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The first symptom of huanglongbing is usually the appearance of a yellow shoot on a tree, hence the name huanglongbing which literally means yellow dragon disease. Progressive yellowing of the entire canopy follows: leaves turn pale-yellow, show symptoms of zinc or manganese deficiency, or display blotchy mottling, and are reduced in size. Blotchy mottle is the most characteristic symptom, but is not specific to huanglongbing. Stubborn disease [Spiroplasma citri], severe forms of Citrus tristeza virus (CTV), species of Phytophthora, waterlogging and the use of marcots can produce similar blotchy mottle patterns. Symptoms of zinc deficiency are also associated with the early stages of citrus blight (a disease of unconfirmed aetiology). Huanglongbing bacteria, however, do not induce the xylem dysfunction and wilting observed in blighted trees. Dual infection of trees with HLB and CTV is common with reports suggesting that these trees have more severe symptoms (Huang et al., 1980).
Chronically infected trees are sparsely foliated and show extensive twig dieback. The fruits are often small, lopsided and poorly coloured (hence the origin of the name greening). They often contain aborted seeds. Similar fruit symptoms are also observed with CTV infection.
Biological control of the two psyllid vectors was achieved successfully in Reunion with hymenopteran psyllid parasites: Tamarixia radiata introduced from India against Diaphorina citri, and Tamarixia dryi, from South Africa, against Trioza erytreae (Aubert and Quilici, 1984; Aubert, 1987; Hoy and Nguyen, 2000). Control of the psyllids, together with destruction of Liberibacter-infected trees and use of Liberibacter-free material for replantings have led to the almost total elimination of huanglongbing in Reunion. Unfortunately, biological control of the psyllids cannot be achieved in most countries because the psyllid parasites are themselves hosts for parasitic insects. Care was taken not to introduce these hyperparasites into Reunion when biological control was initiated.
Research on the biological control of the vectors has also been carried out in Mauritius (Aubert, 1987).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
HLB has been regarded as one of the most important threats to global commercial and sustainable citrus production. It is estimated that globally more than 60 million trees had been destroyed by the disease by the early 1990s (Aubert, 1993). In Saudi Arabia, all sweet oranges and mandarin trees had declined by 1986 leaving only limes (Aubert, 1993). In Indonesia alone, HLB has resulted in the destruction of 3 million trees (Tirtawadja, 1980).
Crop losses from HLB of 30-100% have been reported in Africa (da Graca and Korsten, 2004). A survey was conducted during 1994 to determine the importance of citrus huanglongbing in South Africa (E Juckers and L Korsten, University of Pretoria, South Africa, personal communication). Results indicate that in comparison with a previous survey (Schwarz, 1967), the disease has continued to spread throughout the citrus industry. However, the severity of the disease has been greatly reduced in areas where control strategies are rigorously applied (see Prevention and Control). In huanglongbing-affected areas, lower psyllid populations were observed in the 1994 survey than in 1967, due to effective insect control. However, higher psyllid populations were found in Southern Natal and Eastern Cape where the disease is not present. Several orchards were affected by huanglongbing in the Western Cape (Paarl, Stellenbosch, Vemmershoek) in 1996.
In the Philippines, citrus culture has been drastically limited by huanglongbing. Similarly, in certain areas of Indonesia, the incidence of the disease is high. In northern Bali for instance, almost 100% of mandarin trees planted in 1990-91 were severely affected in 1996. In India, the disease is widespread (Bové et al., 1993; Varma et al., 1993). In the Pokhara Valley of Nepal, a major mandarin-growing area, trees show huanglongbing symptoms before they are 10 years old. The trees are replaced and the disease reoccurs on new trees. This cycle of replanting has occurred several times since the disease was introduced from India in the 1960s (Regmi et al., 1996). In southwestern Saudi Arabia, practically all mandarin and sweet orange trees were destroyed by the disease, but the more tolerant Mexican lime (Citrus aurantifolia) trees managed to survive despite heavy infestation with Diaphorina citri (Bové and Garnier, 1984).
The introduction of L. asiaticus and the discovery of L. americanus in Brazil has resulted in the removal of hundreds of thousands of trees from citrus orchards in Brazil. The eradication of L. asiaticus from Florida, USA, is not considered possible.