Progress 10/01/12 to 09/30/13
Outputs
Progress Report Objectives (from AD-416): Objective 1. Provide BLS phenotype evaluations Lettuce Breeding objectives. Objective 2. Screen bacterial strains available in our collection for their ability to overcome BLS resistance currently deployed. Objective 3. Monitor strains of Xcv isolated from diseased lettuce throughout California for ability to overcome resistance. Approach (from AD-416): We will conduct laboratory and greenhouse experiments of lettuce to advance disease resistance breeding, understanding of mechanisms involved in resistance and to understand how pathogen diversity influences disease. This project relates to objective C: Identify emerging diseases and their etiology and evaluate resistant germplasm for diseases of strawberry, lettuce, and vegetables. Bacterial leaf spot of lettuce caused by the pathogen Xanthomonas campestris pv. vitians (Xcv) results in significant losses throughout California the US. Genetic diversity of over 100 strains collected world- wide was described. Pathogen genotypes present in California elicited a hypersensitive response within 24 hours on host cultivars previously identified as resistant. In contrast, other genotypes of the pathogen from other geographic regions did not. Disease progress was significantly reduced in resistant cultivars compared to disease progress in susceptible cultivars inoculated with the California strains. The relationships between virulence and genotype are being further explored. In order to find sequences useful for the specific detection and quantification of the pathogen, whole genomes of ten strains of the pathogen and related organisms were sequenced.
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Progress 10/01/11 to 09/30/12
Outputs
Progress Report Objectives (from AD-416): Objective 1. Provide phenotypic data for breeding lines essential to the development of BLS resistant germplasm. Objective 2. Determine if resistant, susceptible and important parental cultivars differ in their influence on Xcv populations. Objective 3. Develop a reliable and sensitive scheme for detecting and quantifying bacteria from potential inoculum sources. Objective 4. Evaluate the effect of pathogen diversity on severity of disease on susceptible and resistant cultivars. Approach (from AD-416): We will conduct laboratory and greenhouse experiments of lettuce to advance disease resistance breeding, understanding of mechanisms involved in resistance and to understand how pathogen diversity influences disease. This project relates to objective C: Identify emerging diseases and their etiology and evaluate resistant germplasm for diseases of strawberry, lettuce, and vegetables. Bacterial leaf spot (BLS) of lettuce caused by the pathogen Xanthomonas campestris pv. vitians (Xcv) results in significant losses throughout California. Among the cultivars evaluated, the resistant cultivar was the only one on which Xcv elicited an HR within 24 hours. In plant tissue from these cultivars, bacterial populations levels did not increase at the same rate or to the same level as those on susceptible cultivars. We demonstrated differences in virulence among strain mixtures used to evaluate resistance here and abroad. A mixture of strains from Quebec, Canada, was more virulent than the mixture of strains from California. Preliminary data indicate that this is because two of the three strains are more virulent than strains from California. Only one strain from California appears to be as virulent as the strains from Quebec. Two of the three strains from Quebec belonged to a different genetic group than the third strain and the strains from California. According to analyses with three genes sequenced from 120 Xcv strains, there are three different genetic groups of Xcv. The Quebec strains belong to a group with few representatives and to which new pathogens from radicchio belong. Sequences from these three genes and an additional three genes are being used to design new specific primers for detection and quantification, because the PCR primers and protocols currently available are not specific for Xcv strains.
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Progress 10/01/10 to 09/30/11
Outputs
Progress Report Objectives (from AD-416) Objective 1. Provide phenotypic data for breeding lines essential to the development of BLS resistant germplasm. Objective 2. Determine if resistant, susceptible and important parental cultivars differ in their influence on Xcv populations. Objective 3. Develop a reliable and sensitive scheme for detecting and quantifying bacteria from potential inoculum sources. Objective 4. Evaluate the effect of pathogen diversity on severity of disease on susceptible and resistant cultivars. Approach (from AD-416) We will conduct laboratory and greenhouse experiments of lettuce to advance disease resistance breeding, understanding of mechanisms involved in resistance and to understand how pathogen diversity influences disease. An initial experiment conducted to compare the virulence of Xanthomonas campestris pv. vitians (Xcv) strains from the US verses that of Canadian strains on susceptible and resistant lettuce cultivars demonstrated that the strains differed for virulence on various cultivars. We gathered 70 Xcv strains from around the world and began the sequencing of 16S rDNA and four housekeeping genes in order to determine genetic lineages. In separate experiments, we investigated the influence of host plant diversity on growth and survival of the pathogen. Evaluation of changes in population levels of Xcv on cultivars that were previously characterized as either susceptible or resistant indicated that resistant cultivars support lower numbers of the bacterial pathogen than susceptible cultivars support. Population dynamics are directly related to disease development and cultivars supporting higher populations of the pathogen had higher levels of disease in these experiments. These data indicate that resistance to Bacterial Leaf Spot (BLS) is in part due to differences in the ability of the pathogen to grow or survive on or in the lettuce leaves. Additional experiments will further investigate the location and timing of development of the differences in population dynamics. In addition to understanding how cultivars influence the growth and survival of the pathogen, we began experiments to compare methods for detection and quantification of the pathogen from environmental samples. Maltose Methyl Green (MMG) medium was confirmed to be the best semi- selective medium for isolation and quantification of the pathogen. However, DNA-based detection methods were also improved as part of this research.
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