Rapid and accurate detection of huanglongbing in citrus by elasticity testing using a piezoelectric finger

Rapid and sensitive detection of citrus huanglongbing (HLB) is critical for the control of this devastating disease. In this study, we have evaluated using a piezoelectric finger (PEF) with a 0.4 mm probe to measure the elastic modulus of a leaf to detect HLB in four different species of citrus including grapefruit (GFT), pumelo (PUM), lemon (LEM), and Valencia orange (VAL). Diseased citrus leaves were harvested from trees testing positive for the presence of Candidatus Liberibacter asiaticus (Las), the causal agent of HLB, and included both symptomatic leaves, which were blotchy mottle or yellowing and asymptomatic leaves, which did not display outward symptoms. Healthy leaves were harvested from trees testing negative for Las. The results indicated that the PEF elastic modulus test exhibited an overall 94% sensitivity and 90% specificity against the Las status of the trees for all four citrus types combined. Comparative quantitative real-time polymerase chain reaction (qPCR) tests on the same leaves showed an overall 89% sensitivity and 100% specificity against the Las status of the trees. While a Cohen-Kappa coefficient of 0.81 was obtained between the PEF and qPCR predictions, suggesting a "strong" agreement between the PEF and qPCR tests, a more detailed examination indicated that PEF was more sensitive overall in detecting the Las positive trees than qPCR, particularly from asymptomatic leaves for which PEF was 96% sensitive versus 78% sensitive by qPCR, indicating the potential of using PEF for early detection of HLB.