Identification of Sources of Resistance to Xanthomonas campestris pv. campestris in Brassica napus Crops

Lema, M., Soengas, P., Velasco, P., Francisco, M., and Cartea, M. E. 2011. Identification of sources of resistance to Xanthomonas campestris pv. campestris in Brassica napus crops. Plant Dis. 95:292-297. Black rot, caused by Xanthomonas campestris pv. campestris, is one of the most important diseases affecting Brassica crops worldwide. Nine races have been differentiated in X. campestris pv. campestris, with races 1 and 4 being the most virulent and widespread. The objective of this work was to identify sources of resistance to races 1 and 4 of X. campestris pv. campestris in different Brassica napus crops, mainly in the underexplored pabularia group. Seventy-six accessions belonging to four B. napus groups were screened for resistance to two X. campestris pv. campestris races (1 and 4). The strain of race 1 used in this study was more virulent on the tested materials than the strain of race 4. No race-specific resistance was found to race 1. Most cultivars were susceptible except Russian kale, from the pabularia group, which showed some resistant plants and some other accessions with some partially resistant plants. High levels of race-specific resistance to race 4 were found in the pabularia group, and great variability within accessions was identified. Three improved cultivars (Ragged Jack kale, Friese Gele, and Valle del Oro) and four landraces (Russian kale, MBG-BRS0037, MBG-BRS0041, and MBG-BRS0131) showed plants with some degree of resistance to both races, which may indicate that race-nonspecific resistance is involved. These accessions could be directly used in breeding programs, either as improved cultivars or as donors of race-specific resistance to other Brassica cultivars. Black rot is one of the most devastating diseases in Brassica crops and has worldwide distribution. This bacterial disease is caused by Xanthomonas campestris pv. campestris (Pammel) Dowson. The seedborne pathogen can survive in crop debris or crucifer weeds and is especially damaging in vegetable Brassica crops, mainly in warm and humid climates. In coastal temperate areas of Europe, the disease is not able to kill the plant (35) but it makes V-shaped necrotic lesions on leaf margins, which decrease the quality on the fresh market. Moreover, this disease weakens plant tissues, potentially leading to infection from other pathogens. Infected seed are the primary source of inoculum. When optimal conditions for bacterial development are not present, no symptoms are expressed; however, the pathogen can remain in the vascular system of these asymptomatic plants. Symptoms may be expressed on a susceptible cultivar when conditions favor disease development. In Spain, which is an important Brassica spp. producer and consumer, the presence of X. campestris pv. campestris was described in the 1970s by Urquijo et al. (29) and later by Ortega and López (18). Urquijo et al. (29) determined the presence of X. campestris pv. campestris in Brassica napus, B. oleracea, and B. rapa crops from northwest, central, and eastern Spain based on the symptoms of the disease in infected plants. Ortega and López (18) identified X. campestris pv. campestris from different B. oleracea crops (broccoli, Brussels sprout, cabbage, and cauliflower) from the Mediterranean coast of Spain based on morphological and biochemical characteristics of the pathogen and in a pathogenicity test (isolation, growth of the bacterium in pure culture, and reinoculation of susceptible host plants). In northwestern Spain, black rot has been recently identified in several Brassica crops (14) and a study referred to the incidence of X. campestris pv. campestris races is being carried out (unpublished data). According to our preliminary results, race 4 is the most prevalent in this area, followed by races 1 and 6, although it is possible that other races are also present (races 2, 7, and 9). In this region, the production is mainly by small growers who do not use healthy plant material or diseasefree seed; consequently, the pathogen can spread rapidly. No studies involving either the pathogen or screens for resistance in this area have been conducted. Williams (34) illustrated a number of control measures against this disease; specifically, the use of resistant cultivars and the exploration of new sources of resistance. Currently, disease management is still complicated and limited sources of resistance are available. Recently, the search for new sources of resistance has focused on race-specific resistance since the existence of six races of the pathogen was described by Vicente et al. (31). The authors analyzed 164 X. campestris pv. campestris isolates from various origins worldwide and established a Brassica differential series, thus modifying previous classifications by Kamoun et al. (13) and Ignatov et al. (11). The interaction between differential cultivars and X. campestris pv. campestris races is explained by using a gene-for-gene model. In addition to monogenic race-specific resistance (4,8,31,32), a quantitative race-nonspecific resistance has been described (3,7,23,25,32). Recently, Fargier and Manceau (5) analyzed 47 representative strains of X. campestris and added three new races (races 7, 8, and 9) to the six races described by Vicente et al. (31). It is recognized that races 1 and 4 are the most virulent and widespread, accounting for most black rot cases around the world (6,31). Sources of resistance to X. campestris pv. campestris in Brassica genomes have been examined by Bain (2), Hunter et al. (10), Guo et al. (8), Nogueira and Dias (17), Ribeiro and Dias (19), Westman et al. (33), Taylor et al. (25), Massomo et al. (16), Tonguç and Griffiths (28), Jensen et al. (12), and Griffiths et al. (6). To the authors’ knowledge, the use of resistant cultivars has only had limited success, and available sources with useful levels of resistance are scarce. Moreover, most of this work focused in cabbage (B. oleracea var. capitata) due to its economical value, while the search for resistance in B. napus has been more restricted. Guo et al. (8) identified two highly resistant accessions of B. napus (plant introduction [PI] 199947 and PI 199949) but they were reidentified later as B. carinata (25). In this study, several crops from the BrassicaCorresponding author: M. Lema, E-mail: [email protected] Accepted for publication 22 October 2010. doi:10.1094 / PDIS-06-10-0428 © 2011 The American Phytopathological Society