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ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Subtropical Plant Pathology Research » Research » Publications at this Location » Publication #335687
Title: Risk based HLB survey for Hacienda Heights and San Gabriel in southern CA

Author
item Gottwald, Timothy
item LUO, WEIQI - North Carolina State University

Submitted to: Journal of Citrus Pathology
Publication Type: Abstract Only
Publication Acceptance Date: 12/20/2016
Publication Date: 5/18/2017
Citation: Gottwald, T.R., Luo, W. 2017. Risk based HLB survey for Hacienda Heights and San Gabriel in southern CA. Journal of Citrus Pathology. 4(1):15/45.

Interpretive Summary: Given the recent findings of HLB in Los Angeles, it is critical to efficiently detect HLB/ACP locations to prevent further spread of the disease. A residential survey methodology is proposed where the area surrounding HLB findings is partitioned into different sampling locations each equipped with survey strategies designed on an overall risk. This dynamic risk-based sampling plan maximizes HLB/ACP detection and adjusts to new HLB findings.

Technical Abstract: After the discovery of a single residential HLB tree in the Hacienda Heights area in 2012, another HLB positive tree was confirmed in San Gabriel early July 2015. This is the second time the disease has been identified in Los Angeles. Subsequently 17 additional locations have been discovered with HLB within 800 meters of the initial infected tree. To prevent further spread of the disease, it is necessary to design a high intensity residential survey to quickly identify additional HLB-infected trees in the area. An extended quarantine with approx. 5 mile radius (i.e. 87 square miles) was created around the properties on which HLB was recently detected in the city of San Gabriel. Residential population density does not have a uniform distribution in this part of the Los Angeles basin. For balanced and efficient sampling like the Hacienda Heights area, San Gabriel is divided into eight radiating segments (defined as N, NE, E, SE, S, SW, W, NW) around the centroid of recently found HLB positive properties. Six inner buffer zones (i.e. 400m, 800m, 1200m, 1 mile, 2 miles & 5 miles) were also created to adapt different sampling strategies if needed. The overall sampling proportion in each segment is determined by the number of households and previous ACP finds. Percent sampling effort ranges from 9.2% to 15.6% among the eight directional sectors. Within each segment, sampling locations (i.e. census blocks) are selected based on an overall risk calculation, which is a combination of predetermined residential HLB/ACP risk, distance from HLB finds, census travel risk from Asian connections, previous ACP density, and distribution of CLas+ ACP locations (note, inconclusive CLas+ ACP locations were also considered with less weighting). The residential property density also influences the chance of location selection and modulates sampling intensity. According to the manpower availability and sampling efficiency, total sampling capacity of 52,800 properties is calculated through the survey period. A total of 948 moderate to high risk census blocks (i.e. risk>0.8) were identified within the eight sectors through the risk-based model. Using the overall risk calculation we define, 81% of census blocks in the San Gabriel quarantine area are sampled at least once. Approximately 10% of the properties within the risk-defined census blocks are surveyed to give a reliable representation of HLB dispersal. To maximize HLB/ACP detection in San Gabriel, we provide a detailed sampling plan for three temporally contiguous sampling cycles with approximately the same duration. Because of the intensity of risk in some areas, there is some overlap in the selection of census blocks to sample, which will provide multiple opportunities to detect the disease in higher-risk areas as the tree phenology changes due to flushing and climate. The designed survey methodology can be used to monitor actual HLB situation, check whether HLB has spread out of the quarantine area, and determine the possible pathway for HLB spread. The survey methodology is flexible and dynamic. If additional HLB locations are found outside the original HLB epicenter, adjustments can be made to address sample intensity in the new areas.