. This was caused by the very small response to selection in the B-population in the first two cycles of recurrent selection.|The data clearly indicate that recurrent mild selection against susceptibility is a powerful method to accumulate partial resistance. This occurred most efficiently when no confounding major, race-specific resistance genes were present and when a defined pathogen population was used. Little progress was obtained when the host population contained major race-specific resistance genes and was exposed to a racial mixture.

English

Springer

Carelia Juarez

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