Marie A. C. Langham
South Dakota State University
In recent years, Florida’s $9 billion commercial citrus industry has been damaged by an outbreak of citrus canker (Fig. 1), caused by Xanthomonas axonopodis pv. citri. The industry has lost millions of dollars due to reduced citrus yields, undesirability of infected fruit, loss of markets for nursery stock, and removal of infected citrus trees. Recently, a second disease, citrus greening (Fig. 2), has attacked Florida’s citrus industry.
Citrus greening is commonly found in Africa and Asia where it has been described as the most dangerous disease to affect citrus. In China, it is called huanglongbing or the yellow dragon disease due to the blotchy yellow leaf mottle that it causes. Leaves of infected plants also become hardened and curl outward. The disease also causes chronic decline, stunting, premature defoliation, off-season flowering, root decay, twig dieback, decreased vigor, and death of the tree. Fruit produced on these trees fails to ripen and remains predominately green with a bitter and salty flavor. Thus, both the tree and its fruit are damaged with resulting losses for the citrus growers. Citrus greening has a wide host range among the different species of citrus. It is most damaging to sweet orange (Citrus sinensis) and Mandarin orange (C. reticulata), but it also damages sour orange (C. aurantium), pommelo (C. maxima), lemon (C. limon), and grapefruit (C. X paradisi).
Citrus greening is caused by either of two fastidious phloem-inhabiting bacterial species, Candidatus Liberibacter asiaticus and Ca. L. africanus. These bacteria cannot be cultured in the laboratory (fastidious), and they are transmitted by the Asian citrus psyllid, Diaphorina citri. The Asian citrus psyllid was originally found in Florida during 1998. Concerns about the spread of this insect were confirmed when it was discovered in the citrus production area of Texas during 2001. Now, grower’s concerns are focused on how quickly and where the psyllid will spread the citrus greening bacteria. In addition to insect transmission, these bacteria can also be transmitted by grafting if infected propagation material is used.
There is no cure for citrus greening, and control efforts must focus on management. Early detection is essential for control of citrus greening, because it allows the opportunity to remove infected trees before the bacteria can spread. Management efforts will focus on eradication of infected trees, use of clean stock materials, and control of the Asian citrus psyllid. For the Florida citrus industry, this comes as a double blow since the industry is still struggling to control citrus canker. For those of us who are consumers of citrus fruit and juices, these diseases may affect the supply of citrus available at our local supermarkets. So, when you are eating your next piece of citrus or your next citrus product, take a moment to remember the threat of the yellow dragon and the disease that it represents.
For more information, try the following:
Huanglongbing, Citrus Greening Disease, hosted by the USDA-Animal and Plant Health Inspection Service, provides an overview of the disease, the pathogens that cause it, and the vectors that spread it.
Sponsored by the University of Florida, The Asian Citrus Psyllid and Citrus Greening Disease by Joseph L. Knapp, Susan Halbert, Richard Lee, Marjorie Hoy, Richard Clark and Michael Kesinger also provides an overview of citrus greening and the Asian Citrus psyllid.