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Combating Citrus Greening Disease

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ARS combats greening disease through disease detection, prevention, and mitigation research. Citrus greening represents the greatest threat to the $3.35 billion U.S. citrus industry. It is caused by a bacterial pathogen, Candidatus Liberibacter asiaticus (CLas), which is spread by the Asian citrus psyllid. Since the psyllid’s discovery in Florida in 1998, the industry has lost 60 percent of acreage and closed about 80 percent of juice plants and packinghouses. The disease has spread to Texas, California, Georgia, Arizona, and Louisiana. The following ARS advancements in FY 2020 highlight ongoing citrus greening response efforts.

New approach to solving crop pest and pathogen problems. There is an urgent need for solutions to control whitefly and the diseases transmitted by it. ARS researchers in Fort Pierce, FL, in collaboration with a private industry partner, developed a method of engineering only a group of plant cells that can be attached to other plants (essentially as a new organ) to produce desired molecules that are secreted into the plant vascular tissue and move throughout the plant. This “new organ” cannot survive away from the plant and does not move from the location where it is attached, thus the harvested commodity (i.e., fruit, nut, etc.) is not genetically engineered. It also cannot form whole plants, seed, or pollen, thus there is no escape of genetic material. The scientists are evaluating the ability of this strategy to cure trees infected with Huanglongbing (HLB, aka citrus greening) by engineering similar organs to produce natural peptides and double-stranded RNA that kills the HLB-causing bacterium, and attaching these organs to ornamental and/or horticultural crops. Proof-of-concept has been completed in tomato. This strategy could be adapted as a means to rapidly deliver genetic engineering solutions in an environmentally sustainable and consumer acceptable method.

Enhanced detection of bacteria associated with citrus greening. Huanglongbing (HLB) is the most destructive disease of citrus worldwide and in Florida. HLB is caused by a bacterium that cannot be grown in culture. ARS scientists in Beltsville, MD, developed a novel set of assays based on antibodies that recognize the pathogen when it is pressed on a paper-like surface. The sensitive assay produces colored spots and is easily scaled to large numbers of samples. This simple method matches the current urgent need for accurate, sensitive, and high-throughput screening of HLB and may play an important role, especially for plant inspection and quarantine programs.

Molecular profiling of citrus leaves. Citrus greening is devastating orchards in Florida and the south, and threatens groves in California. A collaboration between ARS scientists in Ithaca, NY, and Riverside, CA, identified molecular changes in citrus leaves resulting from infection with the causative citrus greening bacterium. Leaf samples were collected biweekly from lemon and navel orange trees for a year following graft inoculation with either pathogen-containing or healthy budwood and analyzed for biomarkers of infection. RNA, protein, and metabolite levels were compared over time between healthy and infected plants, revealing molecular profiles that were associated with infection months before visual symptoms of disease appeared. Results from this study reveal differences in the response to infection between these two distinct varieties of citrus and provide insight into how the plant response to the pathogen changes with time. These findings can be used to improve diagnostic tests for citrus greening disease. This high-profile research was featured on the cover of a 2020 issue of Journal of Proteome Research.

A strategy to control Asian citrus psyllid. Growers rely on insecticides to control Asian citrus psyllid (ACP), but insecticide-resistant psyllid populations are emerging, and control costs are high. As an alternative to insecticide control, ARS researchers in Fort Pierce, FL, are developing a control strategy called “Conservation Biological Control” in which certain plants are grown to support insect predators that attack the psyllid. The scientists demonstrated that a statistical method called Response Surface Methodology could be used to optimize mixtures of plants to support the insect predators of ACP. Various plants were used, including mixtures of crown-of-thorns, lima beans, wild poinsettia, flowering buckwheat, partridge pea, and ornamental portulaca. Results showed that when formulating a plant mixture to aid in biological control of the psyllid, consideration should be given to the proportion of each plant species included in the mixture.