How to Identify and Control Citrus Greening Disease (HLB)

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’. 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. It poses a significant economic threat to the citrus industry and many countries have strict quarantine measures to mitigate the spread. 

What Is Citrus Greening Disease?

HLB is caused by uncultured, phloem-restricted alpha-proteobacteria. 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.

One of the first recordings of this disease dates back to China in 1919. In China, 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.

HLB is not just an issue that impacts commercial growers. Because this disease is carried by the Asian citrus psyllid, it can travel quickly. 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.

Types Of Greening Disease

Three species of Candidatus Liberibacter (CL) bacteria cause the disease known as citrus greening:

• CL asiaticus (CLas)
• CL americanus (CLam)
• 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

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. This is caused by 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. They are usually 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.

Control

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.

Infected citrus trees will die within a few years. 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. 

Prevention

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. In home gardens, these insecticides can impact beneficial insect populations, so it’s best to stick with other prevention methods.

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 and encourage beneficial insects to tackle Asian citrus psyllid.