Ecology And Environment

Citrus Greening Disease in the us



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The continental United States had a new type of illegal immigrant arrive in Florida during the 1990s. Asian citrus psyllids, small herbivorous insects sometimes referred to as jumping plant lice, were found in Delray Beach on June 2, 1998. How long they may have been in Florida before they were recognized and brought to the attention of authorities cannot be determined.

The psyllids didn't arrive alone, they brought with them bacteria of three species of Liberibacter, plant pathogens (disease causing microbes) that cause a disease in several species of trees and shrubs in the taxonomic family Rutaceae, also known as the Citrus family. In their native China the disease is called huanglongbing, which means yellow dragon disease; in the West it may be called yellow shoot disease or more commonly: citrus greening disease.

A survey carried out in June, 1998, after the initial discovery, found Asian citrus psyllids (Diaphorina citri Kuwayama)
established in 12 counties of Florida's southeastern citrus growing region. To be that established and spread, they probably arrived several years before their discovery.

By 2001 they had spread to 28 counties in Florida and had been found in Texas, having hitched a ride on a shipment of orange jasmine (Murraya paniculata) plants from Florida to Texas. By 2006 they were throughout the citrus growing region of Florida and found in 32 counties in Texas, and in May of 2008 Asian citrus psyllids were found in the Jefferson and Orleans parishes of Louisiana.

No sign of them has been found in the orange groves of California so far, and quarantines are in effect to try and keep it that way. These psyllids are also found throughout the islands of Guam, Hawaii and Puerto Rico. In 2005 it was confirmed that citrus greening disease was established in Florida; that the Asian citrus psyllids in the United States of America were indeed carriers for the disease.

Citrus greening disease is considered a prime candidate for the most devastating disease of citrus fruit orchards. Its establishment in the continental United States constitutes a severe threat to the economically valuable and important commercial orange, grapefruit and other citrus fruit industries; Florida's 68% share of US production had an "on tree" value of $1.04 billion in the 2005-06 year. This is a significant part of Florida's agriculture, food and natural resource industries, which provided 769,224 jobs in 2005.

The traditional backyard citrus tree might also go, with indeterminate effects on American suburban and rural culture. The popular ornamental, the orange jasmine shrub, is resistant to the disease but is a favorite of the Asian citrus psyllid. This can result in commercial orchards, if successfully cleared, being re-infested from suburban and rural gardens.

If a tree becomes infected with Liberibacter bacteria it will develop the disease over a period of years; clinical signs may first appear on individual branches before spreading throughout the tree. The disease results in yellow colored shoots, hence the yellow shoot disease name, discoloration in older leaves, die-back in smaller branches and twigs, and fruit that are reduced in size, have lopsided interiors, poor color, drop off before ripening and most importantly, have such a poor taste as to make them virtually inedible.

At this time there is no cure for trees infected by this disease. Infected trees need to be felled and incinerated. Trees in the same orchard, showing no clinical signs, need to be tested to determine if they are also infected. Instars (growth stages) and adults of Asian citrus psyllid found on apparently healthy trees should also be collected and tested for these bacteria. In all cases, any suspicion of these bacteria either in the sap of the tree or the psyllids predating it, should result in the felling and incineration of the tree.

This is not a disease to be trifled with. The citrus industry of India has been fighting these invaders since the early 1970s with various insecticides and management practices; recently dieback has been increasing, and the very existence of the commercial citrus fruit industry in that nation is threatened. Infestations in the US need to be controlled aggressively and quarantine measures enforced rigorously if there is to be any hope of maintaining a citrus fruit industry worth billions to the US economy and at least partially supporting millions of jobs across the nation.



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