The Curse of the Yellow Dragon
by Roger Snell
Chinese huáng lóng bìng, literally "Yellow Dragon Disease," is the greatest threat to citrus the world has ever seen. Really, I am totally being serious. During the 18th and early 19th centuries, Phytophthora took the top honors. Then came Citrus tristeza virus in the 1930's wresting the accolades as it caused the death of over 100 million trees. Citrus huanglongbing (HLB) is estimated to have killed 60 million trees, before even reaching the western hemisphere sometime in the 1990's. Now that it is in Brazil and Florida those totals will greatly increase.
Citrus has been grown as an agricultural crop in China for over 4,000 years and India a close second in citricultural antiquity. Nothing resembling the disease HLB had been recorded until 260 years ago. It is possible the disease was present in the wild on indigenous rutaceous plants (the rue family of plants of which citrus belong) and only infected plants as the modern citrus industry expanded in the last two centuries, or the two species of psyllids identified as carriers of the disease spread to these areas and became a vector for the pathogen.
Asian citrus psyllid (Diaphorina citri) eggs.
Adult Asian citrus psyllid (Diaphorina citri) feeding on host.
The vector for HLB in Asia and the Americas is an Asian citrus psyllid, Diaphorina citri and it is assumed the bacteria multiplies in the insect from observations of large numbers of liberibacter seen through electron microscopy. The symptoms of the disease appear as yellowing of the veins and adjacent tissue, followed by yellowing of the entire leaf, with dieback occurring, decay of feeder roots and imminent death. There is no cure. Death is sure. The fruit is also effected by greening and bitter taste.
Through the 20th century, Asian HLB spread through south and southeast Asia becoming endemic from Pakistan to southern Japan, Indonesia and the Philippines, traveling even to the Arabian peninsula. In Australia the psyllids appeared in the 1920's and were eradicated, but with that history of infestation making Australia a high risk.
In 2004 HLB was discovered in Brazil and Florida a year later. All it would take is the transfer of potted plants that are infested with the psyllid to California to wreck a $1.3 billion industry. The second largest producer of oranges in the world is California.
News Flash, Dec. 16th 2010, the Asian citrus psyllid is discovered in a trap in Ventura County. The first Asian citrus psyllid was found in San Diego a little more than two years ago, although to date none have been infected with the disease.
Many years ago the transportation of citrus trees from southern to Northern California was stopped at the Grapevine and that with culling of infected trees has helped keep tristeza in check, but the public must also be made aware of the consequences of the moving of potted materials from infected areas. Glassy winged sharpshooters, a vector for Pierce's disease, have been found at trailer parks in northern California transported on plant material from down south. This challenge to the wine industry has been met with vigor and reasonable effectiveness, but has been very costly monetarily. An ounce of prevention is worth a pound of cure.
Images courtesy of The American Phytophathological Society and Senasa
Roger has been mentioned in The Placer Herald in an article on HLB.
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