Citrus greening disease (; or HLB) is a disease of citrus caused by a vector-transmitted pathogen. The causative agents are motile bacteria, 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. Symptoms 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. Transmission HLB was originally thought to be a viral disease, but was later discovered to be caused by bacteria, carried by insect vectors. HLB infection can arise in various climates and is often associated with different species of psyllid insects. For example, citrus crops in Africa become infected under cool conditions as the bacteria are transmitted by the African citrus psyllid Trioza erytreae, an invasive insect that favors 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