Control of Huanglongbing (HLB) disease with reference to its occurrence in Malaysia
The center of diversity for citrus was originally found on the northeastern India, eastward through the Malay Archipelago and south to Australia. Today citrus is produced in 140 countries, mainly between the north and south 40º latitudes. Citrus ranked first among fruit crops in the international trade based on value. Citrus production of the world is around 105 million tons per year. Orange (
Citrus sinensis) accounts for almost two thirds of the total citrus production (65%), followed by tangerine (Citrus reticulata) (21%), lemon (C. limon) (6%) and grapefruit (C. paradisi) (5.5%). Other significant commercially grown species are lime (C. aurantifolia), pummelo (C. grandis) and citron (C. medica). The largest citrus producers are Brazil (20%), United States (14%), China (12%), Mexico (6%) and the countries of the mediterranean basin (15%). Humidity and day-to-night temperature fluctuations influence which varieties are best adapted to an area. Most citrus fruits are produced for fresh market consumption and only around 30% is processed. Fresh fruits are rich in vitamin C which plays a vital role in prevention of scurvy. After extraction of the juice, the skin and fruit pulp can be used as livestock feed or making compost. The rind acid (oil) of the citrus is considered an expensive commodity in international trade (F.A.O 2003). Unfortunately, the citrus industry is threatened by 2 destructive diseases namely Witches’ Broom disease of lime (WBDL) and Huanglongbing (HLB) disease. WBDL has been known to be caused by Candidatus phytoplasma aurantifolia. It is one of the destructive diseases on citrus industry in the Middle East, India and Pakistan. It was reported in 1970 for the first time from lime orchard of Oman, united Arabian emirate (UAE) (1988), Iran (1998) and India and Pakistan (1999). WBDL is a phloem limited phytoplasma disease of lime. HLB disease causing citrus greening (Candidatus liberibacter spp.) is the second most severe disease on citrus industry all over the world. HLB has destroyed an estimated 60 million trees in Africa and Asia. More than 40 countries were infected by HLB in Africa, Asia and USA (Chau et al., 1996; Bove, 2006; Roux et al., 2006; Batool et al., 2007). The HLB pathogens are highly fastidious phloem-inhabiting bacteria in the genus Candidatus liberibacter. The bacteria have not been cultured yet in laboratory media and do not survive outside the host cells. Three types of phloem limited bacteria causing HLB disease have been described and identified (Bove, 2006). The isolate from South Africa (Candidatus liberibacter africanus) is considered heat-sensitive and found in Africa. It is vectored by African citrus psyllid (Trioza erytreae) and was described by Guercio in 1918. The isolate from Asia (Candidatus liberibacter asiaticus) is more severe and widespread; it is vectored by Asian psyllid (Diaphorina citri Kuwayama) (Garnier et al., 2000). This type of HLB is heat-tolerant (Garnier et al., 2000). It can show the symptom on humid, cool and hot temperature, up to 35ºC (Garnier et al., 2000; Bove 2006; Le Roux et al., 2006). The isolate from America has been named Candidatus liberibacter americanus; it was detected in Brazil and Florida (Coletta-Filho et al., 2005; Texeira et al., 2005).