Huanglongbing: Curse of the yellow dragon

Huanglongbing (literally ‘Yellow Dragon Disease’ in Mandarin), previously known as ‘citrus greening disease’ or CGD, is one of the worst diseases of citrus trees worldwide.

It originated in China at the end of the 19th century. At the time, it was called the ‘Yellow Shoot Disease’.

The disease was later reported in South Africa in 1929 as ‘Yellow Branch Disease’ and later called ‘Greening’, which refers to the green colour that citrus fruits take on at the time of harvest.

The disease is widely distributed in Asia and Africa. The heat-sensitive form (Liberibacter africanum) prevails in Saudi Arabia, Yemen, Burundi, Cameroon, Central African Republic, the Comoros, Ethiopia, Kenya, Madagascar, Malawi, Mauritius, Reunion, Rwanda, Somalia, South Africa, Swaziland, Tanzania and Zimbabwe.

Trioza erytreae occurs in Saudi Arabia, in the eastern and highland areas of the country with a favourable climate, whereas Diaphorina citri is widespread in the western, more coastal areas.

A more heat-tolerant form (Liberibacter asiaticum) is spread across Asian countries, namely Bangladesh, China, India, Pakistan, Bhutan, Indonesia, Japan, Malaysia, Nepal, the Philippines, Taiwan, Thailand, and Vietnam.

In Africa, this form prevails in Mauritius and Reunion.

Transmission of the disease

CGD was originally thought to be a viral disease. Later though, it was discovered that the affliction was caused by bacteria, carried by insect vectors.

CGD infection can arise in various climates and is often associated with different species of psyllid insects. Psyllidae is a family of small, plant-feeding insects that tend to be very host-specific. Thus each species will feed on one plant variety.

Two species of citrus psyllid, Diaphorina citri Kuwayama (Asiatic psylla) and Trioza erytreae Del Guerico, (African psylla) can transmit the greening pathogen.

The ‘acquisition feeding period’ is 30 minutes or longer. Acquisition feeding period means the feeding time during which a vector feeds on an infected plant to acquire a virus for subsequent transmission.

The pathogen remains latent for 3–20 days. These two species of psyllids that transmit CGD are widely distributed. One species, residing in the islands of the Indian Ocean, is associated with the spread of the African Huanglongbing (HLB).

The other vector is the Asian citrus psyllid, which is adapted to warm humid areas and is widespread throughout Asia, the Indian Subcontinent, Saudi Arabia, Reunion and Mauritius. Diaphorina citri has also been reported in South and Central America, Brazil, and is now well established in Florida. Trioza erytreae is sensitive to heat whereas D citri is more resistant to extremes of temperature but more sensitive to high rainfall and humidity.

For example, citrus crops in Africa become infected under cool conditions as the bacteria are transmitted by the African psyllid Trioza ertreae, an invasive insect that favours cool and moist conditions for optimal growth.

Citrus crops in Asia, however, are often infected under warm conditions as the bacteria are transmitted by the Asian citrus psyllid Diaphorina citri.

What actually happens in CGD

Symptoms are displayed on different parts of the plant. In general, the greening-affected trees show the open growth, stunting, twig dieback, sparse yellow foliage, or severe fruit drop.

In some cases, green colour develops on the fruit at the peduncular (A peduncle is a stem supporting flowering or fruiting) rather than the styler (A style is a slender stalk that often connects the ovary and stigma in plants) end, as in normal case.

This is known as ‘colour inversion’ or ‘red nose’. The CGD is distinguished by the common symptoms of yellowing of the veins and adjacent tissues, followed by splotchy mottling of the entire leaf, premature defoliation, dieback of twigs and decay of feeder rootlets & lateral roots, ultimately followed by the death of the entire plant.

Controlling the yellow dragon

A recent study by Virginia Tech in the United States warns that the future supply of cirus products is threatened by Huanglongbing.

Different management strategies are needed to avoid a potential threat of this disease. First, there should be legislative controls to protect the mother stock. This would prevent the spread of pathogens.

Second, thermotherapy has been found to be very effective in treating the disease.

Third, chemotherapy using tetracycline hydrochloride and penicillin carbendazin also helps.

Last, the vector can be controlled by using Dimetoate EC, Malathion EC and Carbofuran EP.