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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). However, Huanglongbing bacteria do not induce the xylem dysfunction and wilting observed in blighted trees.
Chronically infected trees are sparsely foliated and show extensive twig dieback. The fruits are often small, lopsided, can have a sour or bitter taste (Jepson, 2009; ANR, 2010; USDA, 2012) and are poorly coloured (hence the origin of the name greening). They often contain aborted seeds. Similar fruit symptoms are also observed with CTV infection. The lifespan of infected trees is shortened (Miyakawa, 1980; Ammar et al., 2011).
Phytosanitary Measures
In areas where the disease is not present, effective quarantine measures are essential to prevent the introduction of the HLB organism or the vector. Furthermore, the possibility exists that the vector could be introduced 'naturally' or through alternative hosts such as Murraya spp. This poses a potential threat because the adult D. citri can transmit the disease, which can persist in the vector for up to 3 months (da Graca and Korsten, 2004).
Biological Control
In the absence of hyperparaitic wasps, the parasitic wasp Tamarixia radiata significantly reduced populations of D. citri, the vector of HLB, on the Indian Ocean island of Reunion, leaving a strongly limited population of the vector (Aubert and Quilici, 1984).
Chemical Control
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:
Huanglongbing has been regarded as one of the most important threats to global commercial and sustainable citrus production (Garnier et al., 2000; Duan et al., 2009; ANR, 2010; Ammar et al., 2011; Islam et al., 2012). It is estimated that globally more than 60 million trees had been destroyed by the disease by the early 1990s (Aubert, 1993). In West Java alone it was estimated that from 1960 onwards no less than 3 million trees were destroyed by Huanglongbing, and the destruction is still taking place (Tirtawadja, 1980). In Asia, approximately 100 million infected citrus trees have been destroyed by this disease, and 1 million trees were eliminated in Brazil in 2004 (Gottwald et al., 2007; Duan et al., 2009).
