Black rot is a bacterial disease of crucifer species caused by Xanthomonas campestris pv. campestris (Xcc). Xcc is prevalent worldwide and is a destructive disease of Brassica oleracea vegetables such as cabbage, broccoli, and cauliflower (Williams, 1980). Xcc is seed-borne and also can overwinter on cruciferous weeds and wild relatives of cultivated Brassica crops (Cook et al., 1952; Schaad and Dianese, 1981). Symptoms of the disease include blackening of the veins in petioles and characteristic V-shaped lesions originating from the leaf margin, which enlarge causing the plant to wilt and eventually rot. Currently, the most effective approaches to controlling black rot are through good agricultural practices, hot water treatment of seeds, and the use of cultivars with moderate resistance to the disease (Griffiths and Roe, 2005). However, these approaches have a limited effect with unpredictable black rot outbreaks occurring throughout growing regions, highlighting the need for incorporation of effective and stable resistance into cultivated varieties. When grown in a conducive environment, symptoms typically appear 10 to 14 d after infection (Williams, 1980).
Studies have previously focused on resistance derived from B. oleracea, including the cabbage cultivar Early Fuji (Bain, 1952), the inheritance of which was reported to be controlled by a single recessive gene with two modifiers (Williams et al., 1972). Resistance has also been documented in B. oleracea PIs, including the cabbage accession PI 436606 from China, also determined to be controlled by multiple genes (Dickson and Hunter, 1987; Hunter et al., 1987). These sources have been used in the development of black rot-resistant breeding lines, including Badger #16 (Williams, University of Wisconsin); NY4002 (Dickson, Cornell University); and Cornell 101, Cornell 102, and Cornell 103 (Griffiths, Cornell University). However, the resistance is typically incomplete and difficult to incorporate into hybrid cultivars (Camargo et al., 1995). Resistance has been reported in the mustard species B. nigra, B. juncea, and B. carinata (Guo et al., 1991; Taylor et al., 2002; Tonguc and Griffiths, 2004a; Westman et al., 1999), including B. juncea accessions PI 633077 and PI 633078 (previously A 19182 and A 19183), and B. carinata accessions PI 199947 and PI 199949 (Guo et al., 1991) and used in the creation of interspecific hybrids (Tonguc et al., 2003; Tonguc and Griffiths, 2004b).
Xcc has been characterized into at least six distinct races, the most important of these being races 1 and 4, which account for over 90% of black rot disease worldwide (Vicente et al., 2001). B. oleracea accessions have not been identified that exhibit complete resistance to races 1 and 4, but breeding lines have been developed that exhibit incomplete resistance to these races. Related crucifer species, including B. carinata and B. juncea, exhibit resistance to Xcc races 1 and 4, that appear to be controlled by a single gene (Guo et al., 1991; Hansen and Earle, 1995; Taylor et al., 2002; Tonguc et al., 2003; Tonguc and Griffiths, 2004b; Vicente et al., 2002).
To effectively use germplasm resources for interspecific hybridization with B. oleracea, it will be important to determine the presence and frequency of Xcc resistance not just within cultivated Brassica vegetables, but also within related crucifer accessions. Resistance may then be exploited through interspecific crosses of B. oleracea with related species. Crucifer accessions could provide important resistance sources for the development of hybrid Brassica vegetables by contributing new resistance genes for Xcc or genes that can be incorporated into cole crops more effectively. The aim of this research was to evaluate crucifer species at the juvenile stage to quantify the presence, frequency, and potential use of accessions for introgression of Xcc resistance. To achieve this, the crucifer accessions from the NC-7 (Ames, IA) and NE-9 (Geneva, NY) USDA regional PI centers were inoculated and evaluated for resistance to race 1 and race 4 of Xcc using the wound inoculation technique (Griffiths and Roe, 2005).
Camargo, L.E.A., Williams, P.H. & Osborn, T.C. 1995 Mapping of quantitative loci controlling resistance to Brassica oleracea to Xanthomonas campestris pv. campestris in the field and greenhouse Phytopathology 85 1296 1300
Cunha, C., Tonguc, M. & Griffiths, P.D. 2004 Discrimination of diploid Brassica species using PCR-RFLP of chloroplast DNA HortScience 39 481 484
Dickson, M.D. & Hunter, J.E. 1987 Sources of resistance to black rot of cabbage expressed in seedlings and adult plants Plant Dis. 71 263 266
Griffiths, P.D. & Roe, C. 2005 Response of Brassica oleracea var. capitata to wound and spray inoculation with Xanthomonas campestris pv. campestris in juvenile and mature plants HortScience 40 47 49
Guo, H., Dickson, M.H. & Hunter, J.E. 1991 Brassica napus sources of resistance to black rot in crucifers and inheritance of resistance HortScience 26 1545 1547
Hansen, L.N. & Earle, E.D. 1995 Transfer of resistance to Xanthomonas campestris pv. campestris into Brassica oleracea L. by protoplast fusion Theor. Appl. Genet. 91 1293 1300
Hunter, J.E., Dickson, M.H. & Ludwig, J.W. 1987 Sources of resistance to black rot of cabbage expressed in seedlings and adult plants Plant Dis. 71 263 266
Schaad, N.W. & Dianese, J.C. 1981 Cruciferous weeds as sources of inoculum of Xanthomonas campestris in black rot of crucifers Phytopathology 71 1215 1220
Shaw, J.J. & Kado, C.I. 1988 Whole plant wound inoculation for consistent reproduction of black rot of crucifers Phytopathology 78 981 986
Shelton, A.M. & Hunter, J.E. 1985 Evaluation of the potential of the flea bettle Phyllotreta cruciferae to transmit Xanthomonas campestris pv. campestris, casual agent of black rot of crucifers Can. J. Plant Pathol. 7 308 310
Taylor, J.D., Conway, J., Roberts, S.J., Astley, D. & Vicente, J.G. 2002 Sources and origin of resistance to Xanthomonas campestris pv. campestris in Brassica genomes Phytopathology 92 105 111
Tonguc, M., Earle, E. & Griffiths, P.D. 2003 Segregation distortion of Brassica carinata derived black rot resistance in Brassica oleracea Euphytica 134 269 276
Tonguc, M. & Griffiths, P.D. 2004a Evaluation of Brassica carinata accessions for resistance to black rot (Xanthomonas campestris pv. campestris) HortScience 39 952 954
Tonguc, M. & Griffiths, P.D. 2004b Development of black rot resistant interspecific hybrids between B. oleracea L. cultivars and accession A 19182 Euphytica 136 313 318
U, N. 1935 Genome analysis in Brassica with special reference to the experimental formation of Brassica napus and peculiar mode of fertilization Japan J. Bot. 7 389 452
Vicente, J.G., Conway, J., Roberts, S.J. & Taylor, J.D. 2001 Identification and origin of Xanthomonas campestris pv. campestris races and related pathovars Phytopathology 91 492 499
Vicente, J.G., Taylor, J.D., Sharpe, A.G., Parkin, I.A.P., Lydiate, D.J. & King, G.J. 2002 Inheritance of race-specific resistance to Xanthomonas campestris pv. campestris in Brassica genomes Phytopathology 92 1134 1141
Westman, A.L., Kresovich, S. & Dickson, M.H. 1999 Regional variation in Brassica nigra and other weedy crucifers for disease reaction to Alternaria brassicicola and Xanthomonas campestris pv. campestris Euphytica 106 253 259