**10. Durability of resistance to** *L. maculans*

achieved the same level of *B. juncea* resistance as the donor parent. Wide hybrids (interspe‐ cific, intergeneric or intertribal) have also been produced either by sexual crossing fol‐ lowed by embryo culture or by somatic hybridisation as a result of protoplast fusion to transfer genes for blackleg resistance [106, 107]. Previous studies have reported hybrids between *B. napus* and *Arabidopsis thaliana*, belonging to different tribes; the Brassiceae and Sisymbrieae, respectively [108]. These hybrids were further utilized for identifying genetic regions associated with blackleg resistance [49]. Two regions localised on chromosome 3 of

Crouch *et al.* [75] transferred genes for resistance to *L. maculans* derived from *B. rapa* subsp. *sylvestris* into *B. napus*, using a resynthesised amphidiploid, as a result of hybridisation between *B. rapa* subsp. *sylvestris* and *B. oleracea* subsp. *alboglabra*. As a result, several cultivars derived from the re-synthesized *B. napus* lines were released for commercial cultivation in Australia such as Surpass 400, Surpass 404CL, Surpass 501TT, Surpass 603CL, Hyola 43, and Hyola 60. The *R*-genes *LepR1*, *LepR2* and *LepR4* have also been introgressed into *B. napus* via conventional interspecific crosses [75, 109]. Introgression of genes for resistance to *L. maculans* from *Sinapis arvensis*, *Coincya momensis* and *B. juncea* into *B. napus* was attempted [110]. Hybrid derivatives of *B. napus* and *S. arevensis*, and *B. napus* and *C. momensis* showed a high levels of resistance at the seedling (cotyledon) and/or adult plant stages. The offspring from asymmetric hybrids between *B. napus* and *B. nigra, B. juncea* and *B. carinata* were analysed for the presence of B genome markers and resistance to *L. macu‐ lans* [111]. This study revealed that resistance is conserved in one triplicate region in the B genome. Often, the majority of wide-hybrid derivatives exhibit unwanted traits and low frequencies of recombination between the different species which complicate the develop‐ ment of *B. napus* cultivars resistant to *L. maculans* by traditional breeding [43, 47]. Link‐ age drag due to suboptimal/undesired genes can be eliminated using the application of high density genome-wide molecular markers such as SNPs [112]. However, Rouxel and Balesdent [93] cautioned that before important breeding efforts are devoted to introgres‐ sion of resistance genes from distant species into Brassica, there is a need thoroughly to evaluate their genetic control, putative redundancy and potential durability in the field.

Using transgenic technology, *R*-genes from other organisms can also be transferred irrespec‐ tive of natural barriers to crossing. However, it is possible that transferred genes may not always contribute novel resistance specificities to the transgenic crop. Although several approaches have been used to induce host resistance in plants [113, 114] no major breakthrough has been made for an efficient management of blackleg disease. For example, Hennin *et al.* [115] demonstrated the expression of *Cf9* gene, which confers *Avr9*-dependant resistance to *Cladosporium fulvum* in tomato, along with co-expression of *Avr9* produced increased resistance to *L. maculans* in transgenic *B. napus* plants. Manipulation of plant defense responses is resource-expensive [116] and may be deleterious to the plant. Plants need to be selected for both appropriate expression of beneficial defense responses and avoidance of unnecessary ones [117], making artificially-induced constitutive expression of these responses an imprac‐

tical solution to engineering resistance.

*A. thaliana* were shown to be linked with resistance to *L. maculans*.

96 Plant Breeding from Laboratories to Fields

Durable disease resistance can be achieved by utilisation of one or more single dominant *R*genes [118]. However, the effectiveness of the specific *R*-genes depends on the *L. maculans* population structure, i.e. on the frequency of the corresponding *Avr* allele, which is known to differ according to regions/countries [27, 94] and the rapid evolution of virulent pathotypes. For example, the mean number of virulence alleles per isolates was reported to be higher in Australia (5.11 virulence alleles) than in Europe (4.33) and Canada (3.46) [27]. It has been suggested that there is a fitness cost associated with pathogen evolution from avirulence to virulence to overcome host resistance [38, 119].

Previous research has shown that different qualitative gene sources for resistance vary in providing effective durable resistance over period of time. For example, Light *et al.* [120] reported that the adult plant survival of French winter lines such as Doublol (*Rlm1*), Capitol (*Rlm1, Rlm3*), Columbus\*1 (*Rlm1, Rlm3*), Carolus (*Rlm1, Rlm2, Rlm3*) and Rlm\_EX (*Rlm7*) was higher than the Australian cultivar, AV-Sapphire and concluded that French winter canola cultivars have effective resistance under Australian conditions.

Single resistance genes do not always provide a durable resistance as has been shown in a field experiment using the *Jlm1/Rlm6* gene introgressed into *B. napus* from *B. juncea* [121]. Several incidences on the breakdown/ineffectiveness of race-specific resistance genes in Surpass 400 (*(LepR3, RlmS)*), in Vivol and Capitol (*Rlm1),* and *Rlm6* genes in *Brassica* have been reported in literature particularly when they were grown extensively [34, 94, 122]. As a consequence, breeders have to develop new cultivars and replace 'old' cultivars in order to change pathogen specificity of *R-gene* even without the knowledge of comprehensive distribution of *Avr* genes. The latter is now feasible and being used in order to monitor the pathogen population [123]. In order to avoid selection pressure against a particular *Avr* gene in the pathogen population, pyramiding of several host *R*-genes and deployment of quantitative resistance is being practiced in several crops such as in wheat, and barley. However, this strategy has not resulted in greater durability of resistance [124, 125]. In contrast, a recent study [121] demonstrated that a major *R*-gene (*Rlm6*) is more durable when expressed in a genetic background that also has quantitative resistance, indicating the need to identify and combine both qualitative and quantitative loci for blackleg resistance. Although the proposed strategy may be useful for blackleg disease management in areas where 'less' disease pressure and low variability with *L. maculans* populations exists, in Australia polygenic resistance derived from the French cultivar Jet Neuf [87], was reported to become less effective over time [37]. Additionally, several Australian cultivars which are reported to harbour both qualitative and quantitative loci for blackleg resistance are susceptible to natural populations of *L. maculans* Delourme et al [99]. It is difficult to know whether this evolution results from a change in virulence, or in aggres‐ siveness in the pathogen populations since these polygenic-resistance cultivars may also carry specific *R*-genes [99]. In order to keep the frequency of isolates virulent towards any race– specific gene under a 'threshold' level, an integrated approach based upon best farm practices such as crop rotation, stubble management, application of fungicides and deployment of resistance genes including rotation of race-specific genes [126] needs to be implemented for sustainable canola production, especially in areas where *L. maculans* populations are highly diverse and rapidly evolving.
