Asian Citrus Greening Disease
A Potential Tree-Killing Disease Not Currently Known to Be Present in Texas
Julian W. Sauls, Ph. D.
Asian Citrus Greening is a very serious disease of citrus which was discovered in Florida in August, 2005, and has since spread to over 30 counties, covering most of the commercial citrus-producing counties in the state. Because of its presence over such a widespread area of Florida, eradication of the disease is not a viable option. Indeed, in 2007, the USDA placed a federal quarantine on the entire state of Florida. The fact that its insect vector, the Asian citrus psyllid, has been present in parts of Texas since the summer of 2001, and is presently under both federal and state quarantines in 32 Texas counties, the Texas citrus industry is concerned about the possibility of the disease being brought into Texas.
Greening is known in China, where it originated, as "huanglongbing" which roughly translates into "yellow dragon disease". The Asiatic strain present in Florida 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.
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This page created and maintained by Julian W. Sauls, Ph. D
Revised April 24, 2008