Also known as the yellow dragon disease, greening disease is a serious disease of citrus trees that can cause their death within five to eight years of infection. Any fruit the tree produces when this disease is present is usually "unusable," according to the Broward County, Florida, website. Citrus trees in Phoenix, Arizona, can also become the victims of this disease, for which no cure exists.
Oranges were first grown commercially in Arizona in the late 1800s and have become a major economic crop for the state. The primary area of citrus production is in the southwestern region of the state, because frost rarely occurs. Phoenix is located in this part of Arizona, and many home gardeners have a citrus tree or two in their yards or on their patios. Citrus greening disease is a relative newcomer to the list of citrus diseases: it was first reported in south Florida in 2005, where quarantines now exist, as they also do in Arizona. However, the psyllid insect had not established itself as of 2006, according to the document "Biology, History and World Status of Huanglongbing," by John V. da Graça of the Texas A & M University-Kingsville Citrus Center.
Causes and Types of Greening Disease
A bacterium known as Liberibacter asiaticus causes citrus greening disease. It affects the vascular system of the citrus tree it infects and in time kills the plant. An insect known as the citrus psyllid transmits this disease, but tools, wind and rain are not the apparent causes of the disease's spread. This insect also feeds on other plants, including ornamentals. Scientists have identified three strains of greening disease: Asian, African and Brazilian.
Identifying Greening Disease
After a citrus tree becomes infected with greening disease, leaf shoots turn yellow, twigs die back and the plant begins to lose its leaves. The tree will gradually decline in vigor. Older leaves become patchy, discolored or mottled in appearance. Leaf veins can turn yellow and resemble leaves that have been affected by a condition known as iron chlorosis. If the tree produces any fruit, it will be smaller than a healthy citrus fruit and will not taste good. Fruit will typically drop from the tree. The Asian citrus psyllid, which causes this disease, can also cause the edges of citrus leaves to become notched due to its feeding habits.
Challenges of Controlling the Disease
Although no cure exists for greening disease, citrus growers attempt to prevent it by controlling the psyllid insect that causes it. However, this pest is widespread in the areas where it exists, so control is difficult. Also, humans sometimes transport infected trees from a region where the disease exists to one where it has not yet been introduced. When a citrus tree becomes infected, the symptoms are not immediately apparent: several years can pass, by which time the disease is well established.
Preventing This Disease's Spread
Because greening disease has the potential of causing widespread economic damage to citrus-growing regions such as the area near Phoenix, Arizona, state departments of agriculture have instituted quarantines for the areas where the disease is known to exist. Plants that are known to host the psyllid insect may not be moved into citrus-growing regions of Arizona, California and other states, including Hawaii and territories located in tropical regions, such as Guam, Puerto Rico and the Virgin Islands. Because the insect and greening disease are well established in Dade County, Florida, prohibitions exist that forbid all citrus plants from being moved out of that region.