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United States Department of Agriculture

Agricultural Research Service

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Research Project: Next-Generation Lettuce Breeding: Genes to Growers

Location: Crop Improvement and Protection Research

2011 Annual Report

1a. Objectives (from AD-416)
To determine the inheritance of field resistance to downy mildew, tipburn resistance, sclerotinia resistance, and shelf-life in multiple genetic mapping populations. To identify sources of physiological resistance to sclerotinia. To develop genetically improved lines of lettuce for downy mildew resistance, sclerotinia resistance, tipburn resistance, and shelf-life.

1b. Approach (from AD-416)
The research to be conducted involves developing and analyzing multiple recombinant inbred line populations (RILs) to identify genes and QTL for field resistance to downy mildew, scelertinia resistance, tip burn resistance and extended shelf-life. The RILS wil be evaluated in field and greenhouse experiments, genotyped with molecular marker, analyzed to determine the inheritance traits, and markers suitable for markers assisted selection will also be developed. There is also research to develop improved in vitro and greenhouse methods for sclerotinia resistance.

3. Progress Report
Since the initial discovery of Verticillium wilt in lettuce in 1995, the disease has spread within the Salinas Valley. In affected fields, losses have ranged between 30 to 90%. Fumigation is not economically feasible for lettuce, and crop rotation is ineffective due to the broad host range of V. dahliae. Therefore, the development of lettuce cultivars resistant to Verticillium wilt is important to the sustainability of the lettuce industry in California. There are two distinct pathogenic races of V. dahliae causing Verticillium wilt on lettuce. While resistance to race 1 is known, the existence of race 2 isolates in California lettuce production fields is certain. Moreover, it is highly probable that widespread use of race 1 resistant germplasm will select for and increase the economic damage caused by race 2 isolates. Therefore, identification and subsequent development of lettuce cultivars with resistance to race 2 is imperative for sustaining the lettuce industry. We are currently screening the Western Regional Plant Introduction Station (WRPIS) Lactuca collection for resistance to race 2 isolates of V. dahliae. More specifically, our research is using a working collection of the WRPIS located in Salinas, CA. Our strategy uses greenhouse testing to screen up to eight plants of 160 accessions per year in unreplicated plots to identify candidate sources of resistance. This is followed by replicated greenhouse and field-micro-plot experiments to confirm resistance. To date, we have screened 824 L. sativa and L. serriola accessions using race 2 V. dahliae isolate VdLs17. We have confirmed partial resistance (disease incidence significantly lower than ‘Salinas’) in four accessions (PIs 169511, 171674, 204707, 226641). However, all of these PIs have had at least a few symptomatic plants, and all but PI 171674 have had non-symptomatic plants that are nonetheless colonized by V. dahliae. Complete resistance to race 2 has not yet been found, and we will continue to screen the collection in hopes of finding this trait. This includes new L. serriola accessions collected through the efforts of the USDA, Plant Exchange Office.

4. Accomplishments

Last Modified: 07/27/2017
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