Citrus greening also referred to as huanglongbing, is a disease that affects citrus production across the globe (Rumble, 2016). The symptoms of plants with this disease is that they cannot obtain sufficient nutrients from the soil, the twigs die back, the leaves turn yellow,  and the fruits remain green,  small, and unsuitable for harvest. Consequently, the plants die entirely after a few years. A bacterium also known Candidatus Liberibacter asiaticus (CLas) causes these symptoms by spreading the disease from one tree to the other using a tiny insect vector known as the Asian citrus psyllid. Currently, the disease has been detected in China and other parts of the world. Growers and scientists have tried numerous methods to combat the disease, but none has been long-lasting or effective enough (Singerman and Pilar, 2016). However, some publications illuminate strategies that can control the spread of CLas, as this paper will indicate.

Compared to other countries, citrus greening is more common in China, as they use more chemical fertilizer for plants that are planted close to each other. Additionally, growers use random pesticides to eliminate harmful insects.  Therefore, to understand how the disease can be cured, it is essential first to understand how the plants are infected.

The CLas infect the psyllid vector through bacteria that is sucked up when the psyllid consumes an infected tree, later reproduces inside the insect, and infect other healthy trees when the psyllid feeds on them. Arguably, the CLas cannot affect new trees without getting a ride on the insect. Thus, no new trees can die to citrus greening. Research has revealed that the nymphs are better equipped at relaying the disease than adults. CLas need to get through the cell lining of the insect’s gut to effectively spread the psyllids (Zhang, 2016). In adults, the gut cells undergo a severe stress response during infection, as the cell nuclei split and some cells undergo auto-induced cell suicide. However, in the nymphs, the level of disruption is lower, which makes them resistant to the effects of CLas exposure. The following are some of the treatment methods.

Antimicrobials

Scientists at the Boyce Thompson Institute have geared their efforts towards psyllids as the probable link to control (Singerman and Pilar, 2016). They have been searching for a protein that turns the bellies of the psyllids blue and its possible impact on preventing transportation of the virus. The Asian citrus psyllids with the blue abdomen possess elevated levels of an oxygen-transporting protein referred to as hemocyanin. When the psyllid is harboring the CLas, it increases the production of this protein. The protein interaction between psyllid and bacterium perpetuate the transmission of CLas into new trees. The interaction between hemocyanin and the CLas protein comprises of a critical microbial metabolic pathway referred to as the acetyl-CoA pathway (Paula, 2018). The biochemical interactions in the bacteria have previously been targeted from the production of antibiotics. To this end, scientists also believe that the increase in hemocyanin, and the blue color communicated to the abdomen, is the evidence of an immune response to the infection of CLas (Ruth, 2017). There is a possibility from these findings that this immune response can be harnessed to stop psyllids from transporting CLas and assist control the spread of the bacterium.

CRISPR

Several approaches have been proposed, as they show a probability of curing citrus greening. Amongst the proposed method is a gene editing tool referred to as CRISPR. Evidence has shown that the approach is efficient, as similar to the word-processing program, it can find and replace CLas. CRISPR is highly beneficial, as it can eradicate the genes that make the tree susceptible to greening. CRISPR can also substitute other genes from a similar plant with the infected genes, instead of from spinach or jellyfish, which has been the trend for the preceding genetically modified entities. It would be more conventional to replacing a particular gene without hosting a new gene to eradicate the issues that the growers face.

CRISPR originates from a virus-fighting system within the bacteria that use ribonucleic acid and DNA, which relays genetic messages. Researchers in the 1980s realized that bacteria amass stray bits of DNA known as the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) (Wayne and Xiomara, 2018). Later, they discovered that these elements of genetic material are similar to their viral adversaries’ DNA. Microorganisms utilize them as approaches to establish RNA, which consumes enzymes in the mission of seek-and-destroy against the DNA of the virus. Over the years, researchers have discovered ways to replicate the procedure and use it on other organisms (Wayne and Xiomara, 2018). The enzymes need to be fed the appropriate series, referred to as guide RNA, and sections of the DNA can cut and paste into the genome wherever necessary (Ahmed and Reza, 2015). Scientists from Lake Alfred Citrus Research and Education Center have confirmed that CRISPR is a possible cure for numerous genetic issues in humans, as they have already shown that it works effectively in citrus (Zhang, 2016).

Amongst the main weaknesses of CRISPR is that it is time-consuming, it needs more sophisticated tools than flasks and test tubes, in addition to demanding gene guns and sequencing equipment used to discharge DNA into the cells. Further, there some are identical genomes in the numerous varieties of genes, which implies that the RNA will probably only lock on to the favorable gene instead of the one the scientists are pursuing (Paldi, 2017).

Thermotherapy

The first approach is where a tree is covered by fitting the hydraulic arms of citrus carrying trucks with a tenting device. After a steam generator is used to inject wet steam into the tent and raise the temperature to about 135 degrees Fahrenheit (Lewis-Rosenblum, 2015). Similarly, the infected trees can also be treated with solar-heated tents for several days. Since steam treatment does not need chemicals, it does not require regulations. Arguably, it is not a cure, but it is the best treatment for short term solutions, as it can prolong the life of the tree. Advantageously, growers can stay in production without worrying about losing their crops for the next couple of years.

In conclusion, Citrus greening is a viral disease that affects citrus production. A bacterium referred to as Candidatus Liberibacter asiaticus causes citrus greening by spreading it from one tree to the other with the help of a tiny insect vector referred to as the Asian citrus psyllid. Arguably, citrus greening is more common in China because they use more chemical fertilizer for plants that are planted close to each other. Several approaches, which include CRISPR, thermotherapy, and antimicrobials, can offer possible solutions for the cure of citrus greening, but none of them is close to the complete eradication of the disease.

References

Ahmed Al-Jumaili, & Reza Ehsani. (2015). Mobile batch heat treatment system for treating HLB-infected citrus trees. Annual International Meeting, New Orleans, 1-431.

DeWitt, D. (2017). UF research shows promise in finding a cure for citrus greening. Campa Bay Times.

Lewis-Rosenblum, H. (2015). Seasonal movement patterns and long-range dispersal of Asian citrus psyllid in Florida citrus. Journal of Economic Entomology, 3-10.

Paldi, N. (2017). Compositions and methods for reducing pathogen-induced citrus greening.

Paula, B. (2018). Active taste compounds in juice from orange symptomatic for huanglongbing (HLB) citrus greening disease. LWT, 518-525.

Rumble, J. (2016). Saving citrus: does the next generation see GM science as a solution? Journal of Agricultural Education, 160-173.

Ruth, T. (2017). Conversing about citrus greening: Extension’s role in educating about genetic modification science as a solution. Journal of Agricultural Education, 34-49.

Singerman, A., & Pilar Useche. (2016). Impact of citrus greening operations in China. Institute of Food and Agricultural Sciences Extension.

Wayne Hunter, & Xiomara Sinisterra-Hunter. (2018). Emerging RNA suppression technologies to protect citrus trees from citrus greening disease bacteria. Advances in Insect Physiology, 163-199.

Zhang, L. (2016). Varying pulse control schemes for citrus huanglongbing epidemic model with overall incidence. Communications in Mathematical Biology and Neuroscience.