Citrus greening disease

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Citrus greening disease
File:Huanglongbing.jpg
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)[1]
Hosts citrus trees
Vectors Diaphorina citri, Trioza erytreae
EPPO code 1LIBEG
Distribution Asia, Africa

Citrus greening disease (Chinese: 黃龍病; pinyin: huánglóngbìng; literally: "Yellow Dragon Disease"),[2] abbreviated as 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 Kuwayama, and the African citrus psyllid, Trioza erytreae Del Guercio, also known as the two-spotted citrus psyllid. It has also been shown to be graft transmissible.[3] 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.

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 of up to 35 °C. The African form, Ca. L. africanus, and American form, Ca. L. americanus are heat sensitive, and thus symptoms only develop when the temperature is in the range 20-25 °C.[4] Although Trioza erytreae is the natural vector of African citrus greening and Diaphorina 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.[5]

Distribution of the Asian citrus psyllid (Diaphorina citri), that is a vector of the citrus greening disease, is primarily in tropical and subtropical Asia. It has been reported in all citrus-growing regions in Asia except mainland Japan. The disease has affected crops in China, Taiwan, India, Sri Lanka, Malaysia, Indonesia, Myanmar, Sri Lanka, the Philippines, Pakistan, Thailand, the Ryukyu Islands, Nepal, Saudi Arabia, and Afghanistan. Areas outside Asia have also reported the disease: Réunion, Mauritius, Brazil, and Florida in the U.S. since 2005, and in several municipalities in Mexico since 2009[6][7][8][9][10] On March 30, 2012, citrus greening disease was confirmed in a single citrus tree in Hacienda Heights, Los Angeles County, California.[11] The first report of HLB in Texas occurred on January 13, 2012 from a Valencia sweet orange tree in a commercial orchard in San Juan, Texas.[12] Prospects are dim for the ubiquitous backyard citrus orchards of California as residential growers are unlikely to consistently use the pesticides which provide effective control in commercial orchards.[13]

The distribution of the African citrus psyllid (Trioza erytreae) includes Africa Madeira, Saudi Arabia, Portugal and Yemen [14] This species is sensitive to high temperatures and will not develop at temperatures greater than 25 °C. It is also a vector of the African strain of Huanglongbing (Candidatus Liberibacter africanus), which is also sensitive to heat. This strain of HLB is reported to occur in Africa, (Burundi, Cameroon, Central African Republic, Comoros, Ethiopia, Kenya, Madagascar, Malawi, Mauritius, Reunion, Rwanda, South Africa, St. Helena (unconfirmed), Swaziland, Tanzania, Zimbabwe), Saudi Arabia and Yemen. The disease is not reported in the EU or USA.[15]

This disease 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.[16]

There is no cure for citrus greening disease and efforts to control the disease have been slow because infected citrus plants are difficult to maintain, regenerate, and study. Researchers at the Agricultural Research Service have used lemon trees infected with citrus greening disease to infect periwinkle plants in an effort to study the disease. Periwinkle plants are easily infected with the disease and respond well when experimentally treated with antibiotics. Researchers are testing the effect of penicillin G sodium and biocide 2,2-dibromo-3-nitrilopropionamide as potential treatments for infected citrus plants based on the positive results that were observed when applied to infected periwinkle.[17]

In June 2014, the United States Department of Agriculture allocated an additional US$31.5 million to expand research combating citrus greening disease.[18]

Genetic modification

No naturally immune citrus cultivars have been identified; however, creating genetically modified citrus may be a possible solution but there are serious questions of its acceptability to consumers.[19] A researcher at Texas AgriLife Research reported in 2012 that incorporating two genes from spinach into citrus trees improved resistance to citrus greening disease in greenhouse trials.[20] Field tests by Southern Gardens Citrus of oranges with the spinach genes in Florida are ongoing.[19]

References

  1. ↑ Bové, J. M. 2006. Huanglongbing: A destructive, newly emerging, century-old disease of citrus. J. Plant Pathol. 88:7-37.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lin, K. H. 1956. Observation on yellow shoot on citrus. Etiological studies of yellow shoot on Citrus. Acta Phytopathological Sinica 2:1-42.
  4. ↑ Garnier, M., S. Jagoueix-Eveillard, P. R. Cronje, G. F. LeRoux, and J. M. Bové. 2000. Genomic characterization of a Liberibacter present in an ornamental rutaceous tree, Calodendrum capense, in the Western Cape Province of South Africa. Proposal of 'Candidatus Liberibacter africanus subsp. capensis.' International Journal of Systematic and Evolutionary Microbiology 50: 2119-2125.
  5. ↑ Lallemand, J., A. Fos, and J. M. Bové. 1986. Transmission de la bacterie associé à la forme africaine de la maladie du “greening” par le psylle asiatique Diaphorina citri Kuwayama. Fruits 41: 341-343.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Kunta, M., Sétamou, M., Skaria, M., Rascoe, J., Li, W., Nakhla, M., da Graça, J.V. 2012. First report of citrus Huanglongbing in Texas. Phytopathology 102, S4.66.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ (EPPO/ CABI) European and Mediterranean Plant Protection Organization/Centre for Agricultural Bioscience International. 1979. EPPO data sheet on quarantine organisms, No. 46, Trioza erytreae. (http://www.eppo.org/QUARANTINE/insects/Trioza_erytreae/TRIZER_ds.pdf).
  15. ↑ http://www.eppo.int/QUARANTINE/bacteria/Liberobacter_africanum/LIBESP_ds.pdf.
  16. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  17. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ 19.0 19.1 Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ R-Santaana (26 March 2012) Spinach genes may stop deadly citrus disease Agrilife Today, Texas A&M, Retrieved 1 October 2012

External links

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