Citrus greening disease or yellow dragon disease (calque of abbr. HLB) is a disease of citrus caused by a vector-transmitted pathogen. The causative agents are motile bacteria, Liberibacter spp. The disease is 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 no known cure. 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 South 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 (hence the "yellow dragon" name given by observing Chaozhou farmers as early as the 1870s); 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 citri.
The young larval stage is the most suitable for acquisition of Ca. L. asiaticus by the Asian citrus psyllid Diaphorina citri, and some cultivars show greater efficiency in transmitting the disease to the vector than others. Temperature also shows a great influence in the parasite-host relationship between the bacteria and the insect vector, affecting how it is acquired and transmitted by the insects.
The causative agents are fastidious phloem-restricted, Gram-negative bacteria in the gracilicutes clade. The Asian form, Ca. L. asiaticus is heat tolerant. This means the greening symptoms can develop at temperatures up to 35 °C. The African form, Ca. L. africanus, and American form, Ca. L. americanus, are heat sensitive, thus symptoms only develop when the temperature is in the range 20–25 °C. Although T. erytreae is the natural vector of African citrus greening and D. citri is the natural vector of American and Asian citrus greening, either psyllid can in fact transmit either of the greening agents under experimental conditions.