1a. Objectives (from AD-416):
The overall goal of this project is to mitigate losses due to Xylella fastidiosa-caused diseases during crop production and to develop effective, sustainable disease and insect-vector management strategies by characterizing host-pathogen-vector-environment interactions of these complex pathosystems. Specific objectives for the project are outlined below: Objective 1: Characterize genomic and biological diversity of Xylella fastidiosa strains. Objective 2: Elucidate molecular interactions of Xylella fastidiosa with horticultural and model plant hosts that result in disease susceptibility or resistance. Objective 3: Determine biological/ecological/transmission properties of Xylella fastidiosa vectors. Objective 4: Develop and assess strategies to manage diseases caused by Xylella fastidiosa.
1b. Approach (from AD-416):
Pierce’s disease (PD) is an economically important disease affecting US grape production. PD is caused by the xylem-limited bacterium Xylella fastidiosa (Xf), a generalist pathogen also causing disease in numerous horticultural crops and landscape ornamentals. In California, prevalence and incidence of PD increased following introduction and establishment of the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis. Presently, PD is managed in California via an area-wide surveillance and insecticide application program aimed at suppressing GWSS populations in citrus (the most common feeding/oviposition host) and urban landscape plants. Development of an integrated management program for PD requires detailed knowledge of host-pathogen-vector-environment interactions. Such knowledge is limited for this complex pathosystem involving multiple hosts and vectors, a genetically diverse pathogen, and a non-uniform agro-ecosystem. The project objectives are designed to address knowledge gaps in the biology, ecology, and genetics of Xf, sharpshooter vectors, and host plants. Genomic diversity, as well as the evolutionary, biological and epidemiological relationships among Xf strains will be characterized. The genetic and molecular basis of host-pathogen-vector interactions will be determined to better understand PD development and epidemiology. Biotic and abiotic factors that affect sharpshooter ecology and pathogen transmission mechanisms/efficiency will be identified. The nature, basis, and mechanism(s) of host resistance to Xf will be identified, characterized, and incorporated into advanced grape selections. New information and products will facilitate mitigation of PD losses, with the ultimate goal of developing an environmentally friendly, integrated management strategy that may augment or replace the area-wide surveillance and insecticide application program.
3. Progress Report:
FY13 research focused on Xylella fastidiosa (Xf), the bacterium responsible for Pierce’s disease (PD) of grapes, and sharpshooter insects that transmit Xf. The research was performed in the context of four objectives dealing with genetic analyses of Xf and grapes, Xf-host plant interactions, sharpshooter ecology/biology/transmission, and PD resistance/control. Progress on Objective 1 (Characterize genomic and biological diversity of Xf strains) included genome sequencing of Xf strains from red oak and muscadine grapes. Comparison of these two Xf genomes with previously sequenced strains is ongoing. Greenhouse experiments designed to evaluate the potential of Xf strains from olive to cause PD were initiated. Genome sequencing of Xf strains from olive has commenced. Progress on Objective 2 (Elucidate molecular interactions of Xf with horticultural and model plant hosts that result in disease susceptibility or resistance) focused on i) Xf genes associated with virulence/pathogenicity, and ii) genetic differences among grapevines that are resistant or susceptible to PD. Gene structure/function studies were conducted with two toxin-antitoxin systems that regulate Xf growth and biofilm formation. Xf proteins of unknown function were evaluated as potential virulence factors in plants using a transient expression system. A seedling population derived from a cross between PD-susceptible and PD-resistant grapes was evaluated for differences in gene expression. Resulting profiles were compared to identify expression patterns specific to PD-resistance. Gene sequences specific to PD-resistant seedlings were used to design DNA-based assays for marker-assisted selection. Progress on Objective 3 (Determine biological/ecological/transmission properties of Xf vectors) focused on sharpshooters. A blue green sharpshooter colony was established to use as a model system for Electrical Penetration Graph (EPG) monitoring of feeding in relationship to Xf inoculation. Preliminary EPG assays identified waveforms representing key behaviors. A protocol to estimate egg maturation rates of individual glassy-winged sharpshooter (GWSS) females was developed. Experiments to determine the effects of plant sap chemistry on GWSS egg maturation rates were initiated. Two compounds released by GWSS-infested grapevines were shown to be attractive to a GWSS egg parasitoid. Chemicals emitted by red-tip photinia, an alternative GWSS host, also are being analyzed to identify parasitoid attractants. Progress on Objective 4 (Develop and assess strategies to manage diseases caused by Xf) focused on breeding and cultural control. Crosses were performed in both table grapes and raisins to combine fruit quality with PD resistance. Additional crosses combined PD resistance with powdery mildew (PM) resistance; resistant seedlings were transplanted to the field. A new PD-evaluation field plot was established in Monte Alto, TX with 13 advanced table grape selections. Rootstock effects on susceptibility of scions to PD were evaluated. PD-susceptible scions grafted onto rootstock ‘101-14MG’ were more tolerant to Xf infection than when grafted onto other rootstocks.
Voegel, T.M., Doddapaneni, H., Cheng, D., Lin, H., Stenger, D.C., Kirkpatrick, B., Roper, C. 2013. Identification of a response regulator involved in surface attachment, cell-cell aggregation, exopolysaccharide production and virulence in the plant pathogen, Xylella fastidiosa. Molecular Plant Pathology. 14:256-264.