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  • Author or Editor: Carolee T. Bull x
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Bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians (Xcv) is an important lettuce disease in California. No adequate control measures have been found, although resistance exists in several heirloom cultivars. Deployment of cultivars resistant to bacterial leaf spot will reduce these periodic and costly disease events. The objectives of this research were to 1) identify new sources of resistance within modern crisphead cultivars and 2) select for resistance in `Salad Crisp' × `Iceberg' progeny. Field plots were established and grown with overhead irrigation, and a three-strain mixture of Xcv was applied until runoff 1 week after thinning at 1 × 109 CFU/mL. Twenty-six crisphead cultivars were tested in unreplicated field trials and rated on a 1 (susceptible) to 4 (resistant) scale. Selection was carried out between and within families from the F2 to F4 generation. Sixteen F3 families were evaluated in unreplicated plots, and 12 F5 families were tested in replicated plots for disease incidence and severity. No usable levels of resistance were identified in the modern crisphead cultivars tested to date. All F3 families had resistance greater than `Iceberg', and 19 plants from eight families were selected for further breeding. Subsequently, 12 plants from two F4 families were selected. Replicated trials of 12 F5 families indicated that all lines have disease severity comparable to both parents. Breeding lines from crosses to `Salinas 88' are currently being developed.

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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) × ‘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’ × ‘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.

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