Citrus greening disease

Citrus greening disease
Citrus greening disease on mandarin oranges
Common names	HLB, citrus vein phloem degeneration (CVPD), citrus greening disease, yellow shoot disease, leaf mottle yellows in the Philippines, citrus dieback in India
Causal agents	Candidatus Liberibacter spp. (Liberibacter asiaticus, Liberibacter africanus,Liberibacter americanus)
Hosts	citrus trees
Vectors	Diaphorina citri, Trioza erytreae
EPPO code	1LIBEG
Distribution	Asia, Africa, United States
Treatment	Not known

Citrus greening disease (Chinese: 黃龍病; pinyin: huánglóngbìng; literally: "yellow dragon disease"; or HLB), is a disease of citrus caused by a vector-transmitted pathogen. The causative agents are motile bacteria, Candidatus Liberibacter spp. The disease is vectored and transmitted by the Asian citrus psyllid, Diaphorina citri, and the African citrus psyllid, Trioza erytreae, also known as the two-spotted citrus psyllid. It has also been shown to be graft-transmissible. Three different types of HLB are currently known: The heat-tolerant Asian form, and the heat-sensitive African and American forms. The disease was first described in 1929 and first reported in China in 1943. The African variation was first reported in 1947 in South Africa, where it is still widespread. Eventually, it affected the United states, reaching Florida in 2005. Within three years, it had spread to the majority of citrus farms. The rapid increase in this disease has threatened the citrus industry not only in Florida, but the entire US. As of 2009, 33 countries have reported HLB infection in their citrus crop.

HLB 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, decay of feeder rootlets and lateral roots, and decline in vigor, ultimately followed by the death of the entire plant. Affected trees have stunted growth, bear multiple off-season flowers (most of which fall off), and produce small, irregularly shaped fruit with a thick, pale peel that remains green at the bottom and tastes very bitter. Common symptoms can often be mistaken for nutrient deficiencies; however, the distinguishing factor between nutrient deficiencies is the pattern of symmetry. Nutrient deficiencies tend to be symmetrical along the leaf vein margin, while HLB has an asymmetrical yellowing around the vein. The most noticeable symptom of HLB is greening and stunting of the fruit, especially after ripening.