Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: October 22, 2010
Publication Date: November 7, 2010
Citation: Yokomi, R.K., Sisterson, M.S. 2010. Estimation of Incidence and Spatial Temporal Distribution of Citrus Stubborn Disease [abstract]. International Organization of Citrus Virologists Proceedings. 31:s53. Technical Abstract: Citrus stubborn disease (CSD) is caused by Spiroplasma citri, a culturable prokaryote principally vectored by the beet leafhopper (BLH) in California. The objective of this study was to develop a method to estimate incidence of CSD. A 100% sample was compared with a 25% sample collected by a hierarchical sampling (HS) plan. The HS was modified by keeping quadrat samples separate and testing each for infection by S. citri. Plots in five central California citrus groves were established ranging in size from 232 to 768 trees and arranged in blocks or as a whole grove. Sample DNA was extracted from fruit columella or leaf petioles and assayed for the presence of S. citri DNA by real-time PCR. Estimating disease incidence as [-ln(Z/N)]/4 where Z = number of sets without infection and N = total number of sets, a high correlation was found between the observed and predicted values as long as disease incidence was < 0.50. The 95% confidence interval included zero, suggesting that on average a Poisson distribution gave a good estimate. Spatial and temporal distribution of CSD was examined over three years in two groves in 256-tree plots with 3 replications surrounded by citrus plantings. Infected trees in both plots did not have a clumped distribution when incidence was less than 31% (P < 0.001) and no edge effect was detected. No new infections were detected despite infection levels of 3% to 31%. In a different citrus grove next to untilled rangeland/foothills (an overwintering habitat of the BLH) significantly more (P < .001) infections were found in citrus trees adjacent to row crops rather than the foothills. For citrus next to row crops, infected trees were more clumped than expected if infections were distributed randomly. In summary, a method was validated to accurately estimate CSD incidence. Since no tree to tree spread of S. citri was detected, natural spread of S. citri appeared to be due to infective leafhoppers migrating or dispersing from mature or harvested row crops or weeds. These data are consistent with observations of BLH thriving on weed hosts in or around citrus groves and surviving temporarily on citrus at the interface with row crops until new host crops germinate. This information can be integrated with cultural practices to mitigate spread of S. citri.