Saving the orange tree

Citrus greening is devastating Florida citrus. It is caused by a bacterium, Candidatus Liberibacter asiaticus, which is spread by small insects called psyllids.
Citrus greening is devastating Florida citrus. It is caused by a bacterium, Candidatus Liberibacter asiaticus, which is spread by small insects called psyllids.
(Phil Coale / Associated Press)

Americans might soon need to get used to apple or grape juice as their breakfast drink of choice — unless, that is, they’re willing to pay exorbitant prices for orange juice. Or maybe scientists, plant breeders and farmers will manage to save the day, using two critical but often-disparaged technologies: chemical pesticides in the short run and genetic engineering in the longer term.

The pestilence that is devastating Florida citrus is a disease called citrus greening. It is caused by a bacterium, Candidatus Liberibacter asiaticus, which is spread by small insects called psyllids. The bacteria infect the tree’s phloem, thereby blocking the flow of nutrients, causing yellow mottling on the leaves and asymmetrical, bitter fruit that never ripens. Infected trees die within about five years, during which time they serve as a reservoir for psyllids to spread the disease further. There is no known cure, but it can be slowed by frequent spraying with large amounts of chemical pesticides.

Greening has seriously compromised citrus production in a number of countries in Asia, Africa, the Indian subcontinent and the Arabian peninsula and was discovered in July 2004 in Brazil. In 2005, the disease was found in south Florida and has since spread to sites in all counties that grow commercial citrus in the state. It has also been found in California but is not yet widespread here.


Several strategies that use genetic engineering techniques are being pursued to produce orange trees resistant to C. Liberibacter. (Because resistance hasn’t been found to occur naturally in citrus varieties anywhere in the world, conventional cross-breeding techniques are not an option.) The approaches range from incorporating a gene from spinach or pigs into orange plants to using viruses — bacteriophages — that prey on bacteria (but are completely harmless to other organisms).

Some of these strategies have been used with stunning success in other crops to enhance resistance to various pests and diseases and to introduce other characteristics. The vast majority of corn, cotton, canola, sugar beet and soy grown in the United States have been genetically engineered with modern molecular techniques. And for more than a decade, most Hawaiian papayas have been engineered to resist a pest that in the 1990s was devastating the state’s production.

More to the point, neither farmers nor consumers are unfamiliar with foods that have been genetically modified in some way. With the exception of wild game, wild berries, wild mushrooms and fish and shellfish, most everything in our diet has been genetically improved by one technique or another. (Even some heirloom fruits and vegetables.) Since the 1930s, plant breeders have performed “wide cross” hybridizations in which large numbers of “alien” genes are moved from one species or genus to another to create plant varieties that otherwise cannot and do not exist in nature. Common commercial varieties include tomato, potato, oat, rice and wheat — hardly fearsome “Frankenfoods.”

However, even with modern molecular techniques, the genetic engineering of trees is a slow process, and citrus growers are desperate for solutions that will buy them time until disease-resistant trees can be developed and tested and are yielding fruit. One promising approach focuses on killing the prolific psyllids. Currently, there is only one effective treatment: a soil drench of neonicotinoid (“neonic”) pesticide (derived from the naturally occurring nicotine found in plants) at the base of the young citrus tree’s trunk. This enables the chemical to be taken up through the roots, which keeps it from affecting other flying insects or pollinators, as spraying can.

Important research is underway that could expand the use of neonic treatment to large, mature, fruit-bearing trees as well as young trees, once the safety and effectiveness of the treatment have been demonstrated to the Environmental Protection Agency. It must be shown to protect the tree from both psyllid attack and citrus greening infection; and it must not harm bees, show up in harmful amounts in fruit or juice or have other adverse environmental effects.

So far, the results have been promising — excellent news for Florida’s citrus farmers. Genetically engineered trees resistant to citrus greening are no doubt the eventual solution to this scourge, but it may take a decade or more to develop reliably resistant, reproducible orange trees that can serve as a basis for the industry. In the meantime, without the protection currently being offered by neonicotinoid soil treatments, the U.S. citrus industry and its associated infrastructure would probably be gone by the time genetically engineered citrus trees become commercially viable.


If we are to preserve Florida’s $9-billion orange juice industry — and citrus crops in California and Texas when the disease spreads further — we will need to rely on the best technologies available. That means coming to terms with safe and rational genetic engineering and the innovative applications of systemic neonic pesticides.

Henry I. Miller, a physician and former director of the FDA’s Office of Biotechnology, is a fellow at Stanford University’s Hoover Institution.