Citrus greening disease, also known as Huanglongbing (HLB), is a fatal bacterial infection in citrus plants most commonly transmitted by the Asian citrus psyllid (Diaphorina citri). Huanglongbing is Chinese for Yellow Dragon Disease because infected citrus leaves develop an irregular mottled yellow color, similar to the skin of a mystical yellow dragon. In addition, citrus greening is sometimes called yellow shoot disease.
It can be hard to tell when a citrus tree is infected because trees may not show any symptoms immediately. However, this disease spreads quickly and can kill trees within a few short years. This disease poses a significant economic threat for the citrus industry and many countries have strict quarantine measures to mitigate the spread.
HLB is not just an issue that impacts commercial growers. Because this disease is carried by the Asian citrus psyllid, it can travel quickly. The Asian citrus psyllid feeds on plant material. Once it ingests infected plant material, it becomes a carrier for the bacteria. As the citrus psyllids move around, their bite causes citrus greening through bacterial spread. It is possible for your citrus trees at home to harbor an Asian citrus psyllid population and become a source for locally transmitted infections. Check with your local agriculture commissioner’s office for the HLB hotline to call if you suspect you may have an Asian citrus psyllid problem or if you think your tree is infected. We have a separate article focused on the identification and life cycle of this pest for your reference.
What Is Citrus Greening Disease?
HLB is caused by uncultured, phloem-restricted alpha-proteobacteria in the genus Candidatus Liberibacter. Citrus trees in Asia and the U.S. are impacted specifically by the Candidatus Liberibacter asiaticus. This bacterium impacts the development of both trees and fruits of nearly all varieties of citrus trees. This is one of the most damaging plant diseases in food and agriculture.
One of the first recordings of this disease dates back to China in 1919. In China where citrus production is a long and established industry, 11 out of 19 citrus-producing provinces have had major outbreaks of the Asian citrus psyllid and subsequently, the disease. Variations were later reported in Africa in the 1930s. To date, HLB is present in over 50 countries around the world including the United States. One of the first major citrus-producing states in the United States to be affected was Florida. It is estimated that 10% of Florida citrus was killed within fours years of the introduction of the disease. Citrus producers in Texas, California, South Carolina, and many other southern states have also reported this issue. While HLB has spread via the Asian citrus psyllid, it is also possible that it might be introduced to the U.S. through grafting infected branches onto healthy citrus plants. It continues to be a major threat for the citrus industry.
Types Of Greening Disease
There are three species 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 carried by the Asian citrus psyllid and CL africanus is carried by the African citrus psyllid (Trioza erytreae). Out of the three strains, CLas is not as tolerant to heat or moisture and is not found in tropical or subtropical climates.
Symptoms Of Citrus Greening
Different citrus trees will show symptoms of infection at different times. Some of the early symptoms include the thinning of the tree canopy, twig dieback, and yellow shoots and leaves. At a glance, these symptoms are very similar to signs of nutrient deficiency, which can also result from the disease.
Infected citrus trees will also show excessive starch accumulation in the leaves due to the bacteria impacting the tree’s glucose-phosphate transport system. These leaves hold on to starch instead of feeding the roots and transporting sugar into the developing fruits. Citrus greening 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 aborted seeds. The fruits will also show reverse coloration with the fruit getting progressively more green from the stem. These fruits cannot be sold commercially nor processed into citrus products like orange juice.
Controlling Citrus Greening Disease
The main method for controlling HLB is to control the population of their vector, the Asian citrus psyllid. The citrus greening disease does not currently have a cure but researchers from the University of California Riverside are in the process of commercializing a promising new remedy for the disease.
Treatment for Citrus Trees
Infected citrus trees will die within a few years and the current best practice is to cut the trees down. However, in 2020, a research team from the University of California Riverside led by professor Hailing Jin discovered a new treatment that can kill the CLas bacterium. This innovation uses an antimicrobial peptide that is stable in high heat, easy to manufacture and safe for humans. Researchers found this peptide in the Australian finger lime, a citrus-relative that is resistant to the citrus greening disease. The peptide can be applied as a foliar spray or injected into HLB-positive citrus trees. Because this peptide can strengthen a tree’s immune response to HLB, it can even be developed into a vaccine for young plants. Field trials are currently in progress in Florida.
Preventing Huanglongbing (HLB)
Agricultural researchers in Fujian province in China developed a four-step citrus greening mitigation plan and have been actively training farmers on these management techniques. Some of these steps can also be adapted for home gardeners.
First, scientists recommend planting a “quarantine belt” of cedar trees around the citrus orchard to act as a physical barrier against the spread of citrus psyllids. Additionally, the scent of cedar trees confuses citrus psyllids seeking citrus trees.
Next, infected trees must be removed entirely. Citrus growers are encouraged to paint the tree stump with a mix of diesel and herbicides to prevent regrowth so that the old tree does not develop any new branches. If you are a home grower in the U.S., check in with your local agricultural commissioner’s office to document, test, and remove your infected citrus trees.
The third step is to spray all citrus trees with insecticides to control the Asian citrus psyllid populations which coincide with each flushing of new growth for citrus trees. This is typically done in commercial operations by professionals. Citrus trees may have 2-3 vegetative flushes per year which means chemical controls must also be applied several times a year. Relating to this step is good pruning and management of the trees to make sure that the trees are similar in height and pruned correctly. Trees of a uniformed size and at a manageable height helps growers to more easily inspect and control pest insects.
Lastly, purchase new citrus trees that are at least two years old and yellow dragon disease-free. A healthy sapling will start off strong and produce fruits more quickly. If you are replacing a tree in an orchard, purchasing an older citrus tree can also help that tree catch up to the others and maintain a consistent height throughout.
Apart from these four steps, it is important to maintain healthy soil life to ensure citrus tree roots can grow strong and take up nutrients efficiently. There are natural predators for the Asian citrus psyllid and it’s also important to take an integrated pest management approach to invite these beneficial insects.
Frequently Asked Questions
Q: Is citrus greening still a problem?
A:Yes, citrus greening, known as huanglongbing (HLB), is still a major problem for citrus producers across the world. Citrus psyllids and the greening they cause are constantly being monitored to reduce the risk of infected trees for food and agriculture chains.