SELECTION METHODS FOR BLACKLEG RESISTANCE IN AUSTRALIA

W.A. Burton¹, S.J. Pymer¹, S.J. Marcroft¹, P.A. Salisbury ^1,2 and D.J. Ballinger³

¹Agriculture Victoria, Victorian Institute for Dryland Agriculture, Private Bag 260, Horsham Victoria 3401 Australia

²Institute of Land and Food Resources, The University of Melbourne, Parkville Victoria 3052 Australia

³DOVURO Pty Ltd, RMB 3775, Horsham Victoria 3401 Australia

email: wayne.burton@nre.vic.gov.au

ABSTRACT

Blackleg, caused by the fungus Leptosphaeria maculans, is the most important disease of canola in Australia and is a major breeding priority for the Australian breeding programs. The major component of resistance in Australia has been resistance to the adult stem canker. A lack of correlation between seedling resistance and adult plant resistance highlighted the need to select under field conditions against the prevailing mixture of races. Blackleg nurseries are sown on canola stubble from the previous season to maximise the level of infection. In the Agriculture Victoria breeding program, a modified pedigree method of selection has generally been used to improve resistance, although recurrent selection is also being used. Parents for crossing are usually single plant selections from the blackleg nursery. Segregating F₂and F₃ material is screened in blackleg nurseries, allowing a high proportion of material to be rejected due to insufficient resistance. The selections are either totally free from, or have very low levels of, stem infection. After two generations of single plant selection, material is then evaluated as bulks. However, further single plant selection to enhance resistance can also occur in F₅ and F₆generations. The polygenic nature of resistance means that the level of resistance in progeny of single plant selections can be variable. Through breeding, the current Australian varieties are now the most resistant spring varieties in the world. With the increasing pressure from blackleg associated with the major increase in canola area in Australia, studies to identify resistance sources which combine enhanced seedling resistance with good adult plant resistance, have been initiated.

KEYWORDS: Leptosphaeria maculans, canola, seedling resistance, adult plant resistance, modified pedigree method, polygenic

INTRODUCTION

Blackleg caused by the fungus Leptosphaeria maculans, is the major disease of canola in Australia. In the early 1970’s blackleg almost decimated the rapeseed industry (Barbetti, 1975; McGee and Emmett, 1977). The introduction of blackleg resistant varieties has been one of the major factors of the resurgence of the canola industry in Australia. In 1993 the canola area was 175,000 ha and in 1999 the area is expected to reach 1.8 million ha. Australian varieties are the most resistant spring B. napus varieties grown worldwide.

The Agriculture Victoria breeding program is based on the selection for resistance to blackleg at the adult plant stage (i.e. resistance to adult stem canker). Selection for resistance at this stage has been due to a lack of correlation between seedling (i.e. cotyledon) resistance and adult plant resistance under Australian conditions (Ballinger and Salisbury, 1996). Salisbury and Ballinger (1993) and Ballinger and Salisbury (1996) showed that seedling resistance and adult plant resistance were under different genetic control. A lack of correlation between seedling and adult plant resistance has also been observed by Roy (1984), Badawy et al. (1991) and Zhu et al. (1993). As seedling and adult plant resistance to blackleg are independent, selection for blackleg resistance needs to be based on adult plant reaction to races that are present in field conditions. The selection of seedling resistance will only be useful if both characters are required (Ballinger and Salisbury, 1996).

This paper explains the methods used by the Agriculture Victoria breeding program to successfully develop blackleg resistant canola cultivars.

BREEDING METHODS AND DISCUSSION

A modified pedigree method of selection is used in the Agriculture Victoria breeding program (Figure 1).

Potential parents for crossing are generally selected as single plants from blackleg nurseries sown on canola stubble from the previous season to maximise the level of infection (Salisbury et al., 1995). Multiple selections are taken from parental lines in the nursery and the highest quality, resistant selections from each particular line are bulked together to be used as a parent in crossing.

Single plant selections are taken at the F₂ and F₃ stages (Figure 1). These selections are free or virtually free of the disease. Individual plants are cut at ground level and stems examined for absence of disease. This selection method is based on the work of Ballinger and Salisbury (1995), where progeny testing was used to evaluate the relative effectiveness of canola parental plants selected for differing in degrees of infection, as a means of improving blackleg resistance. They showed that the progeny of selections with little or no blackleg had higher levels of resistance compared with progeny of plants that survived but had higher levels of infection.

Selections with a higher level of infection are accepted when:

(a) developing a new type (eg. new maturity type, quality type or herbicide resistant type), where the most resistant plants are kept for further crossing or selection, even though they have lower levels of resistance.

(b) crossing with overseas spring lines for quality improvements where very few if any disease free plants are produced, hence the need to select plants with some level of infection.

During the F₂ and F₃ stages of selection, quality and agronomic characteristics are also tested before the selections are re-sown. The F₄’s are preliminary yield tested and seed increased. Breeding lines are then yield tested at multi-locations in the F₅ and re-tested for blackleg resistance (Figure 1).

A significant proportion of F₅ – F₉ lines are rejected as having insufficient resistance (i.e. less than commercial controls), even though they have been selected as free of disease in F₂ and F₃ nurseries (Table 1). Data from blackleg nurseries sown in Victoria in 1996 – 1998, has shown that less than 50% of advanced F₅ – F₉ lines have resistance that is equal or better than the industry standard, the resistant cultivar Dunkeld.

Table 1. The number of Agriculture Victoria breeding lines (F₅ – F₉ generation) evaluated for blackleg resistance from 1996 – 1998 and the proportion of lines which were highly resistant.

	1996^a	1997^a	1998^b	Mean
Number of F₅–F₉ lines evaluated	225	122	153	167
Number of lines with adequate blackleg resistance (i.e. ³ cv. Dunkeld)	105	58	79	81

Percentage (%) of highly resistant lines	47%	47%	52%	49%

^aBlackleg nursery sown at Lake Bolac, Victoria, (Australia)

^bBlackleg nursery sown at Black Range, Victoria, (Australia)

Figure 1. Schematic overview of the Agriculture Victoria breeding program.

The consistently high proportion (on average 51%) of F₅ – F₉ lines that are rejected as having insufficient resistance (Table 1) can most likely be attributed to the polygenic nature of adult plant resistance (Thompson, 1983).

Reselection is also used at F₅ – F₉ stages to further enhance the resistance of these lines. These selections then re-enter the program at the preliminary yield trial stage, and if competitive enough they have the potential to be further re-selected (Figure 1).

In conjunction with the breeding program, research to identify alternative sources of blackleg resistance is also underway. Resistance sources which combine enhanced seedling resistance with good adult plant resistance are being sought to cope with the increasing pressure from blackleg associated with the major increase in canola area in Australia.

CONCLUSIONS

Blackleg, caused by the fungus L. maculans, is the most important disease of canola in Australia and is a major breeding priority for Australian breeding programs. A modified pedigree method of selection developed at VIDA and based on field selection has enabled the development of Australian varieties which are now the most resistant spring varieties in the world.

ACKNOWLEDGEMENTS

This research project has received financial support from the Grains Research and Development Corporation. The assistance of Mr David Robson, Mr Darryl Pearl, Ms Rachael Sullivan and Ms Leisa White for the field operations is acknowledged.

REFERENCES

Badawy, H.M.A., Hoppe, H.-H., and Koch, E. (1991). Differential reactions between the genus Brassica and aggressive single spore isolates of Leptospheria maculans. Journal of Phytopathology 131, 109-119.

Ballinger, D.J. and Salisbury, P.A. (1995). Selection techniques for blackleg resistance in canola. In: Proc. 10th Australian Research Assembly on Brassicas, Struan, South Australia, pp. 130-132.

Ballinger, D.J. and Salisbury, P.A. (1996). Seedling and adult plant evaluation of race variability in Leptospheria maculans on Brassica species in Australia. Australian Journal of Experimental Agriculture, 36, 485-488.

Barbetti, M.J. (1975). Effect of temperature on development and progression in rape of crown canker caused by Leptospheria maculans. Australian Journal of Experimental Agriculture and Animal Husbandry 15, 705-708.

McGee, D.C. and Emmett, R.W. (1977). Blackleg of rapeseed in Victoria: crop losses and factors which affect disease severity. Australian Journal of Agricultural Research 28, 47-51.

Roy, N.N. (1984). Interspecific transfer of Brassica juncea-type high blackleg resistance to Brassica napus. Euphytica 33, 295-303.

Salisbury, P.A. and Ballinger, D.J. (1993). Evaluation of race variability in Leptospheria maculans on Brassica species in Australia. In: Proc. 11th Australian Research Assembly on Brassicas, Wagga Wagga, NSW pp. 107-111.

Salisbury, P.A., Ballinger, D.J., Wratten, N., Plummer, K.M. and Howlett, B.J. (1995). Blackleg disease on oilseed Brassica in Australia: a review. Australian Journal of Experimental Agriculture, 35, 665-672.

Thompson, K.F. (1983). Breeding winter oilseed rape, Brassica napus. Advances in Applied Biology 7, 101-104.