Asian Citrus Greening Disease
Asian Citrus Greening is a very serious disease of citrus which was discovered in Florida in August, 2005, and has since inflicted widespread damage to the commerical citrus industry in that state. The fact that its insect vector, the Asian citrus psyllid, has been present in parts of Texas since the summer of 2001, makes the potential for similar disease spread in Texas a real threat. CG infected plants were discovered near San Juan in Hidalgo County, Texas in 2012. That area has and continues to be under state and federal quarantine. In July 2014, nursery trees in Harris County tested positive for CG and it is believed that infected plants may have been disseminated in the Houston metro area and possibly surrounding counties. The TDA website here provides the latest updates of quarantines in Texas: Citrus Greening TDA. Texas citrus growers must be diligent to contain the spread of CG by controlling its vector--the Asian citrus psyllid.
Asian citrus psyllids reproduce in cycles throughout the growing season, and then overwinter as adults on the tree. Each year as temperatures warm up, psyllids disperse and seek new flush citrus growth to feed on. All people with citrus trees in Texas should monitor for psyllids and control them when present using any number of recommended controls available. Healthy citrus trees can avoid infection by CG if psyllid contol is maintained. A list of recommended organic and non-organic insect control options for psyllids is shown on the preceding web page.
Greening is known in China, where it originated, as "huanglongbing" which roughly translates into "yellow dragon disease". The Asiatic strain present in Florida and Texas is caused by Candidatus Liberibacter asiaticus, but there are two other strains of the disease--the African strain is present in South Africa; the American strain is present in Brazil.
All species of citrus are susceptible, regardless of rootstock. Sweet oranges, mandarins and mandarin hybrids are considered most susceptible. Limes and pumelos are supposed to be least susceptible, but these citrus have proven to be highly symptomatic in Florida.
Greening has a number of symptoms which can help to identify suspicious trees for further testing to confirm its presence. Blotchy mottling of older leaves precedes general chlorosis, which often appears in only one or two sectors of the tree. Fruit are commonly small, misshapen or lopsided, and have a salty or bitterish taste; the seeds are normally blackened due to seed abortion.
Fruit and leaf drop increase as the disease progresses, usually accompanied by off-season bloom and flushes of growth. Stunting and dieback become more pronounced with time. The tree may survive for several years, but death is inevitable.
The bacterium may exist in the tree for two or more years before visible symptoms are apparent. Even then, the early symptoms of blotchy mottling and chlorosis may be confused by other chlorosis symptoms common to citrus, including nutritional deficiencies, and those which result from some insects and diseases--such as Phytophthora foot rot. For assistance in diagnosing Asian citrus greening, the following pictorial includes the most common symptoms as well as other symptoms with which greening can be confused.
If you suspect Asian citrus greening after having reviewed the images contact Dr. Olufemi Alabi Texas A&M AgriLife Research & Extension Center, Weslaco, TX or Dr. Kevin Ong at the Texas A&M Plant Disease Clinic in College Station.
This photo gallery was developed by Dr. Julian Sauls, Extension Specialist, Retired.
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This page created by Julian W. Sauls, Ph. D and maintained by Monte Nesbitt
Revised July 24, 2014