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Baby leaf lettuce cultivars with resistance to bacterial leaf spot (BLS) caused by Xanthomonas campestris pv. vitians (Xcv) are needed to reduce crop losses. The objectives of this research were to assess the genetic diversity for BLS resistance in baby leaf lettuce cultivars and to select early generation populations of lettuce with BLS resistance. Greenhouse experiments using artificial Xcv inoculations were conducted to assess BLS resistance in 35 cultivars of 10 lettuce types used in baby leaf production and in F2 through F3:4 progeny from ‘Batavia Reine des Glaces’ (BLS-resistant, green leaf color) 3 ‘Eruption’ (BLS-susceptible, red leaf color). Higher disease severity was identified in red leaf and red romaine cultivars compared with other types, indicating the need to target these types for resistance breeding. Selection for BLS resistance and red-colored leaves was therefore conducted among 486 F2 plants, 38 F2:3 families, and two populations of F3:4 families from ‘Batavia Reine des Glaces’ 3 ‘Eruption’. Two populations were identified with uniform levels of BLS resistance equivalent to ‘Batavia Reine des Glaces’ and variable leaf morphology and color. These populations can be used by private and publicly employed lettuce breeders to select for diverse types of lettuce cultivars suitable for baby leaf production and with BLS resistance. The popularity of commercially produced and packaged salads has increased in the United States (Glaser et al., 2001). Baby leaf lettuce (Lactuca sativa L) is the primary component of spring mix salads, which includes some mixture of a number of different leafy green vegetable species such as spinach (Spinacia oleracea L.), radicchio (Cichorium intybus L.), frisee (Cichorium endivia L.), arugula (Eruca vesicaria L.), tatsoi and mizuna (Brassica rapa L.), and mache [Valerianella locusta (L.) Laterr.]. Spring mix is 90% more valuable per ton than bulk iceberg or leaf lettuce and 4400 ha of production was worth over $140 million to Monterey County, CA, in 2010 (Anonymous, 2010). Significant amounts are also grown in four other California counties, although the amount of this production is not known. In 2005, Yuma County in Arizona produced 2800 ha of spring mix. The crop is grown at extremely high densities, up to 7.4 million seeds per hectare (three million seeds per acre) on 203to 213-cm wide (80 to 84 inches) raised beds with 24 to 32 seed lines per bed (Nolte, 2010; Smith et al., 2009). The crop is mechanically harvested when the first four true leaves are 5 to 13 cm (2 to 5 inches) long, typically 20 to 30 d after planting. Approximately 15 to 20 different types of lettuce with unique combinations of leaf shape, margin serration, lobing, undulation, pliability, savoy, and color have been used for baby leaf lettuce production, although most commercially prepared salads contain less than 10 types of lettuce. Some cultivars of romaine, leaf, Batavia, and Latin-type lettuce are used for baby leaf and whole head production. However, it is increasingly common for cultivars of these types to be bred specifically for use in baby leaf production. Other specialty leaf-type lettuce such as oak leaf, lolla rossa, and tango are used nearly exclusively for baby leaf production in the United States. A limited number of characters is considered critical to the success of a cultivar intended for baby leaf production in California and Arizona. At a minimum, the cultivar should have an interesting leaf shape and color as a seedling and possess good post-packaging shelf life. Pest resistance is increasingly important, because the short timeframe from planting to harvest constrains pesticide use and considerable amounts of baby leaf production is organic. The U.S. Department of Agriculture (USDA) lettuce breeding program in Salinas, CA, primarily operates as an enhancement breeding program, introgressing new economically valuable characters from unadapted germplasm into inbred lines of iceberg, romaine, or leaf lettuce. The inbred lines are dispersed to other private and public breeding programs for further selection, seed increase and sales, or crossing to develop new breeding populations. The diversity of types used in baby leaf production creates a challenge for public plant breeders attempting to enhance the cultivar gene pool with new traits such as pest resistance. Introgressing a new trait into a new inbred cultivar of each type is too resource-intensive to be feasible. In this case, a strategic approach is needed that balances the need to address the diverse types used in baby leaf production while ensuring that the types with the greatest susceptibility are prioritized. This could be accomplished through surveys of existing baby leaf cultivars of each type to determine which types have the greatest need for resistance. Additionally, breeders could select and release earlygeneration (F2 to F4) populations genetically fixed for disease resistance but with sufficient variability for leaf characters to encompass two or more lettuce types used in baby leaf production. External breeding programs could then select multiple lettuce types from a single resistant population. Development of these populations would rely on crossing parents that differ for resistance and leaf morphology. The high planting densities of baby leaf lettuce exacerbate the severity of many lettuce pests, including leafminers (Liriomyza langei), downy mildew (Bremia lactucae), and bacterial leaf spot (BLS). Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians (Xcv), is an increasingly damaging disease in California during spring and fall production (Barak et al., 2001; Bull and Koike, 2005) and is a sporadic problem Received for publication 13 Sept. 2013. Accepted for publication 11 Nov. 2013. This research was supported by the California Leafy Greens Research Program and the California Department of Food and Agriculture. Partial funding for work by S. Gebben was provided by an NSF Science Talent Expansion Project (STEP) grant #0525444 Hartnell College, Salinas, CA. We thank Shamrock Seed Company and Synergene Seed & Technology for contributing baby leaf lettuce cultivars used in this research. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Current address: Department of Plant Pathology, University of California, One Shields Avenue, Davis, CA 95616-8751. To whom reprint requests should be addressed; e-mail Ryan.Hayes@ars.usda.gov. 18 HORTSCIENCE VOL. 49(1) JANUARY 2014 throughout North America (Carisse et al., 2000; Pernezny et al., 1995; Sahin and Miller, 1997; Toussaint, 1999) and globally (Myung et al., 2010; Sahin, 2000; Wallis and Joubert, 1972). The pathogen causes small angular leaf spots, which are initially water-soaked and later become necrotic (brown to black) and papery. The leaf spots can coalesce, forming large necrotic regions (Bull and Koike, 2005). The symptoms reduce the quality and yield of lettuce and increase the potential for postharvest losses (Carisse et al., 2000). Lettuce crops grown for whole heads that experience severe disease levels may compel harvest crews to cull infected heads, leaving unmarketable plants in the field and only packing the less diseased or healthy plants. The dense plantings and mechanical harvest of baby leaf crops severely reduce the feasibility of culling infected leaves. Consequently, baby leaf crops with BLS may be abandoned resulting in loss of the entire crop. Initial symptoms on cultivars with red-colored leaves can be difficult to detect in the early stages of disease development but may develop further during post-harvest shipping. Therefore, cultivars with red-colored leaves, which are common in baby leaf crops, may contribute to increased post-harvest losses of spring mix crops. Host resistance is an efficient and costeffective tool to manage BLS of lettuce. Diverse sources of resistance are known including the Latin-type cultivar Little Gem and the Batavia-type cultivars Batavia Reine des Glaces (synonym Reine des Glaces), Iceberg, and La Brillante (Bull et al., 2007; Hayes et al., 2013). The USDA in Salinas, CA, developed and released seven resistant iceberg-type breeding lines from the cross (‘Salad Crisp’ · ‘Iceberg’) · ‘Salinas 88’ (Hayes et al., 2008). ‘Little Gem’ and its numerous derivatives are used in baby leaf production, whereas the remaining known sources of resistance are not suitable for baby leaf production. Additional germplasm with resistance to BLS that is suitable for baby leaf production is needed. In this article, we report on the genetic diversity for resistance to BLS in cultivars used for baby leaf production and the selection of early-generation baby leaf germplasm that are genetically fixed for resistance to BLS. The populations were further characterized for salad shelf life, resistance to leafminers, and resistance to downy mildew. Materials and Methods Bacterial leaf spot disease evaluation. All experiments to assess BLS resistance were conducted with greenhouse-grown seedlings in plug trays with plant populations of 2380 plants/m. The results from the greenhouse assay are closely correlated with results in field experiments (Bull et al., 2007; Hayes et al., 2008) and simulate the dense plant populations used in baby leaf production. Plug trays (22 cm wide and 65 cm long) with 31 rows of 11 cells that are 20 mm · 20 mm · 60 mm were filled with commercial plug-mix potting soil. The perimeter cells of plug trays in all experiments were seeded with the susceptible iceberg cultivar Vista Verde, whereas the remaining 29 rows of nine cells were seeded with the experiment entries (experimental families, populations, parents, or control cultivars). Each entry was assigned to and seeded in plots,

hortscience

Baby Leaf Lettuce Germplasm Enhancement: Developing Diverse Populations with Resistance to Bacterial Leaf Spot Caused by Xanthomonas campestris pv. vitians