Screening for resistance to black rot in Brassica oleracea crops M ARGARITA L EMA 1,2 , P ABLO V ELASCO 1 , P ILAR S OENGAS 1 , M ARTA F RANCISCO 1 and M ARI ´ A E LENA C ARTEA 1 1 Department of Plant Genetics, Misio´n Biolo´gica de Galicia (CSIC), PO Box 28, E-36080 Pontevedra, Spain; 2 Corresponding author, E-mail: margarita.lema@usc.es With 4 tables Received October 1, 2010/Accepted March 9, 2012 Communicated by A.-M. Chevre Abstract Xanthomonas campestris pv. campestris (Xcc), causal agent of black rot, is widely distributed around the world in cabbage and kale crops causing severe yield losses. Nine races of the pathogen were described, being races 1 and 4 the most aggressive and widespread. The objective of this work was to identify new sources of resistance to races 1 and 4 of Xcc in different Brassica oleracea crops. Two hundred and twenty- one landraces and 35 commercial and improved cultivars were evaluated. Most accessions were susceptible to both races, except cabbage cultivars ÔBalo´nÕ and ÔQuintal de AlsaciaÕ and kale landrace MBG-BRS0070, which showed some plants with different level of resistance to races 1 and 4, thus indicating that race-non-specific resistance can be involved. Kale landrace MBG-BRS0286 showed an intermediate mean disease score for race 1. These accessions can be crossed with cabbage cultivars and may provide new combinations of resistance genes with protection against black rot. Key words: bacterial disease — Xanthomonas campestris pv. campestris — cabbage — genetic resistance — kale — landrace — tronchuda cabbage Black rot is one of the most devastating and widespread diseases in Brassica crops, and an especially serious problem in leafy-green vegetables in several countries around the world (Chakravarti et al. 1969, Alvarez and Cho 1978, Schaad and Thaveechai 1983, Bandyopadhyay and Chattopadhyay 1985, Onsando 1987, Vicente 2004, Massomo et al. 2005, Mirik et al. 2008, Jensen et al. 2010). This bacterial disease is caused by Xanthomonas campestris pv. campestris (Pammel) Dowson (Xcc). The seed-borne pathogen can survive in crop debris or crucifer weeds (Schaad and Dianese 1981), introducing itself in the plant through hydathodes and wounds (Williams 1980). While the bacteria can cause plant death and consequently great economic losses in warm and humid regions, they produce V-shaped necrotic lesions on leaf margin in coastal temperate areas, which decrease the value of the product in fresh market (Williams 2007). Besides, the disease deteriorates plant health and favours the attack of opportunist organisms, thus limiting seed and root yields. The pathogen can remain in the vascular system of the infected asymptomatic plants until favourable environment for bacterial growth takes place and lets the disease appear. Genetic resistance was proposed by Williams (1980) as an efficient control measure against this disease, along with the utilization of healthy plant material and disease-free seeds. Nowadays, the disease control is compli- cated, and few sources of resistance are available. Nine races of Xcc have been described. Initially, Vicente et al. (2001) identified six races (1–6), and later, Fargier and Manceau (2007) added three additional races (7–9). Race identification was based on avirulence/virulence patterns to six differential host genotypes, involving five resistance genes in the plant and five avirulence genes in the pathogen. Races 1 and 4 have been found to be the most virulent and widespread, accounting for more than 90% of black rot disease around the world (Vicente et al. 2001). Specific resistance to races 1 and/or 4 has been previously reported in different Brassica species (Guo et al. 1991, Ribeiro and Dias 1997, Tonguc and Griffiths 2004a,b, Griffiths et al. 2009). According to Vicente et al. (2002) and Taylor et al. (2002), the R1 gene conferring resistance to race 1 is present in the B genome of Brassica carinata (BBCC), Brassica juncea (AABB) and Brassica nigra (BB), while the R4 gene conferring resistance to race 4 is present in the A genome of Brassica rapa (AA), Brassica napus (AACC) and B. juncea (AABB). In Brassica oleracea (CC) with the C genome, specific resistance to races 1 and 4 is not expected, and it has been never found. However, race-non- specific resistance has been identified in this species (Camargo et al. 1995, Taylor et al. 2002). Numerous studies have been carried out to identify new sources of resistance to black rot, most of them being unaware of the existence of Xcc races, which means an added difficulty to incorporate black rot resistance in new-released materials. Genetic resistance was reported in genotypes of diverse B. oleracea crops in field and greenhouse screenings. In cabbage, Bain (1952) initially found resistance in the cv. ÔHuguenotÕ and in the Japanese cv. ÔEarly FujiÕ and made selections and derived lines from them; also, Hunter et al. (1987) reported resistance in the introduction from China PI 436606 (ÔHeh Yeh da Ping TouÕ). This genotype together with ÔEarly FujiÕ has been extensively used in cabbage breeding resulting in cabbage lines like NY 4002 and Badger Inbred-16 utilized in the development of several resistant cabbage cultivars. Henz and Demelo (1994) considered several cabbage cvs. resistant under greenhouse conditions, including ÔMaster AG-325Õ, ÔArixos F1Õ, ÔMaxim F1Õ and ÔEmpax F1Õ, among others. Jensen et al. (2005) found field resistance to black rot in hybrid cultivars ÔT-689 F1Õ, ÔGianty F1Õ, ÔNo. 9690 F1Õ, ÔN 66 F1Õ and ÔSWR-02 F1Õ. Williams et al. (1972) highlighted the tolerance of Japanese cabbage hybrids and of individual plants in open-pollinated North American ball-head types. Massomo et al. (2004) found partial resistance in several cabbage hybrids under field conditions in Tanzania. Griffiths and Roe (2005), testing different inoculation methods at different growing stages of the plant, verified resistance in breeding lines Badger Inbred-16, Cornell 101, Cornell 102 and NY 4002 and in the Plant Breeding doi:10.1111/j.1439-0523.2012.01974.x Ó 2012 Blackwell Verlag GmbH wileyonlinelibrary.com