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  Bacterial leaf spot
Prepared by R. Hayes et al.
Bacterial leaf spot
  Pathogen Responsible
Xanthomonas campestris pv. vitians
  Description and Sympotoms
Bacterial leaf spot (BLS) is a foliar disease of lettuce caused by Xanthomonas campestris pv. vitians (Xcv). Symptoms include black watery angular spots, typically less than 1 cm in diameter. In severe infections, lesions coalesce into large necrotic patches. Old lesions may dry out and become papery. Bacterial leaf spot afflicts all market types of lettuce and is dependent on cool, wet conditions. Consequently, it occurs predominantly during spring and fall production in coastal California (Barak et al., 2001). It also sporadically occurs throughout the U.S. and internationally. Bacterial leaf spot infected lettuce is unsightly and reduces the marketability of lettuce heads. Commercial fields with low disease pressure may be harvested by stripping off infected leaves, resulting in lower weight and smaller heads. Severely diseased heads are typically left un-harvested, resulting in yield loss. Packing infected lettuce may predispose the crop to post-harvest losses (Koike & Gilbertson, 1997).
  Current Management
The pathogen can survive between crops on contaminated plant debris; therefore, production of lettuce in fields with a recent history of BLS should be avoided (Barak et al., 2001). The bacteria causing BLS may be seed borne (Sahin & Miller, 1997; Carisse et al., 2000), although it has not been recovered from seed lots used for commercial production in California (Umesh et al., 1996). Regardless, several treatments to reduce seed-borne inoculum have been reported (Sahin & Miller, 1997; Carisse et al., 2000; Pernezny et al., 2002). Bactericide and biopesticide treatments applied to transplants or direct-seeded lettuce crops can reduce disease levels (Carisse et al., 2000; Bull & Koike, 2005).
  Genetics of Resistance
Genetic variation for resistance or the degree of susceptibility to bacterial leaf spot has been reported across a diverse range of lettuce cultivars (Sahin & Miller, 1997; Carisse et al., 2000; Bull et al., 2007; Lu et al., 2014). The level of resistance may be dependent on the strain of Xcv. The USDA in Salinas, CA developed and released six iceberg breeding lines with resistance derived from 'Salad Crisp' and 'Iceberg' as well as two baby leaf populations with resistance from 'Batavia Reine des Glaces' (Hayes et al., 2013). The USDA-bred material and their parents express incomplete resistance (IR) to BLS, where they have reduced disease severity compared to other cultivars but are not disease free in high disease pressure environments. Nothing is known about the inheritance of this IR to bacterial leaf spot. The cultivars La Brillante, Pavane, and Little Gem express high levels of resistance to specific Californian isolates of Xcv involving a hypersensitive reaction (HR) (Bull et al., 2015). This HR is conferred by the Xanthomonas resistance 1 (Xar1) gene (Hayes et al., 2014). The presence of Xar1 can be detected through a simple leaf infiltration assay. Plants with the dominant allele for resistance develop dry, tan lesions while those with the recessive allele for susceptibility develop bacterial leaf spot (Bull et al., 2015).
  Genetic Marker Development
Xar1 is present on linkage group 2 in a genomic region that contains numerous NB-LRR encoding resistant gene candidates (RGCs) and functional pathogen resistance loci in the RGC2 family (Hayes et al., 2014). However, molecular markers for Xar1 have yet to be developed and validated based on this information.
  References
  • Barak, J. D., Koike, S. T., and Gilbertson, R. L. 2001. The role of crop debris and weeds in the epidemiology of bacterial leaf spot of lettuce in California. Plant Dis. 85:169-178.
  • Bull, C. T., and Koike, S. T. 2005. Evaluating the efficacy of commercial products for management of bacterial leaf spot on lettuce. Online. Plant Health Progress doi:10.1094/PHP-2005-1121-01-RS.
  • Bull, C.T., Gebben, S., Goldman, P.H., Trent, M.A. and Hayes, R.J. 2015. Population dynamics of Xanthomonas campestris pv. vitians is dependent on the host genotype. Phytopathology 105:316-324.
  • Carisse, O., Ouimet, A., Toussaint, V., and Philion, V. 2000. Evaluation of the effect of seed treatments, bactericides, and cultivars on bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians. Plant Dis. 84:295-299.
  • Hayes, R.J., Trent, M.A., Mou, B., Simko, T., Gebben, S.J., and Bull, C.T. 2013. Baby leaf lettuce germplasm enhancement: developing diverse populations with resistance to bacterial leaf spot caused by Xanthomonas campestris pv. vitians. HortScience 49:18-24.
  • Hayes, R.J., Trent, M.T., Truco M.J., Antonise, R., Michelmore, R.W. and Bull, C.T. 2014. The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars. Horticulture Research 1:14066, doi:10.1038/hortres.2014.66.
  • Koike, S.T. and Gilbertson, R.L. 1997. Bacterial Leaf Spot. Pages 27-28 in: Compendium of Lettuce Diseases. R. M. Davis, K. V. Subbarao, R. N. Raid, and E. A. Kurtz, eds. The American Phytopathological Society, St. Paul, MN.
  • Lu, H., Hu, J., and Kwon, S.J. 2014. Association analysis of bacterial leaf spot resistance and SNP markers derived from expressed sequence tags (ESTs) in lettuce (Lactuca sativa L.). Molecular Breeding 34: 997-1006.
  • Pernezny, K., Nagata, R., Raid, R. N., Collins, J., and Carroll, A. 2002. Investigation of seed treatments for management of bacterial leaf spot of lettuce. Plant Dis. 86:151-155.
  • Sahin, F., and Miller, S. A. 1997. Identification of the bacterial leaf spot pathogen of lettuce, Xanthomonas campestris pv. vitians, in Ohio, and assessment of cultivar resistance and seed treatment. Plant Dis. 81:1443-1446.
  • Umesh, K. C., Koike, S. T., and Gilbertson, R. L. 1996. Association of Xanthomonas campestris pv. vitians with lettuce seed. (Abstr.) Phytopathology 86:S3.
  • Wang, Y., Lu, H., Raid, R., Nuessly, G., and Faroutine, G. 2015. Diverse responses of lettuce cultivars and germplasm lines to infections of three isolates of Xanthomonas campestris pv. vitians. HortScience 50:650-655.
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