2013 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 parent project. The project seeks to develop breeding lines of lettuce that are resistant to downy mildew, leafminers, and bacterial leaf spot (BLS) caused by the bacteria Xanthomonas campestris pv. vitians (Xcv). Genotypic analysis of Xcv strains: Four genes were sequenced for 130 Xcv strains or related strains. The genes were assembled, consensus sequences selected, and multiple comparisons completed. Of the 520 sequences, 12 need to be resequenced. Within the Xcv strains that cause Bacterial leaf spot (BLS) on lettuce there are four genetically distinct groups. Ninety-six strains have the genotype corresponding to group B; most California strains belong to this group. Groups C and D represent unique genotypes known only in California. Group A includes international strains and strains from the Eastern United States and Canada. Development of Xcv detection and quantification protocols: The genetic regions were sequenced for representative strains to find DNA sequences that could distinguish Xanthomonas campestris pv. vitians from related plant pathogens. The genes were sequenced, assembled and multiple alignments were constructed to look for useful regions for primer design. We demonstrated that the current protocols detect Xcv strains that belong to the species X. hortorum as well as X. hortorum pv. taraxaci and X. hortorum pv. carotae. BLS resistance breeding: We completed development of two BLS resistant lettuce populations derived from Batavia Reine des Glaces (resistant) × Eruption (susceptible). In two field experiments we determined that both populations have BLS resistance equivalent to Batavia Reine des Glaces, commercially acceptable salad shelf-life, and are susceptible to leafminers and downy mildew. These populations were approved for public release. A single dominant gene on conferring high level resistance to California Xcv strains was identified in the cultivars La Brillante, Little Gem, and Pavane. The gene was mapped to linkage group 2 of lettuce. We crossed two breeding lines carrying the resistance gene to ‘Crimson’ (red oak leaf) and ‘Merlot’ (red leaf) to initiate breeding of BLS resistant leaf and oak leaf lettuce. Leafminer resistance breeding: We analyzed data on leafminer resistance, downy mildew resistance, and horticultural traits collected from three field trials conducted at Spence Farm and the USDA-ARS station in Salinas in the summer of 2012. Some lines possessed downy mildew resistance, bacterial leaf spot resistance, and acceptable salad shelf-life. Downy mildew resistance breeding: Downy mildew resistance was evaluated in a late fall 2012 field experiment on 458 accessions that included breeding lines and cultivars from seven horticultural types of lettuces (iceberg, Romaine, leaf, Latin, butterhead, Batavia, and oil), and three wild relatives of lettuce. Based on multiple replicated trials the highest levels of resistance were observed in leaf lines GR x Ice – 1078, GR x Ice – 1046, and GR x Ice – 1057. Very high levels of resistance were also observed in leaf lines GR x Ice – 1018, and GR x Ice – 1053, and romaine line RH08-0464. Release statements for resistant germplasm are being prepared.