The Yellow Dragon Plague

—by Roxy Bergeron, HPC volunteer

Is it too dramatic to call it cancer, this insidious affliction pursuing citrus trees?

Fruit warps into impossible green bitter phantoms of what they might have been, raining from a tree in despair, lying by the dozen on the ground. The tree itself becomes stunted, with deformed roots and yellowed leaves, rife with the lethal bacteria that invaded its phloem—its vascular system—months or even years before. It’s a not-so-subtle end-stage failure, with nothing to be done but thank a tree for its years of fragrant delicious yields, then destroy it.

The infection, spread by a tiny louse, first appeared in China in 1911. The globetrotting traveler made port in Miami in the late 1990s. Florida growers began reporting the appearance of the stowaway disease in 2005, and now all 32 citrus-growing counties are affected.1 So far commercial losses in Florida add up to billions, and job loss has mounted into the thousands.2

One infected bug is all you need to bring down a citrus tree. And an entire grove can succumb in five to eight years. No known antibiotic or post-infection strategy is effective. Prevention is the only cure.3

The bug in question is a jumping plant louse, the Asian citrus psyllid, Diaphorina citri Kuwayana.4 This 3- to 4-millimeter-long insect serves as a transmission agent for the bacterium Candidatus Liberibacter americanus (or asiaticus or africanus, depending on the continent). Called huanglongbing (HLB, yellow dragon disease) and citrus greening disease, the infection has been identified over time in citrus trees worldwide.5

The dragon has pressed on and now has spread to Georgia, Texas, South Carolina, and into southern California. Proactive treatment in the form of psyllid eradication is the only strategy currently known to prevent infection. Besides grove sprays and soil drenches with pesticides, the use of a natural enemy, a tiny parasitic wasp called Tamarixia radiata, which lays its eggs on doomed psyllid nymphs, is being tried. The wasps have been released in California in an attempt at biological control.6

Article References:

1. http://www.theledger.com/article/20140113/news/140119676
2. http://www.scientificamerican.com/article/citrus-can-scientists-wasps-sa...
3. http://issg.org/database/species/ecology.asp?si=1496
4. http://entnemdept.ufl.edu/creatures/citrus/acpsyllid.htm
5. http://www.newyorker.com/online/blogs/elements/2013/04/the-green-death.html
6. http://www.sciencedaily.com/releases/2014/02/140204123726.htm