Introduction

A collaborative project involving the Brandon Research Centre, University of Saskatchewan and Cereal Research Centre was initiated in 1992 with funding from the Western Grains Research Foundation, Cargill Research Fund, and Brewing and Malting Barley Research Institute to develop barley germplasm with resistance to QCC and other common stem rust (Puccinia graminis) races as quickly as possible for use in breeding programs. After three years, the project was extended for another three-year term with funding from the Western Grains Research Foundation. Since 1997 was the final year of our project, the purpose of this report will be to briefly summarize our results.

Procedure

The main component of the project was a barley stem rust nursery established in the field at the Brandon Research Centre during the first year of the project and operated each year for the remainder of the project. The purpose of the nursery was to identify and evaluate potential new sources of QCC resistance and screen breeding material developed from crosses to these new sources of resistance. An average of approximately 9,900 multi-seeded hill plots and 9,600 spaced plants were grown each year in the nursery. The multi-seeded hill plots consisted of breeding lines from F₃ to advanced lines in replicated yield tests and foreign accessions. In general, lines with promising stem rust resistance in previous nurseries were replicated 2 to 3 times, while lines being tested for the first time were included only once. Spaced plants were usually from F₂ to F₄ bulk populations. Selection was generally less reliable for spaced plants than multi-seeded hill plots because the stem rust usually overwhelmed the spaced plants to a greater extent.

The barley stem rust nursery at Brandon was successful during all 6 years of its operation in screening barley germplasm from the Brandon Research Centre and the University of Saskatchewan for resistance to stem rust race QCC. The stem rust nursery was artificially inoculated with QCC spores every year since there was little natural inoculum. In all years, a severe epidemic of QCC developed in the nursery with much of the barley germplasm being susceptible. However, a differential response was obtained making it possible to select for QCC resistance. Initially, efforts were focussed on identifying and evaluating potential new sources of QCC resistance both within our breeding material and from foreign accessions. By the end of the project, the focus was on screening breeding material derived from our current sources of QCC resistance.

Results and Discussion

When this project was started in 1992, stem rust race QCC comprised a high proportion of the natural stem rust population and posed a serious threat to barley production in the eastern prairie region of western Canada since all of our registered barley cultivars were susceptible. However, no stem rust epidemics have occurred in western Canada since that time due in part to unfavourable conditions further south in the stem rust corridor. Since stem rust overwinters in the southern Great Plains region of the United States, environmental conditions between that region and western Canada strongly influence the spread of stem rust northward. In 1996 and 1997, the proportion of QCC declined sharply in the natural stem rust population to less than 5%. This was probably due to the introduction of QCC resistant wheat cultivars into the southern Great Plains region. Since little barley is grown there, QCC inoculum appears to have little opportunity to build up again. Although QCC no longer appears to be a serious threat to western Canadian agriculture, it has shown us how vulnerable the barley crop can be when a new virulent race of stem rust appears. The Rpg1 gene has protected barley from stem rust for over half a century so efforts to broaden the genetic base of stem rust resistance in barley were long overdue. This project has enabled us to make a good start in this process.

Over the course of the project, the number of QCC resistant lines identified in all breeding programs increased dramatically. We have made good progress in incorporating both primary sources of resistance, such as Q21861, PC11, Diamond and PI382313, and secondary sources, such as SB90139, SB90149, SB90585, SB91702, SB91709 and Maud, into our germplasm. Many of the resistant lines identified in the nursery were derived from Q21861, although Q21861 itself has performed inconsistently over the years. Q21861 is a barley line developed at CIMMYT in Mexico, but selected in Australia for stem rust resistance. It has the Rpg1 gene, rpg4 and possibly a third recessive gene. The breakdown of Q21861's resistance to QCC under some conditions may be due to temperature sensitivity of the rpg4 gene. It is encouraging that an increasing number of resistant lines were obtained using other sources of resistance, which will diversify the genetic base of resistance and reduce the vulnerability of barley to stem rust in the longer term. Background genetic effects from both the resistant and susceptible parents appear to be important in the expression of QCC resistance. Some derivatives of Q21861 have better QCC resistance than it does, while in other crosses few lines are as resistant as Q21861. A number of lines with good QCC resistance have been obtained from crosses involving PC11 or PI382313 derivatives at the Brandon Research Centre. IBON18-75, another CIMMYT line which shows promise as a new source of QCC resistance, has been successfully used by the Crop Development Centre.

All breeding programs have some lines with at least partial resistance to QCC in replicated yield trials, but a new cultivar with QCC resistance will not be released for at least another year or two. Improvements in agronomic characteristics, quality traits and disease resistance may be needed to complement their QCC resistance. Some of these QCC resistant lines that failed to become registered cultivars are being used as parental material in the breeding programs.

One of our concerns at the outset of the project was that the stem rust resistance sources being used in barley did not confer the same level of resistance to QCC as found in the hard red spring wheat cultivars grown in western Canada. We did not know if this level of resistance would be adequate to protect barley from yield and quality losses due to stem rust. The study that we conducted to answer this question revealed that under conditions of severe stem rust epidemics, yield losses in Q21861, Q/SM-041 (Q21861 derivative with Rpg1 and rpg4) and BM8923-46 (PI382313 derivative with Rpg1 and Rpg3) were about 12%, while those for susceptible cultivars, such as Harrington, Bonanza and Robust, ranged from 30 to 37%. Klages was the most susceptible barley cultivar with yield losses of 53% approaching that of the susceptible wheat cultivar AC Reed. Kernel plumpness was the most sensitive kernel characteristic affected by stem rust. Only Q21861 was able to maintain its kernel plumpness, although Q/SM-041 and BM8923-46 did not suffer as greatly as the susceptible cultivars. This has important implications since kernel plumpness is an important trait in the selection of malting barley by the malting and brewing industry. Reduction in kernel weight was the trait most closely related to stem rust resistance, while test weight seemed to be the least affected. Since natural stem rust epidemics would not likely be as severe as in this test, it was concluded that the levels of stem rust resistance found in such lines as Q21861, Q/SM-041 and BM8923-46 would be adequate to protect barley from yield and quality losses.

One way to bolster the stem rust resistance of barley may be gene pyramiding. We attempted to test the utility of gene pyramiding by evaluating a number of different combinations of QCC resistance developed at Brandon and Saskatoon. Although the data were inconclusive, our tests suggest that the frequency of resistant lines in these combinations is greater than in crosses segregating for only one source of resistance, and the actual level of QCC resistance may also be enhanced in some combinations beyond the parental values. However, the magnitude of this improvement does not appear to be very large in most cases, and may not be any better than the best lines obtained in crosses with only one resistant parent. Some of the lines from these combinations were very poor agronomically which may have affected their resistance ratings. Background genetic effects may also determine how effective different combinations are in improving QCC resistance. Of note were several segregants from a Brandon cross involving Q21681 and PC11 that had lower ratings than either parent. Other promising combinations were Q21861 and PI382313 derivatives from Brandon, and Q21861 derivatives and SB90585 from the Crop Development Centre. Further testing over a number of years will be needed to confirm the utility of these and other combinations.

In 1997, the sixth and final backcross was completed for the modified backcrossing program being conducted at Brandon to incorporate the Q21861 source of resistance into AC Metcalfe. Molecular marker assisted selection will greatly facilitate the process. We plan to extract QCC resistant lines for agronomic and quality evaluation as soon as possible.

Screening breeding populations by inoculating seedlings with QCC spores indoors was not effective enough to utilize in a practical barley breeding program. In general, only a small number of the selected lines showed resistance to QCC in the stem rust nursery during the following summer.

An indirect benefit of this project was the identification of a novel scald resistance gene in PC11 which confers resistance to the highly virulent Rhynchosporium secalis isolate, WRS 1860, recently found in Alberta. This new isolate attacks many of the scald resistant cultivars currently grown in western Canada.

Conclusions

The results of the barley stem rust project indicate that we have made significant progress over the past 6 years in improving the stem rust resistance of our barley germplasm. However, QCC resistant barley cultivars will not likely be available for several years. The level of protection offered by our sources of QCC resistance appears adequate to protect barley. Although there has not been a stem rust epidemic in western Canada over the past 6 years and QCC has declined significantly in the natural stem rust population due to the introduction of QCC resistant wheat cultivars in the southern Great Plains region of the United States, this project has enabled us to broaden the genetic base of stem rust resistance in barley. QCC resistant germplasm will be available from the collaborators upon request.