Citrus Greening Illness: A irritating fruit bacterium

Citrus greens disease, also known as huanglongbing (HLB), is a deadly bacterial infection in citrus plants, most commonly transmitted by the Asian citrus flea seeds (Diaphorina citri). Huanglongbing is Chinese for Yellow Dragon Disease, as infected citrus leaves develop an irregularly mottled yellow color, similar to the skin of a mystical yellow dragon. In addition, citrus greening is sometimes referred to as yellow sprout disease.

It can be difficult to tell when a citrus tree is infected because trees may not show symptoms right away. However, this disease spreads quickly and can kill trees within a few years. This disease poses a significant economic threat to the citrus industry and many countries have strict quarantine measures to contain the spread.

HLB is not just a problem that affects commercial producers. Because this disease is transmitted by the Asian citrus flea seed, it can move around quickly. The Asian citrus flea seed feeds on plant material. As soon as it ingests infected plant material, it becomes a carrier for the bacteria. As the citrus flea seeds move, their bite causes the citrus fruits to turn green through the spread of bacteria. It is possible for your citrus trees at home to harbor an Asian citrus flea seed population and become a source of locally transmitted infections. Contact your local Agriculture Commissioner's office to call the HLB helpline if you suspect you may have an Asian citrus flea seed problem or if you think your tree is infected. We have a separate article on the identification and life cycle of this pest for your reference.

What is Citrus Greening Disease?

The researchers are working to investigate control methods for huanglongbing. Source: TaiwanICDF

HLB is caused by uncultivated, phloem-restricted alpha proteobacteria in the genus Candidatus Liberibacter. Citrus trees in Asia and the USA are particularly affected by Candidatus Liberibacter asiaticus. This bacterium influences the development of trees and fruits of almost all types of citrus trees. This is one of the most harmful plant diseases in food and agriculture.

One of the first records of the disease is from 1919. In China, where citrus production is a long and established industry, 11 out of 19 provinces that produce citrus have had major outbreaks of the Asian citrus flea and later the disease. Variations were later reported in Africa in the 1930s. To date, HLB is represented in over 50 countries worldwide, including the USA. One of the first major states affected to produce citrus fruits was Florida. It is estimated that 10% of Florida citrus fruits were killed within four years of the disease's introduction. Citrus growers in Texas, California, South Carolina, and many other southern states have also reported this problem. While HLB has spread via the Asian citrus flea seed, it is also possible that it could be introduced into the US by grafting infected branches onto healthy citrus plants. It continues to be a major threat to the citrus industry.

Types of greening diseases

There are three types of Candidatus Liberibacter (CL) bacteria that cause the disease known as Citrus Greening: CL asiaticus (CLas), CL americanus (CLam), and CL africanus (CLaf). CLas and CLam are borne by the Asian citrus flea seeds and CL africanus by the African citrus flea seeds (Trioza erytreae). Of the three strains, CLas is not as heat or moisture tolerant and does not occur in tropical or subtropical climates.

Symptoms of Citrus Greening

Different citrus trees show symptoms of infection at different times. Some of the early symptoms include tree canopy thinning, branch death, and yellow shoots and leaves. At a glance, these symptoms are very similar to signs of nutritional deficiencies that can also result from the disease.

Infected citrus trees also show an excessive build-up of starch in the leaves due to the bacteria that affect the tree's glucose-phosphate transport system. These leaves hold onto starch instead of feeding the roots and carrying sugar into the developing fruit. Greening citrus fruits also disrupts additional metabolic functions of the leaves.

Infected trees produce small, misshapen fruits that are asymmetrical with a sour or bitter taste and blackened and chipped seeds. The fruits also show a reverse color, with the fruits becoming increasingly green from the stem. These fruits cannot be sold commercially or made into citrus products such as orange juice.

Combating Citrus Greening Disease

Damage from citrus greenDamage from citrus green disease carried by the psyllids. Source: USDAgov

The main method of controlling HLB is to control the population of its vector, the Asian citrus flea. There is currently no cure for citrus green disease, but researchers at the University of California Riverside are in the process of commercializing a promising new drug for the disease.

Treatment of citrus trees

Infected citrus trees will die within a few years and the current best practice is to cut the trees down. In 2020, a University of California Riverside research team led by Professor Hailing Jin discovered a new treatment that can kill the CLas bacterium. This innovation uses an antimicrobial peptide that is stable at high heat, easy to manufacture and safe for humans. The researchers found this peptide in Australian finger lime, a citrus relative that is resistant to citrus green disease. The peptide can be applied as a foliar spray or injected into HLB positive citrus trees. Since this peptide can boost a tree's immune response to HLB, it can even be developed into a vaccine for young plants. Field trials are currently underway in Florida.

Prevention of Huanglongbing (HLB)

Agricultural researchers in China's Fujian Province have developed a four-step plan to reduce citrus greening and actively trained farmers in these management techniques. Some of these steps can also be customized for home gardeners.

First, scientists recommend planting a "quarantine belt" of cedar around the citrus garden to act as a physical barrier against the spread of citrus flea seeds. Additionally, the scent of cedar confuses the citrus flea seeds looking for citrus trees.

Next, infected trees must be completely removed. Citrus growers are encouraged to paint the stump with a mixture of diesel and herbicides to prevent regrowth so the old tree does not develop new branches. If you are a home grower in the United States, contact your local Agriculture Commissioner's office to document, test, and remove your infected citrus trees.

The third step is to spray all of the citrus trees with insecticides to control the Asian citrus flea seed populations, which coincide with each citrus tree new growth flush. This is usually done in a commercial setting by professionals. Citrus trees can have 2-3 vegetative flushes per year, which means chemical controls must also be applied several times a year. Regarding this step, good pruning and management of the trees is required to ensure that the trees are of a similar height and are properly pruned. Trees of uniform size and at a manageable height help breeders to inspect and control pest insects more easily.

Lastly, buy new citrus trees that are at least two years old and free of yellow dragons. A healthy sapling starts out strong and produces fruit faster. If you're replacing a tree in an orchard, buying an older citrus tree can also help that tree catch up with the others and maintain an even height throughout.

Aside from these four steps, it is important to maintain healthy soil life to ensure that citrus tree roots grow vigorously and can absorb nutrients efficiently. There are natural predators for the Asian citrus syllable and it is also important to take an integrated pest control approach to inviting these beneficial insects.

frequently asked Questions

Asian citrus syllableAn Asian citrus flea seed on a stem. Source: Hankplank

Q: Is citrus greening still a problem?

A: Yes, citrus greening, known as huanglongbing (HLB), is still a huge problem for citrus growers around the world. Citrus flea seeds and the greening they cause are constantly monitored to reduce the risk of infected trees to food and agriculture chains.

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