Mandarin orange (Citrus reticulata Blanco) with symptoms of yellow shootand citrus greening, caused by Candidatus Linberobacter asiaticum,which is vectored by the Asian citrus psyllid, Diaphorina citri.
Photograph courtesy Tim R. Gottwald and Steve M. GarnseyUSDA, ARS, U.S. Horticultural Research Laboratory2120 Camden Road, Orlando, FL 32803
Top panel: Naturally infected young Ponkan mandarin tree in Taiwan with "yellow shoot"symptoms.Bottom left panel: Satsuma mandarin fruit in Taiwan with the nonuniform coloration typical of "greening."Bottom right panel: Diaphorina citri, the Asian citrus psyllid. Photo by B. Aubert.
Background: Citrus Huanglongbing (HLB), also commonly called "citrus greening," is one of the most serious diseases of citrus and has severely affected production of all types of citrus in many areas of Asia. The causal agent of HLB is a phloem-limited bacterium, Liberobacter asiaticum, which has never been successfully cultured. HLB severely affects phloem function and tree health. Initial symptoms of chlorosis and leaf mottling often first appear on one shoot or limb and gave rise to the Chinese name, huanglongbing, meaning "yellow shoot." As the disease progresses, trees turn chlorotic, develop twig dieback, and rapidly decline to a nonproductive state. Fruit on diseased trees is small, frequently misshapen, and fails to color uniformly as it ripens. This symptom is responsible for the other common name, "greening." A distinct form of HLB is found in Africa, and the causal agent, L. africanum, has a lower optimum temperature for expression. The HLB agent is spread by propagation and by two citrus psyllids. Diaphorina citri Kuwayama is the primary vector for the Asian form of HLB and Trioza erytreae is the primary vector for the African form, but both can vector either agent. Introduction of infected plants has triggered HLB epidemics in several countries, and once well established, it is difficult to control. Nearly all orange, mandarin, and grapefruit cultivars are severely affected by HLB, and control measures are limited to the use of disease-free propagating stock, roguing of infected trees, and chemical or biological control of the vectors. While the HLB agent has not been found as yet in the Western Hemisphere, D. citri was introduced into Brazil in the early 1980s and was found in Del Ray Beach, Florida, on 2 June 1998 by the Florida Department of Agriculture and Consumer Services. Presence of the vector in the United States raises concerns that any unrecognized introduction of HLB-infected citrus in the past and any future introduction of the pathogen could lead to spread of the disease by a vector now in place. An ongoing USDA, ARS-supported research program on exotic citrus diseases is designed to address concerns about HLB and other exotic pathogens and includes development and evaluation of diagnostic, survey, and control methods.
APS publication number: IW00006
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