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Dunkeld and Rainbow: A tale of two Victorian canola cultivars

Nelson Gororo1, Phillip Salisbury1,2 and Steve Marcroft1

1 Department of Primary Industries, Private Bag 260, Horsham, Vic 3401, Australia. www.dpi.vic.gov.au
Email nelson.gororo@dpi.vic.gov.au
2 Institute of Land and Food Resources, University of Melbourne, Vic 3010, Australia. Email psalisbury@optushome.com.au

Abstract

Canola production in Australia has traditionally been dominated by cultivars with polygenic blackleg resistance. This polygenic resistance has generally been considered a durable resistance. However, there are suggestions that polygenic blackleg resistance in some cultivars has become less effective over time. Data from advanced breeding experiments and blackleg nurseries conducted by the Department of Primary Industries, Horsham, Victoria between 1996 and 2001 indicated that the grain yield and blackleg resistance of cultivar Dunkeld declined from 1999 to 2001, relative to the cultivar Rainbow. A change in pathogen virulence in the blackleg fungus is presumed to be the key factor in this decline.

Media summary

Changes in Leptosphaeria maculens populations enables the fungus to attack previously resistant cultivars. To maintain canola production levels, new resistant cultivars may be required at regular intervals.

Key Words

Blackleg, polygenic resistance, durability

Introduction

Canola (Brassica napus) is an important crop in Australia, with production of over 1.6 million tonnes each year, predominantly for the export market. A key objective of the National Brassica Improvement Program has been to develop lines adapted to a range of Australian environments, with emphasis on minimizing yield reduction caused by both biotic and abiotic stress. Breeding for resistance has been an effective strategy to reduce the impact of the fungal disease blackleg (Leptosphaeria maculans) in Australia. Breeding programs have traditionally relied on polygenic resistance derived from Japanese spring lines and French winter types (Roy 1978; Buzza 1979). This resistance is incomplete and high disease pressure can sometimes result in significant yield losses (Salisbury et al. 1995; Khangura and Barbetti 2001). This polygenic resistance has generally been considered a durable resistance, in contrast to the highly effective, major gene resistance derived from B. rapa ssp. sylvestris, that has been overcome by more virulent strains of L. maculans in areas of widespread use after only three years (Li and Sivasithamparam 2003).

Monitoring of resistance over time has shown that cultivars such as Rainbow (released in 1993) have been grown widely in Victoria for 10 years with no firm indications that the level of blackleg resistance has deteriorated, although regional and seasonal fluctuations in resistance have been observed. However, there are suggestions that polygenic blackleg resistance in other cultivars such as Dunkeld (also released in 1993) has become less effective over time. This paper compares the performance of two cultivars viz. Rainbow and Dunkeld, widely grown in Victoria from 1996-2001, with a view to detecting any shifts in disease resistance.

Methods

Blackleg resistance data

Data on blackleg resistance were derived from Victorian blackleg nurseries at Lake Bolac and Wonwondah, managed by the Department of Primary Industries (DPI), Horsham, Victoria between 1996 and 2001. Each nursery had 3 to 4 replicates. The characteristics of the cultivars Dunkeld and Rainbow are summarised in Table 1. Designation as resistant was based on commercial classification according to the Blackleg Resistance Ratings, BRR (Anon., 2001). The cultivars were screened for blackleg resistance according to the survival method described by Marcroft et al. (2002). Establishment counts were carried out at the seedling stage and survival counts were taken at maturity.

Grain yield data

For the yield assessment, data were used from advanced breeding experiments (Lake Bolac and Horsham) managed by the DPI, Victoria, breeding program between 1996 and 2001. The Victorian locations have a Mediterranean-type of climate, with canola sown in late autumn or early winter and harvested late spring or early summer.

Table 1. Characteristics of the cultivars Rainbow and Dunkeld used in these experiments

Cultivar

Pedigree

Blackleg resistance a

Designation b

Dunkeld

79N47-58/BLN329 [BJ168/Cresus-o-Precose//Norin20/ Tower /6/Chikuzen*2//Zephyr/Bronowski/5/Sv.62.371/Zephyr//Norin20/3/ Erglu/4/ BJ168/Cresus-o-Precose]

6.0

R

Rainbow

RZ6/Eureka [Mutu/3/Chisaya//Zephyr/Bronowski/4/ Haya//Zephyr/Bronowski/3/Chisaya//Zephyr/Bronowski]

6.5

R

a Blackleg Resistance Ratings based on BRR (Anon., 2001). 1=susceptible; 9=resistant

b R-Resistant

A linear model to describe the survival performance of plants under high blackleg disease pressure for the different genotypes was determined and included 2 factors, genotypes and environments. A random model was assumed and means and variance components for genotypes, environments and genotype x environment were estimated using the REML algorithm in GENSTAT (Payne et al. 1995).

Results and Discussion

The level of plant survival of the cultivars, in the presence of blackleg inoculum, was generally lower at Lake Bolac (high rainfall) than at Horsham. The level of plant survival under blackleg pressure, was influenced more by environmental factors than differences in plant genotype (Table 2). This reinforces previous conclusions that region and year have a major influence on the plant survival of the Victorian canola crop (Marcroft et al. 2002).

Table 2. Estimates of variance components (σ2) of environmental (σ2E), cultivar (σ2G), cultivar x environment (σ2GE) and residual (σ2e) factors for plant survival under blackleg disease pressure

Parameter

Plant Survival

Environmental variance component (σ2E)

395.2 ±118.3*

Genotype variance component (σ2G)

166.9 ± 85.9*

G x E interaction component (σ2GE)

57.3 ± 12.0*

Residual variance component (σ2e)

125.7 ± 9.5

* P<0.05

Although the environmental effect on plant survival exceeded the cultivar effect in this study (Table 2), significant variation between the two cultivars was also observed. Figure 1 demonstrates the variation in survival of Rainbow and Dunkeld across sites and years, with Dunkeld displaying higher plant survival than Rainbow from 1996 to 1998. By 1999, the survival of Dunkeld was significantly less (P<0.05) than that of Rainbow. This possibly indicates a change or shift in the pathogen population that rendered the previously resistant cultivar Dunkeld less resistant. The pedigrees of Dunkeld and Rainbow are quite distinct, each contain several possible sources of blackleg resistance. Dunkeld resistance is traced back to Chikuzen, Norin 20 and BJ168 (B. juncea) while Rainbow derives its blackleg resistance from Mutu and Chisaya (Salisbury and Wratten 1999).

Figure 1. Comparison of blackleg resistance (% plant survival) of Dunkeld and Rainbow in six environments from 1996-2001. Survival differences non-significant (ns) or significant at P <0.05 (*).

There were no significant differences in grain yield between Dunkeld and Rainbow from 1996 to 1998, particularly in the high rainfall environment (Lake Bolac) (Figure 2). Significant differences between the two cultivars (P <0.1) were observed at this location in 2000 and 2001 with Rainbow yielding significantly higher than Dunkeld. These results suggest that the grain yield decline of Dunkeld is due to its reduced resistance to blackleg. The superiority of Rainbow on grain yield over Dunkeld is not associated with better adaptation to the growing areas but is due to the existence of genes conferring superior performance under blackleg pressure.

Figure 2. Differences in grain yield (t/ha) of Dunkeld and Rainbow in six environments in 1996-2001. Yield differences non-significant (ns) or significant at P <0.1 (*).

Conclusions

There appears to have been a change or shift in the blackleg pathogen population causing the previously resistant cultivar Dunkeld to be less resistant, indicating that the pathogen population contains the necessary diversity of virulence to overcome such widely used polygenic resistances in Australia. This is also a strong indication that the polygenic resistance genes present in cultivars vary among genotypes. Despite these observations, the use of polygenic resistance is still the best strategy for breeding for durable resistance (Eagles 1988). However, while polygenic resistance should be more durable than monogenic resistance, evolutionary trends would favour more virulent or aggressive strains with higher levels of reproductive fitness on cultivars such as Dunkeld that were widely grown. Production losses will gradually increase as pathogen populations shift towards more virulent or aggressive strains. Concern regarding this narrow genetic base of commercial B. napus cultivars for blackleg resistance has spurred the search for new sources of resistance, particularly in related Brassica species. Examples include the release, in 2000, of cultivars derived from B. rapa ssp. sylvestris incorporating a single major gene for blackleg resistance. The cultivars were widely adopted by farmers because they performed well relative to other commercial cultivars. However, in 2002, this sylvestris resistance was overcome by L. maculans (Li and Sivasithamparam 2003) resulting in 90 % grain yield loss on the Eyre Peninsula, South Australia.

Reliance on a limited number of resistance genes against a highly diverse pathogen such as L. maculans may risk shortening the useful life of a resistance source. Efforts should be made to extend the life of those sources of resistance that have proven to be effective. Currently, cultivars are continually replaced by different cultivars, with few modern cultivars, including Rainbow, being used for longer than 5 years. An alternative strategy is to keep rotating cultivars with different resistance genes, so that the frequency of strains able to attack one particular resistance gene does not build up.

References

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Marcroft SJ, Purwantara A, Salisbury PA, Potter TD, Wratten N, Khangura R, Barbetti MJ and Howlett BJ (2002). Reaction of a range of Brassica species under Australian conditions to the fungus, Leptosphaeria maculans, the causal agent of blackleg. Australian Journal of Experimental Agriculture 42, 587-594.

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