1a.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.
1b.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.