Project Number: 5302-22000-011-00
Project Type: Appropriated

Start Date: Apr 09, 2012
End Date: Apr 08, 2017

Objective:
Citrus tristeza virus and Spiroplasma citri, causal agent of citrus stubborn disease, are important production-limiting, insect-vectored pathogens in California. Early host response (e.g. over-expressed microRNAs and small interfering RNAs) associated with single and multiple strain graft inoculations of these pathogens in greenhouse tests will be characterized and developed as biomarkers for early-stage pathogen-specific detection. The overall goal is to elucidate mechanism of mild strain cross-protection of Citrus tristeza virus and pathogen genetic bottlenecks resulting from insect vector transmission. New knowledge gained will lead to improved sustainable management strategies for these citrus disease agents and their insect vectors. 1. Develop field deployable systems that provide rapid, sensitive detection of Citrus tristeza virus and Spiroplasma citri in citrus. 2. Determine and characterize genetic variations in Citrus tristeza virus and Spiroplasma citri strains before and after vector passage. 3. Determine changes in host gene expression in Citrus tristeza virus cross-protected citrus that can be used to screen for cross-protective strains of Citrus tristeza virus.

Approach:
Citrus tristeza virus (CTV) and Spiroplasma citri are both phloem-limited pathogens of citrus and are transmitted by insect vectors. The California Department of Food and Agriculture and stakeholders manage CTV in commercial groves and urban areas by an eradication program and maintenance of pathogen-free budwood sources and citrus propagations in commercial nurseries. CTV eradication was modified in 2009 to detect and eliminate only citrus trees infected with severe strains of CTV. This is achieved by screening field strains of CTV by serology with a monoclonal antibody and polymerase chain reaction (PCR) assays with genotype-specific sequence markers. Genetic diversity of field strains CTV and S. citri will be characterized with respect to the molecular and genetic basis of their host-pathogen-vector relations and disease epidemiology. Further improvement in pathogen surveillance and control requires filling knowledge gaps in host response to pathogen infection as mild CTV strains, which no longer are being eradicated, continue to spread by indigenous aphid vectors. The nature, basis and mechanism(s) of cross-protection will be identified and characterized. CTV symptom phenotype with mixed infections will be monitored for post-translational gene silencing through analysis and characterization of small interfering RNAs, micro RNAs and macro RNAs. New information will facilitate mitigation of losses and misidentifications of CTV and S. citri with the ultimate goal of developing sustainable, integrated management strategies for tristeza and stubborn in California.