Detecting HLB-infected trees

Finding HLB-infected trees and eliminating them before ACP picks up the disease and spreads it to neighboring trees is a major challenge. The pathogen in the tree cannot be detected by leaf testing for three to nine months after infection, and the symptoms don’t show up in the tree for a year or more after infection. Meanwhile, the disease can be spread by ACP. Research is under way to develop early HLB detection so that infected trees can be rapidly removed.

Electronic “sniffer” for determining HLB

Scientists at UC Davis are refining a mobile chemical sensor that can detect diseased citrus trees by sniffing their volatile organic compounds (VOCs). VOCs are emitted by all types of plants and contribute to their distinctive odors—such as the perfume of orange blossoms and pungent scent of garlic in the air. VOCs must exist at very high levels for humans to smell them, and there are some VOCs people cannot smell at all. Cristina Davis, professor in the UC Davis Department of Mechanical and Aerospace Engineering, and Abhaya Dandekar, professor in the UC Davis Department of Plant Sciences, collected samples of VOCs emitted from HLB-infected trees in Florida every month for a year in order to “train” the mobile sensor to recognize the “smell” of HLB. “The idea is to extract a group of compounds that create the signature for the presence of HLB,” Dandekar said. A software program develops an algorithm that lets the machine know it is detecting HLB. Davis is working with Applied Nanotech, Inc., in Texas to commercialize this artificial nose.

Metabolic changes can signal presence of disease

When pathogens infect plants or people, changes begin almost immediately to the metabolism of the host. Carolyn Slupsky, UC Davis assistant professor with a split appointment in the Department of Nutrition and Department of Food Science and Technology, is looking at the metabolism of citrus trees infected with the pathogen that causes HLB. “We want to see what chemical or metabolic processes change when the tree is infected,” Slupsky said. “Once we understand the metabolic changes induced by the pathogen, the information may help with development of earlier detection methods and new treatments.”

Small RNAs could be early diagnosis markers of HLB

Bacteria, such as the one that is associated with HLB, can cause plants to produce unique molecules called small RNAs, which repress gene expression in plants, animals, and many fungi. These small RNAs signal the plant to produce host-response proteins to protect itself from pathogens. Hailing Jin, associate professor in the Department of Plant Pathology and Microbiology at UC Riverside, is working to identify the HLB-induced small RNAs that will indicate whether a citrus tree is infected with the disease long before the plant expresses symptoms.

Protein secretions may hold the key to early HLB identification

Because the pathogen that is associated with HLB doesn’t spread throughout infected citrus trees right away, selecting a branch to test is a shot in the dark. Wenbo Ma, associate professor in the Department of Plant Pathology and Microbiology at UC Riverside, believes that the proteins secreted by the bacterium that is associated with HLB are moved rapidly throughout the tree in the phloem, the food-conducting tissue of the plant. Pathogen-specific proteins in the phloem could be a more reliable disease detection tool than the pathogen itself. Ma and her research associates developed a simple and fast method to sample the phloem. “Choose a couple of branches, cut them off and blot the cut ends on filter paper,” she said. “Back in the lab, antibodies are used to detect proteins on the membrane.” Ma has used this disease detection technique successfully to detect citrus stubborn disease.