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Description of a novel mild strain of Citrus tristeza virus in California that reacts with monoclonal antibody MCA13

Raymond Yokomi: USDA, ARS, SJVASC

<div>Quick decline caused by <em>Citrus tristeza virus</em> (CTV) destroyed citrus on sour orange rootstock in southern California during the 1930’s -40’s. However, use of resistant/tolerant CTV rootstocks, certified pathogen-free budwood, and quarantines have limited further economic damage from CTV. Multi-locus marker profiles of California CTV isolates revealed presence of multiple genotypes but bioindexing showed two general phenotypes: mild; and various degrees of seedling yellows/stem pitting. To examine the genetic diversity involved, full-length genomes of three California CTV isolates were determined by siRNA sequencing. Phylogenetic analyses differentiated these isolates into the VT genotype (CA-VT-AT39) and a new genotype called S1 (CA-S1-L and CA-S1-L65). The three isolates all reacted with MCA13, a monoclonal antibody used by the Central California Tristeza Eradication Agency (CCTEA) to identify potentially harmful CTV isolates. Bioindexing showed that CA-VT-AT39 was severe but CA-S1-L and CA-S1-L65 were mild. Screening the <em>in planta</em> CTV accessions collected from 1968 to 2011 by the CCTEA for S1 strains resulted in identification of 42 other S1 suggesting it has been present since before 1968 in California. BLAST and phylogenetic analysis of the S1 <em>p</em>25 gene region with other extant CTV sequences from NCBI suggested putative S1-like isolates may occur elsewhere in the Mediterranean and Asian regions. This information is useful for the management of CTV.</div>