Overview
Huanglongbing, also known as citrus greening or HLB, is a devastating disease of citrus trees. It is responsible for much of the tree decline seen in citrus groves around the state of Florida. The disease was first discovered in South Florida in 2005 but has since spread to nearly all production regions in the state. The bacterial pathogen associated with the disease is moved or vectored by an insect called the Asian citrus psyllid. This is the main means in nature the pathogen can move from plant to plant.
Facts about HLB
- First described a little over a century ago, it is one of the oldest known citrus diseases.
- Initially, early in the 19th century, due to chlorotic mottling on the affected leaves, HLB was thought to be nutrient deficiency maladies.
- The bacterial pathogen associated with HLB in Florida is an uncultured bacterium called Candidatus Liberibacter asiaticus.
- The pathogen also can be transmitted from tree to tree by grafting with infected plant materials.
- The bacteria are located in the phloem vessels of citrus trees and go systemically throughout the tree once the tree is infected, affecting all the organs.
- A tree that is infected by the pathogen when it is young rarely becomes productive over its lifetime.
- The Asian citrus psyllid arrived in Florida in 1998, followed by rapid spread throughout the state, establishing the insect in all main production areas.
Why is HLB so difficult to manage?
Huanglongbing has been extremely challenging for growers world-wide to manage despite years of work towards finding solutions that are practical and profitable. Multiple methods of protecting trees have been tried since the discovery of the disease in Florida. One challenge is that a tree can be infected with the pathogen without symptoms for up to a year, yet the Asian citrus psyllid is able to move the pathogen to other trees. This hampered the strategy of removing infected trees. Removing infected trees eliminated a source of HLB-causing bacteria, but more trees were affected than were easily identifiable and the disease was not slowed as much as needed to be able to manage it. Another approach was to use frequent insecticide applications to reduce the number of Asian citrus psyllids, but the insect vector reproduces very quickly for most of the year in Florida and again the effect was not strong enough, even when used alongside other methods. Once the bacterium infects citrus trees after being introduced by the insect vector, it moves, multiplies, and lives throughout the phloem tissues of citrus trees. This means that the pathogen is protected from most methods of direct management.
Additional challenges for HLB management in Florida
- HLB had already spread to many southern Florida groves before the disease was discovered and there were no symptoms on many affected trees. This caused the strategy of removing affected trees low efficacy even when managing the insect vector, the Asian citrus psyllid.
- Citrus trees in urban landscapes can be affected by HLB and act as source of the disease for other citrus trees.
- Citrus trees decline faster when they are exposed to the HLB-causing bacterium frequently over their lifetime.
- When the disease was first discovered in Florida, the technologies for detection of the pathogen were not as sensitive as what is used now. Improvements have been made over the years, but work is still ongoing to make them even more sensitive.
- The Asian citrus psyllid became better able to tolerate some frequently used insecticides in some regions of Florida. Management of the insect vector became more challenging in those locations.
All commercial citrus cultivars are susceptible to HLB, and there are no tolerant or resistant cultivars are currently available.
Promising avenues for the future
Scientists from many fields continue to work hard to manage this difficult citrus disease. New ideas, based on knowledge generated previously, continue to be tested. One area of short-term promise is the use of injectable crop protection materials. New cultivars of citrus, both scion and rootstocks, have been continuously developed and tested for tolerance to HLB. After exposure to the disease in the field, some cultivars are still looking promising. Edited citrus is also under development to make trees more tolerant to HLB. Read more about these approaches below.
Huanglongbing is not a disease that has developed with its host for many years. It is one of the potential reasons that citrus succumbs so easily to the disease. Prior to the early 20th Century, there were few reports of any symptoms similar to HLB from the major citrus growing regions of Southeast Asia. This region has successfully grown citrus for centuries, which would not have been the case if HLB were present.
- 1700s: Symptoms resembling HLB, called dieback, were described in central India.
- 1915 and 1920: First evidence of HLB in Southeast Asia from Husain and Nath in the Punjab region of India
- 1919: Disease first reported in southern China
- 1921: First report of disease in the Philippines where it was thought to be related to zinc deficiency
- 1928: A disease under the names, yellow shoot or greening depending on region, was described in South Africa. They were assumed to be mineral toxicity until at least 1937.
- 1941-1955: Surveys in in southern China and Taiwan were conducted to find extent of disease. From survey work, the Chinese researcher Lin Kung Hsiang, determined that farmers in areas of Guangdong province had reported the symptoms of HLB since 1870 and that this was likely the area where the disease was introduced.
- 1956: Lin Kung Hsiang demonstrated by experimentation that HLB is a graft transmissible infectious disease, not related to physiological disorders (e.g. nutrient deficiencies, water logging, etc.) or soil borne diseases (e.g. phytophthora, etc.). The work was not well known outside of China until later in the 20th Century
- 1960's: HLB first appeared in Thailand and decimated the citrus industry in some parts of the country
- 1965: Researchers in South Africa also demonstrated HLB (greening) was transmissible by graft inoculation and by the African citrus psyllid, Trioza erytreae
- 1966: Philippine and Indian researchers recognized the similarities between the 'mottle leaf' or 'citrus dieback' disease and HLB in China and Taiwan and 'greening' in South Africa
- 1967: Philippine and Indian researchers demonstrated 'mottle leaf' or 'citrus dieback' could be transmitted by the Asian citrus psyllid, Diaphorina citri
- 1967-1971: Researchers demonstrated that the pathogen was not a virus, as had been presumed, but an unculturable bacterium
- 1984: Monique Garnier and José Bové showed that the bacteria in citrus phloem of symptomatic trees was Gram negative
- 1995: The official name of the disease became huanglongbing (HLB) at the International Organization of Citrus Virologists (IOCV) at the 13th conference of the Organization in Fuzhou (Fujian, China) in recognition of the seminal work of Lin Kung Hsiang
- 1998: Asian citrus psyllid arrived in Florida
- 2004: The disease was confirmed to be in Brazil
- 2005: The disease was confirmed to be in south Miami-Dade County, Florida. A federal order was issued to restrict the interstate movement of all citrus greening and Asian citrus psyllid host plant material from Florida's quarantined areas
- 2005-2012: HLB spread through much of the commercial citrus production regions of Florida
- 2012: HLB found in Texas and California
References
Bové, J. M. 2006. Huanglongbing: A destructive, newly-emerging, century-old disease of citrus. Journal of Plant Pathology 88: 7-37. https://www.jstor.org/stable/41998278
Gottwald, T. R. 2010. Current epidemiological understanding of citrus huanglongbing. Annual Review of Phytopathology 48: 119-139. https://doi.org/10.1146/annurev-phyto-073009-114418
Vector biology
CLas is transmitted primarily by the Asian citrus psyllid, Diaphorina citri. Transmission is the process where an insect picks up a pathogen from one host, for example a citrus tree, and moves it to a new host, a different citrus tree. The psyllid acquires, or picks up, the bacterium during early nymphal stages when feeding on infected trees. The pathogen stays in the insect’s gut and salivary glands and is inoculated, or put, into healthy trees when the adult psyllid feeds. Psyllids prefer to feed on the phloem of young growing leaves (flush), and during this feeding they inoculate the bacteria into the phloem of the new tree.
Infection route and phloem colonization
Upon inoculation, Ca. Liberibacter migrates through the phloem sieve elements to other parts of the tree, always staying within the phloem. The bacterium accumulates and stimulates host defenses that cause phloem blockage. The blockage leads to the blockage of sugar transport and signaling molecules. The bacteria release proteins to interfere with the plant defense response, engaging in an ‘arms-race’ between the pathogen and the host.
Physiological disruptions
- Sugar transport failure: The pathogen blocks the phloem and reduces phloem sap flow, leading to carbohydrate (mostly sugars) starvation in expanding tissues.
- Hormonal imbalances: Triggering the defense hormones inhibits the accumulation of plant growth hormones such as auxin and cytokinin, disrupting normal plant development.
- Oxidative burst: Elevated reactive oxygen species damage cell membranes and starch‑binding proteins.
- Water transport: Blockage of sugar transport and hormonal imbalances create a cascade of toxic effects for the root system. This reduces water uptake.
Symptom cascade
- Root Damage - The root mass shrinks as tissues die, resulting in a dieback pattern that dampens the tree’s overall vigor and accelerates collapse. The first roots to disappear are the finer, fibrous roots that are responsible for the uptake of water and nutrients.
- Chlorosis (yellowing) of young leaves – Leaves abnormally accumulate starch and develop a characteristic asymmetric mottled pattern often called “blotchy mottle”.
- Stunted canopy – The length between leaf nodes is reduced on twigs and branches, leading to a reduction of overall size.
- Damaged fruits – fruit become small, lopsided, green‑shouldered, with aborted seeds, and low juice content. The flavor resembles unripe fruit and the fruit are generally considered unpalatable.
- Fruit drop – Fruit are prone to premature drop, usually within a month of harvest. These fruit are considered unharvestable.
- Tree decline and death – Cumulative effects of carbohydrate starvation, structural weakness, and secondary infections lead to the death of the tree.
The Asian citrus psyllid (ACP) is the vector for HLB in Florida. This means it is responsible for the movement of the bacterium associated with HLB from citrus tree to citrus tree under natural conditions like in groves or residential locations. These insects are piercing and sucking insects that have mouth parts that act like a syringe, allowing them to feed directly on the phloem of citrus. As they feed, they regurgitate into the plant, and it is during this process that the bacterium travels from the salivary glands of the insect to the phloem. They particularly like to feed on young tender flush (new growth), and it is the only place they reproduce. Young nymphs feed on the young flush and ingest the bacterium into their bodies. They carry the bacterium for the rest of their lives.
It is important to manage populations of the vector to keep citrus trees as productive as possible. When trees are infected with the bacterium over and over, the trees decline more quickly and are not able to produce many viable fruit. The reason is thought to be associated with the way the bacterium travels through the phloem of the trees. When there are only a small number of bacteria in the tree, they are unevenly distributed throughout the tree. The bacterium multiplies relatively slowly and the disease symptoms take longer to occur. If there are multiple points of feeding by ACP, then the bacterium can colonize the whole tree more quickly.
There are multiple strategies to manage ACP populations. These include natural predators, chemical control, and multiple forms of exclusion as described below. Each management method has drawbacks and benefits that must be balanced.
Individual Protective Covers
Individual protective covers (IPC) are white mesh bags that are placed over individual newly planted trees. These bags need to have a mesh size of 50 or smaller so that the very small Asian citrus psyllid cannot pass through the mesh. They come in two common sizes of 6 or 8 feet, and each have benefits and drawbacks. There is still debate as to whether to fasten the bottom of IPCs around the trunk. Wind can be very problematic and if the IPC is lifted so the tree is exposed, the investment is wasted. If IPCs are to be used, they need to be deployed on the day of planting. Young healthy trees are highly attractive to the Asian citrus psyllid, and they will become colonized quickly and likely infected with the bacterium without immediate protection. This defeats the purpose of excluding ACP from the trees. The trees will outgrow their protection after 1.5 to 2 years. We are still working on recommended practices for when the IPCs are removed. With no additional ACP management, the trees will be affected by HLB in less than a year.
It is especially important to protect young trees from ACP, thereby HLB. Young trees are very susceptible to the effect of being infected by the bacterium. The bacterium more quickly becomes systemic within the trees when they are young and small compared to trees infected when they are fully grown. This is in part because of the effects on the root system of the citrus trees. Once a tree becomes infected and the bacterium is systemic, the root system begins to shrink. It is unlikely to become bigger than the size it is when the tree is infected. The effect is particularly dramatic on the smaller fibrous roots that gather water and nutrients for the trees. For this reason, trees affected by HLB at an early stage are unlikely to be ever profitable and may never produce edible fruit. Some will die before fruiting.
It is tempting to put an IPC on trees and not worry about other pests and diseases. This is not good practice as other insect pests and diseases are problematic in these new environments. The insect pests to watch for are discussed here. The disease that is particularly difficult within IPCs is greasy spot, a fungal disease. The reasons for greasy spot challenges include the mesh retaining humidity and fallen leaves where the fungus reproduces accumulate. More about greasy spot and greasy spot management can be found here. One additional benefit of IPCs is that we see a reduction in the amount of citrus canker on the leaves. The netting reduces amount of windblown rain coming in contact with the trees and the leaves are not damaged by the citrus leaf miner, which makes them less vulnerable to citrus canker.
Citrus Under Protective Screen (CUPS)
Citrus under protective screen, known as CUPS, is a newer way of growing citrus in Florida. Production is focused on the fresh fruit market. The primary reason and benefit for growing citrus in these structures is that the Asian citrus psyllid can be kept out, and the trees remain safe from HLB. The exclusion principle is the same as for individual protective covers (IPCs), but on a much larger scale. These are large, screened structures normally covering multiple acres. The trees remain in these structures their entire lives unlike the IPCs that are removed after a few years. It means that unless there is some sort of opening to the outside, Asian citrus psyllids should not be able to enter the structures and therefore no HLB. Breaches do happen in extreme weather events like high winds or hurricanes. The Asian citrus psyllid can enter while repairs are underway but once the openings are closed, insect management can rapidly eliminate the population. Trees may be infected by the bacterium during this time, but they tend to be isolated trees. If reinfection is minimized by eliminating the Asian citrus psyllid, trees will take many seasons to decline and for the fruit to become poor quality. Once trees decline past a certain point, they can be easily replaced.
Growing citrus within these modified environments is different from open groves. Among other differences, the interior of these structures gets much hotter than outside during the summer. This means there can be different disease and pest issues within these structures. We are still learning about these new challenges but as we have new recommendations, we will share with the citrus community. More information about growing citrus in CUPS can be found here.
Irrigation
Since HLB weakens the roots, the tree becomes very sensitive to water stress. A consistently moist top-soil layer lets the compromised root system keep working. Therefore, using the same amount of water, frequent, shorter irrigation keeps phloem turgor high, supports nutrient uptake, limits fruit drop, and helps the canopy remain productive.
Fertilization
Since HLB weakens the root system and clogs the phloem, fertilization will work better with steady, low-dose feeding that’s easy to absorb, ensuring all nutrients are available to the tree and keeping soil pH at ~6.0–6.5.
- N and K: Frequent, low‑dose fertigation or controlled‑release fertilizer keeps a steady supply, aiming for N:K roughly 1:1 to 1:1.3.
- Micronutrients: Regular low‑dose sprays of Zn, Mn, B, Mg, and Ca help bypass compromised roots.
- pH: Keep soil pH about 6.0–6.5 and acidify high‑alkalinity irrigation water.
Plant Growth Regulators
Plant growth regulators can alleviate some HLB symptoms, such as premature fruit drop, canopy decline, and unregulated flush, so trees can stay more productive.
- Preharvest fruit drop: HLB elevates ethylene and weakens the abscission zone. Preharvest 2,4-D (auxin analog) and GA3 (Gibberellic acid) cuts drop, especially in Valencia-type oranges.
- Leaf/fruit senescence: Gibberellic acid (GA3) delays senescence, improves peel integrity, and can reduce drop. Multiple GA sprays over the season improved canopy density and yield.
- Flush and partitioning: Cytokinins (e.g., 6-BA) can stimulate budbreak and leaf retention.
Trunk Injection
Trunk injection allows for the direct application of a product to the vascular system where the pathogen is located within the tree.
- Cultivar Selection
- Biotechnology
- Nanotechnology
- Peptides
- Plant defense inducers
