Engineered oranges needed to resist disease -- NAS

If the Florida orange industry wants to resist a sweeping tide of disease that has ravaged its trees for the past five years, it will likely have no choice but to genetically engineer its crops, according to a report released today by the National Academy of Sciences.

Since its discovery in 2005, citrus greening disease -- also known from its Asian origins as huanglongbing -- has spread to nearly every orange-growing county in Florida, carried by an invasive relative of the aphid, the Asian citrus psyllid. The bacterial disease has already cut the state's orange juice production by several percentage points, leaving swaths of the $9.3 billion industry to sprout misshapen, sour fruit unsuitable even for juicing. Once a tree is infected, there is no cure.

Researchers and farmers have scrambled to halt the disease, culling the insects and infected trees, but they have so far fallen short in finding citrus varieties that resist the disease. This lack of a breeding option means that for a long-term solution, the industry has few options, the academy concluded.

"Conventional plant breeding is unlikely to deliver resistant varieties," given the little resistance currently found, the report says. "This situation renders genetic engineering ... as more viable for developing citrus with resistance."

Though there are non-citrus plants that demonstrate resistance to greening, due to expense or incompatibility, the genes required for creating the modified trees would likely stem from "animal, plant, microbial or bacteriophage origin" or from components of venom or bacterial spore proteins, the report says.


Introducing genetically modified citrus trees would face substantial hurdles. While engineered crops using bacteria genes have been widely accepted in corn, soy and other cash crops, few minor crops have been deregulated for commercial use, with Hawaii's papayas, engineered to resist the ringspot virus, being the notable example.

Groups opposed to modified food would certainly oppose the use of foreign genes in the country's oranges and, subsequently, orange juice. The United States, unlike Europe, requires no labeling of food carrying transgenic material.

"Several well-known activist organizations have taken a stance against genetically modified foods in general and would likely attempt to raise opposition in the general public to juice from transgenic oranges," the report says.

Field tests of trees engineered to resist greening are already under way, with a small grove planted by Southern Gardens, one of Florida's largest citrus producers. The company, a division of U.S. Sugar, developed the trees in collaboration with Texas A&M University. The trials are still too early to provide tangible results, the company said (Greenwire, July 28, 2009).

One of the largest challenges in introducing modified citrus varieties will be the hybrid planting system employed by farmers, which typically sees different varieties grafted onto common rootstocks. While scientists should target these rootstocks in developing resistance strategies, USDA has never tackled the complicated approval process such hybrids would entail, the report says.

Unless early test groves provide an unlikely breakthrough, it will take 10 to 15 years to develop resistant citrus, the report predicts. Until then, orange farmers must continue their vigilant efforts to control the psyllid population and destroy infected trees. Wariness should be employed in increasing insecticide use, the report adds, lest the psyllids develop resistance.

Click here to read the report.

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