Spread of Citrus tristeza virus in citrus orchards in central California

Authors: Yokomi, Raymond - Ray; Sisterson, Mark; HAJERI, SUBHAS - Central California Tristeza Eradication Agency

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2020
Publication Date: 5/12/2020
Citation: Yokomi, R.K., Sisterson, M.S., Hajeri, S. 2020. Spread of Citrus tristeza virus in citrus orchards in central California. Plant Disease. 104(7):1925-1931. https://doi.org/10.1094/PDIS-08-19-1791-RE.
DOI: https://doi.org/10.1094/PDIS-08-19-1791-RE

Interpretive Summary: Citrus tristeza virus (CTV) is one of the most destructive viruses of citrus. Some strains of CTV are associated with severe disease symptoms and loss of tree productivity, whereas other CTV strains result in asymptomatic infections. To assess infection status, trees are often tested with two enzyme-linked immunosorbent assay (ELISA) antibodies. The first is a general antibody that reacts with all CTV strains, regardless of severity. The other antibody, referred to as MCA13, is primarily associated with severe CTV strains. Citrus growing areas in California are divided into pest control districts. Some pest control districts mandate surveying citrus orchards and removing all trees that have a positive reaction to the MCA13 antibody. Other pest control districts do not survey or remove any CTV infected trees. To assess the benefits of removing trees that react to the MCA13 antibody on CTV spread, rates and patterns of CTV spread were compared between an area that surveys and removes CTV infected trees that react with the MCA13 antibody to a district that does not survey or remove CTV infected trees. In the district that surveys orchards and removes MCA13-positive trees, only two MCA13-positive trees were identified in study orchards over a ten-year period (2008-2018). In addition, rate of spread of mild CTV strains (i.e., strains that do not react to MCA13) was also limited, most likely due to mandated insecticide sprays to suppress vector populations. In the district that does not survey and remove MCA13-positive trees, rates of spread of CTV strains that do not react with MCA13 was 9 times greater than the rate of spread of CTV strains that react with MCA13. As aphids may move long distances, the reduction in rate of spread of MCA13 strains of CTV suggests that there may be a regional benefit to removing trees that have a positive reaction to the MCA13 antibody. Results of this study provide stakeholders, regulators, and scientists with the information necessary to evaluate the benefits of the survey and tree removal program and is useful for making recommendations to improve management.

Technical Abstract: In California, citrus tristeza virus (CTV) is regulated by quarantines with central California divided into five Pest Control Districts (PCDs). Two PCDs do not abate CTV-infected trees; whereas three maintain an abatement program that surveys and removes trees infected with CTV that reacts with MCA13, a monoclonal antibody that purportedly reacts to severe CTV isolates. In this study, CTV spread was monitored by a universal polyclonal antibody and MCA13 in four plots in an abatement zone and three plots in an adjacent non-abatement district. In abatement plots, a 25% hierarchical sampling protocol was used, with samples from four adjacent trees pooled. Over a ten-year period, only two MCA13-positive samples were observed in abatement district plots. While MCA13-negative CTV strains were observed, spread was limited presumably due to a grower-supported aphid control program. In non-abatement district plots, trees were individually tested. Although incidence of MCA13-positive and -negative strains did not differ in 2015, rate of spread between 2015 and 2018 was nine times greater for MCA13-negative than MCA13-positive strains. Analysis of the spatial distribution of CTV infected trees suggests that spread of MCA13-negative CTV strains was due largely to primary spread; whereas spread of MCA13-positive CTV strains was due largely to limited secondary spread. Collectively, the results suggest that spread of MCA13-positive CTV strains has been limited by a combination of tree removal and aphid vector suppression. In non-abatement plots, additional molecular analysis of MCA13-positive trees determined that the majority consisted of VT strains or a mixture of VT and T30 strains. Evaluation of disease symptoms on MCA13-positive trees in non-abatement plots indicated that all were asymptomatic as the citrus was planted on a CTV-tolerant rootstock.