2012 Annual Report
1a.Objectives (from AD-416):
Develop spring mix lettuce cultivars with disease resistance, good appearance and extended shelf-life.
1b.Approach (from AD-416):
Field, greenhouse, laboratory, and molecular marker experiments to develop and select lettuce lines with improved performance for fall, winter, and spring planting slots in California.
The agreement supports objective 1 of the in-house project, the goal being to develop lettuce cultivars suitable for baby leaf production and possessing resistance to downy mildew, leaf miners, and bacterial leaf spot. We have made progress towards developing scientific information on Xanthomonas campestris pv. vitiansXcv detection, differentiation, and quantification that can be communicated in peer reviewed publications. This project is monitored using conference phone calls, email, and biennial progress reports. We analyzed the sequences for four genes for the majority of Xcv strains in our collection (approximately 100 strains). Four distinct MultiLocus Sequence Type groups have been identified. These groups represent genetically distinct pathogens. Identification of these groups provides the information needed to test the hypothesis that distinct genotypes have different levels of virulence on susceptible and resistant lettuce cultivars. To date we have employed two interns from Hartnell College supported by funds from this grant. At least one more will be employed this summer.
We grew, harvested, cleaned and cataloged seeds from leafminer-resistant plants selected from our field trials last fall. An experiment was designed and repeated to identify two S2 red romaine populations from the cross of ‘Batavia Reine des Glaces x Eruption with BLS resistance equivalent to ‘Batavia Reine des Glaces'. Resistance to Xcv in ‘Little Gem’ is due to a hypersensitive response (HR), a rapid leaf necrosis at the site of infection. We determined that a single gene is responsible for the HR (hypereactive response) derived from the cross Clemente x Little Gem. We also identified HR to Xcv in the ‘La Brilliante’. A set of 91 RILs from the cross Salinas 88 x La Brillante to a greenhouse inoculation test to quantify the resistance levels among the RILs. This population has an existing genetic map, which was used in quantitative trait loci analysis to identify a single region on chromosome 2 responsible for BLS resistance in ‘La Brillante’.
We evaluated over 1,800 accessions for their resistance to downy mildew in at least one trial. In addition, more than 300 accessions were tested for their deterioration after minimal processing for salad and storing in the modified atmosphere environment Evaluations of resistance and post-harvest quality were performed on all breeding lines that are targeted for release in 2013: two leaf-type lines GRxIce-1052 and GRxIce-1053, and the romaine type line RH08-0464. So far, all evaluations confirmed good resistance to downy mildew and acceptable levels of post-harvest deterioration in all three breeding lines.