Preparation for future white rust epidemics in Brassica juncea in Western Australia C. X. Li A,, K. Sivasithamparam B, G. Walton C, P. Fels C, M. J. Barbetti A, C A School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia. B School of Earth and Geographical Sciences, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia. C Department of Agriculture and Food Western Australia, Baron-Hay Court, South Perth, W.A. 6151, Australia ABSTRACT White rust (Albugo candida) is a highly destructive disease of cruciferous vegetable and oilseed crops. It has caused serious yield losses in Brassica juncea in India and is a potential threat to the emerging canola quality B. juncea industry in Australia. To prepare for future white rust epidemics in Brassica juncea in Australia, sources of host resistance urgently need to be identified. Forty-three B. juncea genotypes, viz. 22 from India, 12 from Australia and 9 from China, were tested in field trials in Western Australia. Varying levels of host resistance to Australian strains of A. candida (race 2) were identified among the genotypes from the three countries. Genotypes CBJ-001, CBJ-003 and CBJ-004 from China consistently showed high levels of resistance to A. candida across all trials and these could be used as sources of resistance for B. juncea breeding in Australia. Differentiation of resistance among these genotypes under field conditions after artificial or natural inoculation was similar to that obtained under glasshouse conditions. Our studies demonstrated that controlled environmental conditions are suitable for rapid identification of resistant genotypes. In addition, we found that disease screening was equally successful in characterizing varying levels of host resistance when assessments were either made using disease incidence as the assessment trait or when using disease severity as the assessment trait. Keywords: Brassica juncea, germplasm, Albugo candida, white rust, resistance, screening INTRODUCTION White rust is caused by fungal pathogen Albugo candida. It is a highly destructive disease of many economically important cruciferous vegetables (Williams and Pound 1963) and oilseed crops (Harper and Pittman 1974). Significant yield losses from this disease have been reported on Brassica juncea and B. rapa and on some susceptible B. napus lines in India, China and Australia (Harper and Pittman 1974; Verma and Petrie 1980; Barbetti 1981; Fan et al. 1983; Mukherjee et al. 2001). The estimated yield losses caused by combined infection of leaf and inflorescence are up to 60% or more in India (Lakra and Saharan 1989), and up to 20% in Australia (Barbetti 1981; Barbetti and Carter 1986). B. juncea is the predominant oilseed Brassica species sown in India (Kumar et al. 2000) and an important crop in some regions in China (Wang et al. 2007). Most commercial varieties of B. juncea in China and India are susceptible to this pathogen. The area grown B. juncea in Australia is increasing rapidly as the canola-quality B. juncea has been developed to extend oilseed production into the lower rainfall areas (Burton et al. 2003; 2007b). It has been demonstrated that the most efficient and cost effective way of disease management is through host resistance. Therefore, it is essential to identify useful sources of resistance in B. juncea, not only for Australia, but also for China and India (1999; 2007a). The aims of this paper were: 1), to determine the differential host responses to white rust in genotypes of B. juncea from Australia, China and India, under Australian field conditions; 2), to determine if these host responses were consistent across different field trials where different inoculation techniques (natural and artificial inoculation) were applied; 3), to compare the results from field trials with those obtained in previous glasshouse studies.
MATERIALS AND METHODS Seed was obtained from India, China and Australia through the Australian Centre for International Agricultural Research (ACIAR) programme. Genotypes of B. juncea from three countries were screened under the Western Australian field conditions against white rust disease (A. candida). Twenty seeds per genotype were sown in single rows of 1 m length and plants were not thinned after germination. There was 0.6m spacing between rows. Rows of testing genotypes were arranged in a randomized complete block design with five or six replications. Field trial one was undertaken in an experimental field block at the University of Western Australia, Crawley, Perth, during the 2006 cropping season. Forty-three rows of seedlings were spray-inoculated twice with a white rust zoosporangial suspension with concentration of 1 x 10 5 zoosporangia ml -1. Field trial two was carried out in a screen house at the University of Western Australia, Shenton Park Field Research Station. Thirty-eight lines of B. juncea were tested to natural white rust infection in the field. Disease incidence (percentage of leaves infected) and the disease severity (percentage of leaf area covered by white rust pustules) were assessed five/four times during the growing season using a 0-6 scoring system modified from Singh et al. (1999) and Mukherjee et al. (2001). The number of plants with staghead for each row was counted and the percentage of plants with staghead was calculated at the end of growing season. The area under disease progress curve (AUDPC) was calculated separately for each parameter according to formula described in (Campbell and Madden 1990). A single factor analysis of variance was conducted using Genstat (9 th Edition, Lawes Agricultural Trust, UK) for AUDPC of disease incidence, of disease severity and of the percentage of plants with staghead. Fisher s least significant difference (LSD) at 95% significant level was used to test the differences among genotypes. The correlation of disease levels across the different field trials were predicted by regression analysis. RESULTS Field trial one: There were significant differences (P < 0.001) among genotypes in relation to the AUDPC for both disease incidence and disease severity following artificial inoculation. Based on disease incidence and severity, the most resistant genotypes were CBJ-001, CBJ-003 and CBJ-004 from China, and JR049 and JR042 from Australia. Overall, the genotypes from India were more susceptible than those from Australia and China, with RL1359 and Seetha the most susceptible genotypes (Table 1). There were significant differences (P < 0.001) among genotypes in relation to the percentage of plants with staghead. Genotypes MPIR and TABP-15 from China, JO006 from Australia, Sanjucta Ascsh and Vardan from India were among the genotypes that had the lowest percentage (< 74%) of plants with staghead. In contrast, genotypes XINYOU 9, XINYOU 8 and CBJ-004 from China were among the genotypes that had a high percentage of plants with staghead (Table 1). Field trial two: There were significant differences (P < 0.001) among genotypes in relation to both the disease incidence and the disease severity following natural field inoculation. Genotypes CBJ-001, CBJ-002, CBJ-003 and CBJ-004 from China were the most resistant genotypes, with zero scores for both disease incidence and disease severity. The next most resistant genotypes were MPIR and XINYOU8 from China and JM18 from Australia. The most susceptible genotypes were RH30, RL1359 and Seetha from India (Table 2). There were significant differences (P < 0.001) among genotypes in relation to the percentage of plants with staghead. Genotypes MPIR, XINYOU 5 and TABP-15 from China, Vardan and Sanjucta Ascsh from India were among the genotypes that had lowest AUDPC percentage of plants showing staghead (< 23). Genotypes Prakash from India, XINYOU 8 from China and JN033 from Australia were among the genotypes that had greatest AUDPC percentage of plants with staghead (> 100) (Table 1).
Table 1: White rust development in 43 or 38 Brassica juncea genotypes from Australia, China and India grown under field conditions after inoculation with zoospangia suspension of Albugo candida or after exposure to natural inoculation of Albugo candida. The disease incidence and disease severity were assessed five times throughout the growing season and the area under disease progress curve (AUDPC) was calculated. The percentage of the plants with staghead was calculated based on the total number of plants of a genotype and the number of plants with staghead of the genotype at plant maturity. Genotype Source Field Trial One Field Trial Two AUDPC AUDPC AUDPC AUDPC AUDPC AUDPC Incidence Severity staghead Incidence Severity staghead CBJ-001 China 23.6 5.9 211.8 0 0 0 CBJ-002 China 79.5 71 292.8 0 0 5.9 CBJ-003 China 13.8 5.9 352.1 0 0 9.1 CBJ-004 China 16.4 4.9 462.3 0 0 9.1 GM1 India 328.9 190.8 111 141 138.08 13.2 JM16 Australia 225.5 80.1 401.2 - - - JM18 Australia 229.6 94.4 151.7 79.33 32.33 16.5 JN004 Australia 245.3 135 144.5 123.42 77.33 22.2 JN010 Australia 243.3 141.8 231.6 122.42 81.25 24.9 JN028 Australia 278.9 144.5 88.8 93.08 50.92 25.6 JN031 Australia 227.2 107.4 395.4 - - - JN032 Australia 262.6 113.2 259.3 -- - - JN033 Australia 262.9 126.4 396.6 99.92 48 26.6 JO006 Australia 258.7 122.8 61.4 93.08 54.92 28.9 JO009 Australia 198 88.3 38.5 - - - JR042 Australia 97.3 30.8 87.1 - - - JR049 Australia 68.8 25 88.6 - - - Kranti India 357.4 298.2 146.1 152.5 136.08 37.2 MPIR China 220.7 66.5 30.9 68.58 28.42 37.2 NDR8501 India 345.1 235.1 160.7 148.67 140.92 38.4 PBR91 India 346.3 254.1 88.8 152.5 138.92 39.9 PBR97 India 337.4 230.4 129.8 142.92 131.25 42.7 PCR7 India 344.6 270.7 210.5 142.92 129.25 47.2 Prakash India 321.7 272.7 174.1 141 118.58 47.2 RH30 India 372.1 243.8 86.1 156.33 152.5 48.7 RH781 India 340.3 243.9 87.6 144.83 137.08 49.4 RH8113 India 351.6 270.6 73 146.75 139 50.7 RH819 India 321.2 225.1 154.1 146.75 138 53.1 RH8812 India 315.1 170.5 103.2 141 113.67 56.7 RL1359 India 374.5 300.8 146 150.58 143.83 57.4 RLM619 India 367.8 238.9 105.1 146.75 134.08 59.3 Rohini India 356.9 264 201.3 144.83 135.08 62.6 RRN-593 India 340.5 212 159.6 141 110.75 63 Sanjucta Ascsh India 362.2 268.1 34.5 148.67 144.83 150 Seetha India 367.8 309.5 96.4 146.75 142.92 65.8 Sej2 India 302.1 202.5 99.7 144.83 141.92 66.1 TABP-15 China 239.5 129.9 74 88.17 35.25 71.8 Vaibhav India 319.5 231.8 71.3 142.92 128.33 72.9 Vardan India 364.4 270.6 73.9 141 129.33 84.9 Varuna India 344.4 237.9 85.3 142.92 134.17 89.5 XINYOU4 China - - - 80.42 36.33 89.7 XINYOU5 China 174.5 43.6 240.7 105.75 50.92 98.1
XINYOU8 China 170.3 53.4 411.2 74.5 33.33 110.6 XINYOU9 China 292.4 134.6 480 121.5 72.5 146.1 Significance (P <) 0.001 0.001 0.001 0.001 0.001 0.001 l.s.d.(p 0.05) 71.06 62.42 167.25 14.783 14.712 42.45 Correlations of disease levels from the different trials Disease incidence from field trial one was significantly and positively correlated with the disease incidence in field trial two (r = 0.96, P < 0.001, n = 38). Similarly, disease severity in field trial one was significantly and positively correlated with the disease severity in field trial two (r = 0.94, P < 0.001, n = 38). The percentage of plants with staghead from two trials was also significantly correlated (r = 0.39, P < 0.05, n = 38). DISCUSSION This study showed that the host response of B. juncea genotypes to A. candida could be readily differentiated from screening germplasm using either artificial or natural field inoculation. The overall results from field trial one was significantly correlated with that from field trial two, indicating that the level of host response of B. juncea genotypes to A. candida to artificial inoculation was similar to that of natural inoculum. The key findings from our two separate field trials also conformed with the results of our previous glasshouse studies (Li et al. 2007b). Hence, either field or glasshouse screening can be utilised for the effective characterisation of genotype responses to A. candida. Genotypes of CBJ-002, CBJ-003 and CBJ-004 from China consistently showed the lowest level of disease incidence and disease severity in both field trials one and two. And they were also the most resistant genotypes in our previous glasshouse study (Li et al. 2007a; 2007b). These genotypes could potentially be developed as new cultivars or used as parental material to develop white rust resistant genotypes for Australia. Together with other host resistances identified in this study, they could be useful sources of resistance in preparation for future white rust epidemics in B. juncea in Australia, particularly in Western Australia. The most susceptible genotypes such as RH30 and RL1359 from India in the current study, were also among the most susceptible genotypes in previous glasshouse studies (Li et al. 2007a; 2007b). Further, the result indicates that staghead development may not be a reliable criterion to screen different genotypes for resistance to A. candida from our current and previous study. The degree of staghead formation is likely dependent upon the particular environmental conditions occurring at particular field locations. ACKNOWLEDGEMENTS This project is funded by ACIAR and Grains Research & Development Corporation (GRDC). REFERENCES (1999) Oilseed Brassica Improvement in China, India and Australia, Australian Centre for International Agricultural Research Project Document CIM/1999/072, pp. 43. (2007a) Australian Centre for International Agricultural Research, 2006 Annual Report, March 2007, Oilseed Brassica Improvement in China, India and Australia. University of Melbourne, Australia, pp. 36. (2007b) Dune brings canola to low rainfall zones. Canola News March 07, 7-8. Barbetti MJ (1981) Effects of sowing date and oospore seed contamination upon subsequent crop incidence of white rust (Albugo candida) in rapeseed. Australasian Plant Pathol. 10, 44-46. Barbetti MJ, Carter EC (1986) 'Disease of rapeseed.' West. Aust. Dept. Agric., Bull. No. 4105, Perth. Burton W, Salisbury P, Potts D (2003) The potential of canola quality Brassica juncea as an oilseed crop for Australia. Proc. 13 th Biennial Aust. Res. Assembly on Brassicas'. Tamworth, N.S.W. Australia pp. 62-64
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