This award recognizes outstanding contributions to regulatory plant pathology, crop security, and trade enhancement efforts by APS members.
A huge challenge in plant regulatory diagnostics is speed, accuracy, and large sample volumes. Often, with disease outbreaks the testing laboratory must rapidly adapt to cope with the surge in samples. The detection of Asian citrus psyllid (ACP) and citrus huanglongbing (HLB) disease in California triggered federal and state quarantines requiring many samples to be tested using USDA-validated protocols. These protocols were single-tube PCR tests suitable for processing a maximum of a few hundred samples a week. This was the situation in 2011 when Lucita Kumagai, senior plant pathologist, took over as the team leader for HLB diagnostics at the California Department of Food and Agriculture (CDFA) Plant Pest Diagnostics Center. Kumagai was born in Cavite City, Philippines, and received her bachelor's (1986) and master's (1992) degrees in biological sciences from California State University, Sacramento.
In California, the insect vector ACP was first detected in 2008, and by 2010, it had spread to eight counties. Initially, with statewide surveys, the laboratory processed 200–300 plant samples a week. Kumagai anticipated the sample number would rise dramatically when infected trees or ACP were found. To address this inevitability, she conducted experiments and validated a high-throughput protocol using the same diagnostic primers as the USDA-validated low-capacity protocol. The USDA protocol tested for Candidatus Liberibacter americanus and Ca. Liberibacter asiaticus in separate reactions, requiring that each tube be processed individually. In contrast, the proposed high-throughput format was set up to grind, extract DNA using robotic liquid handlers, and perform the PCR reaction in a 96-well plate format. The validated high-throughput protocol demonstrated that the reactions could be run in a 96-well format with both detection reactions and internal controls present in the same reaction mixture. Kumagai and her team also validated a new set of primers, which increased the specificity and sensitivity of the HLB qPCR assay. These changes were subsequently approved by the USDA. The improved protocol reduced the cost of consumable supplies by more than 50%, significantly increased lab sample capacity, reduced nonspecific amplifications, and minimized repetitive motion injuries to the staff. Since the first detection of an HLB-infected tree in 2012, the number of samples tested has increased significantly each year. Through continued innovation, testing efficiencies, and additional employees, the CDFA laboratory can now process up to 2,500 plant samples a week. The lab has successfully tested an exponentially increasing number of plant and ACP samples over the last several years (from 23,078 samples in 2011 to over 154,159 samples in 2018).
Early detection of Ca. Liberibacter asiaticus in citrus trees is a major challenge due to uneven distribution of the pathogen and the difficulties of sampling large trees and finding the symptomatic foliage. Infected trees may be asymptomatic, or the symptoms may be barely visible in the dense canopy. In addition, data from researchers in Florida and Texas showed that, in the initial phase of disease spread, many samples had higher background Ct values but did not test in the positive range. Normally, these high Ct-value samples would be considered as inconclusive or negative and could possibly miss being detected. Therefore, Kumagai proposed resampling the high Ct-value citrus trees by quadrants and testing each quadrant separately. She worked with the survey crew and helped to optimize the sampling protocol while considering practical challenges in the field. Using this protocol, researchers on numerous occasions have found that only one of the four quadrant samples tested positive, which probably would have been missed by the previous sampling method. Quadrant sampling of high-risk trees increased detection and removal of infected trees (up to 2,303 residential trees so far) by at least 30% and most likely slowed the spread of HLB inoculum.
Kumagai has collaborated with researchers in the USDA, universities, and other states to advance HLB diagnostics. She continues to share samples and data with researchers to answer questions about genotypic variation of both the psyllid vector and the pathogen, help researchers experiment with early-detection technologies (including working with the F1K9 dog team trained to sniff out HLB-infected trees), and improve risk models so that HLB statewide surveys, tissue-sampling strategies, and assays can be further refined. These collaborations have resulted in 11 scientific publications. A tireless educator on HLB, she has presented webinars to groups at the USDA and frequently speaks about the disease to county staff, master gardeners, foreign agriculture officials, and citrus growers.
To address the continually increasing demand for more trees to be tested, the department initiated a program that would issue permits to allow private and grower-funded laboratories in California to test non-regulatory samples for HLB. Kumagai and her team helped develop and implement this program and established minimal facility requirements and testing protocols for interested laboratories. Participating laboratories were inspected to evaluate equipment, personnel, and planned workflow to ensure the testing was done per protocol and minimize the risks of contamination and false results. A proficiency testing program was deemed necessary to ensure best laboratory practices and reliability of test results to take regulatory action. Kumagai played a key role in procuring validated proficiency samples from the USDA, administering the proficiency test to participating laboratories biannually, and finalizing the permit and laboratory facility inspection guidelines.
In addition to her regular duties, Kumagai also inspected candidate laboratories, followed up on technical lab issues, and advised on needed improvements, including on the design for an addition to a lab for molecular diagnostics and provided advice on laboratory decontamination, reorganization, and remodeling. Due to her efforts and the determined staff at the Citrus Research Board (CRB) Jerry Dimitman Laboratory, the CRB lab received USDA accreditation in August 2018, which allows them to test ACP and plant samples in California. This collaboration with CRB and the accreditation of their lab will support the CDFA HLB testing program and, thus, help to contain the spread of this disease and protect the multibillion-dollar citrus industry in California.