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Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/7/2011 Publication Date: 1/20/2012 Citation: Yokomi, R.K., Sisterson, M.S. 2012. Validation and comparison of a hierarchal sampling plan for estimating incidence of citrus stubborn disease. International Organization of Citrus Virologists Proceedings. Available: http://www.ivia.es/iocv/archivos/Proceedings_XVIII_Conference/Yokomi_and_Sisterson.pdf. Interpretive Summary: Citrus stubborn disease (CSD) is a production-limiting disease caused by Spiroplasma citri, a culturable wall-less prokaryote. The pathogen is transmitted mainly by the beet leafhopper in California. Previously, a polymerase chain reaction (PCR) assay to detect S. citri in infected samples was developed. The objective of this study was to use PCR to detect the pathogen in field trees and validate a sampling method to estimate disease incidence. To accomplish this, incidence of CSD was determined in 19 citrus plots by testing each tree by PCR for infection by this pathogen. The data were used to simulate two hierarchical sampling plans where plots were grouped into quadrats of 4-adjacent trees and samples counted from only 25% of the quadrats: i) bulk-screened by combining all four trees in a quadrat to determine the proportion of quadrats that had one or more infected trees: ii) screened all four trees per quadrat individually, with incidence calculated as the proportion of infected trees. The accuracy of the two sampling plans was assessed using a computer program to simulate the two sampling plans using the field-collected data. With disease incidence < 30%, accuracy of incidence estimates from both sampling plans was similar. However, with disease incidence >45%, screening trees individually was more accurate at estimating disease incidence than bulk screening samples. Although visual symptoms of CSD can be confused with other pathogens or horticultural maladies, previous estimates of CSD incidence were based on visual symptoms because bacterial isolation and culturing of a large number of samples was impractical. Disease diagnosis using PCR was a great improvement along with high throughput testing procedures. With validation of the sampling methods reported here, the procedure becomes useful for epidemiological studies of CSD as well as a management tool for disease mitigation. Technical Abstract: Citrus stubborn disease (CSD) is a production-limiting disease that is caused by Spiroplasma citri, a culturable wall-less prokaryote. The pathogen is transmitted mainly by the beet leafhopper (BLH) in California. The objective of this study was to validate a method to estimate incidence of CSD in the San Joaquin Valley of California. To accomplish this, 100% of trees from 19 field plots located within 5 citrus groves were screened via real-time PCR to determine true incidence within each field plot. The data were used to simulate two hierarchical sampling plans. Both sampling plans divided groves into quadrats of four trees and collected samples from only 25% of quadrats. The first sampling plan, bulk-screened with all four trees in a quadrat and determined the proportion of sampled quadrats that contained one or more infected trees. This value was used to estimate incidence assuming that infections were distributed randomly. The second sampling plan screened all four trees in a quadrat individually, with incidence calculated as the proportion of infected trees. To evaluate the accuracy of the two sampling plans at estimating incidence, a computer program simulated the two sampling plans using the field collected data. With disease incidence < 30%, accuracy of incidence estimates from both sampling plans was similar. However, with disease incidence >45%, screening trees individually was more accurate at estimating disease incidence than bulk screening samples. |
