|Yokomi, Raymond - Ray|
|HAREJI, SUBHAS - Central California Tristeza Eradication Agency|
Submitted to: Citrograph
Publication Type: Trade Journal
Publication Acceptance Date: 11/30/2022
Publication Date: 3/1/2023
Citation: Yokomi, R.K., Hareji, S. 2023. Surveys of Citrus tristeza virus in commercial citrus orchards in California. Citrograph. available: https://citrus-research-board-static.sfo2.digitaloceanspaces.com/citrograph/pdf/CRB-Citrograph-Mag-Q2-Spring-2023-Web.pdf
Interpretive Summary: A Florida Citrus tristeza virus vector (CTVvv) expressing spinach defensin has been approved for field release in Florida commercial citrus to help combat Huanglongbing (HLB). To use CTVvv in California, regulatory approval is required to ascertain CTV risk. Therefore, a survey of commercial citrus orchards in the twelve major citrus-producing counties in California was evaluated. CTV surveys conducted from 2010 to 2013 in Tulare, Kern and Fresno County Pest Control Districts (PCDs) estimated the CTV incidence on average was 0.2%. Our new survey, conducted in 2020-2022, showed that CTV incidence in southern California counties ranged from 34.9% to 6.3% in San Diego, San Bernardino, Riverside and Ventura. T30 was the predominant strain comprising over 95% of CTV detected either alone or in mixtures. T36 was rarely detected in our survey. T30 was also found to be the predominant CTV strain found in central California. Overall, no evidence was found that CTV was affecting citrus production in California. Since T30 is endemic, symptomless in all citrus cultivars on non-sour orange rootstocks, a mild strain of California T30 is the safest choice to develop as a viral vector.
Technical Abstract: California citrus orchards were surveyed to estimate the incidence and strains1 of Citrus tristeza virus (CTV) present in the state. The objective of this survey was to document the presence of CTV in commercial orchards and determine the CTV strain most appropriate to develop as a future CTV viral vector to help fight Huanglongbing (HLB) in California. CTV surveys conducted from 2009 to 2013 in Tulare, Kern and Fresno County Pest Control Districts (PCDs) estimated the CTV incidence ranged from 0.2% to 1.3% with an overall average of 0.2%. Surveys conducted from 2020-22 in portions of Ventura, Riverside, San Bernardino, and San Diego counties estimated county-wide CTV incidence ranged from 6.3% to 34.9%. CTV was rarely found in Santa Barbara, Madera, Imperial San Luis Obispo, and Monterey counties. CTV strain evaluation of 550 CTV accessions maintained in-planta by the Citrus Pest Detection Program, Tulare, California found over 59% of the representative isolates tested were T30. Other strains detected included VT, RB, and S1 but the T36 strain was rarely detected. In the 2020-2022 survey, over 95% of CTV detected was T30 alone or in mixtures with other strains. No sign of economic impact of CTV was observed in orchards sampled though citrus on sour orange rootstock was not included in the survey. These data indicate T30 is the most common CTV strain in the state and, hence, is the most appropriate strain to develop as a CTVvv for citrus in California.