2009 Annual Report 1a.Objectives (from AD-416) 1) Develop qPCR probes for the different Dekopon sub-isolates and quantify each in single and mixed infections to assess interference in accumulation/replication of each sub-isolate. 2) Examine P20, P23, and CP (suppressors of gene silencing) for each sub-isolate for genetic variation and associate with symptom expression. 3) Characterize siRNA profiles in mixed and single infected plants. 1b.Approach (from AD-416) Aphid transmission, reverse transcription (RT) polymerase chain reaction (PCR), real time PCR (qPCR), cloning, sequencing, western blots, northern blots, hybridizations. Documents Reimbursable with Citrus Research Board. Log 37119. 3.Progress Report Mixed infection with two tristeza complexes were compared to determine if replication of one isolate affects symptom expression. In this study, symptom severity was decreased and associated with higher replication of the mild component in the mixture. A gene P20 which codes for the major component of the amorphous inclusion bodies in CTV-infected cells was also examined. Mild symptoms were associated with low accumulation of P20 from the severe strain. This research was conducted in Parlier in collaboration with a visiting scientist. Research is monitored by frequent internet correspondence and site visits. This research supports ARS mission to examine pathogen biology and virulence determinants. U.S. citrus growers and citrus industries around the world are stakeholders that will benefit from research that should lead to more durable and predictable cross protection.