By Tom Burfield

The most effective method of controlling the devastating citrus greening disease that has ravaged Florida’s orange groves also may be the most controversial.

A report from the National Academy of Sciences says the most powerful long-time management tool for the bacterium that causes the disease and, possibly, for the Asian citrus psyllid that spreads it, may be genetic engineering.

“Genetic engineering, in the form of transgenic citrus or citrus inoculated with a transgene-expressing virus vector, holds the greatest hope for generating citrus cultivars resistant to (the causal bacterium and the psyllid),” the report says.

At the same time, the report warns that groups opposed to genetically modified foods of any kind may try to dissuade the public from turning to genetically engineered orange juice.

Work goes on

Meantime, efforts continue toward finding a solution to citrus greening, also called huanglongbing—or HLB—which now is present in nearly all of Florida’s citrus-producing counties but is most prevalent in the southern areas of the state.

The state’s citrus industry is well worth preserving, according to the report.

“There would be great repercussions for Florida’s economy if the estimated $9.3 billion annual economic benefit of the citrus industry were to be lost or significantly diminished,” the NAS report says.

T. Erik Mirkov, professor of plant virology at Texas A&M University in Weslaco, has been at the forefront of the search for a genetic solution to greening disease and appears to be making progress.

“We’ve found some genes in spinach that we’ve transferred into citrus that provide resistance,” he says.

Mirkov has receiv ed a permit from the U.S. Department of Agriculture to conduct field testing of trees in Florida.

“We’ve got good results in the greenhouse, and now we’re making sure it holds up in the field,” he says.

Although his work appears promising, he still has a way to go.

“We don’t have what I would call immunity quite yet,” he says.

Nationwide threat

Although the disease so far has wreaked havoc in Florida, it also poses a threat to the citrus industry in Texas, California and other states.

“It’s the citrus grower’s worst nightmare,” Mirkov says.

One of the most challenging aspects of the disease is that a tree can be a source of infection even before it shows any symptoms.

“The disease can be spreading before you even see it,” he says.

But researchers are making progress to resolve that situation, as well.

“We need to find cheap, effective, high throughput methods to test thousands and thousands of trees,” he says.

Mirkov guesses conservatively that it will be a year before he’ll know for sure how successful his field test is. But he’s optimistic because of the positive results he’s already seen in the greenhouse.

In the greenhouse, trees are inoculated with the disease, which is a more severe process than they would undergo with the natural infection process in the field.

“We’re pretty comfortable that we have something here,” he says.

Mirkov is working with the Environmental Protection Agency, the Food and Drug Administration and the U.S. Department of Agriculture to get approval to grow the trees commercially.

“We have to jump through a lot of hoops,” he says.

He’ll even have to ensure that the process won’t hurt the honeybees used to pollinate the trees.

Speedy process

Randall Niedz, research geneticist at the U.S. Horticultural Research Laboratory in Fort Pierce, Fla., hopes to speed up the research process by using mature trees rather than seedlings that scientists typically use for testing and can take many years to bear fruit.

“If we can get mature tissue transformation in place, we can generate transgenics and will be able to evaluate the fruit much sooner than with seedlings,” he says.

Niedz says most experts would like to see a “very elegant solution” that, rather than kill the bacterium, might interrupt the biology and make it die, rendering the solution completely safe.

Texas A&M’s research is funded by Clewiston, Fla.-based Southern Gardens Citrus, where Mirkov’s field trials are taking place.

“It’s really working out well,” Mirkov says. “It’s good industry-university collaboration.”

It will be several years at best before the trees can be sold commercially, and replacing existing trees with disease-resistant ones won’t be cheap.

“There will be a cost to get into it, but it won’t be near what we’re all dealing with in the groves today,” says Rick Kress, president of Southern Gardens.

Growers already have seen costs go up 40 percent to cover things, such as additional spraying and tree removal, he says.

The research process itself is costly.

“It is extremely expensive to carry this work all the way through the regulatory process,” Kress says.

No outcry…yet

So far, Mirkov says he hasn’t heard from any groups who typically oppose genetically modified products.

“I think when you have something so critical, we may not see big pushback in terms of public perception,” he says.

But Kress isn’t so optimistic.

“It’s going to be a definite challenge,” he says.

Southern Gardens already has started the education process, working with customers and explaining that, unlike many other genetically modified products, there may be no other alternatives to genetically modified citrus.

“A GMO orange tree might be the only solution to protecting the citrus industry,” he says.

And using genetically modified trees would result in a significant reduction in chemical and pesticide use, which is important for the environment, Kress adds.

At least one major orange juice manufacturer who, in the past, would not have considered using genetically modified oranges, is looking forward to disease-resistant trees, Mirkov says.

“They can’t wait for these things to get in the field,” he says, “because they know, without it, they are going to end up being out of business.”

Contact The Grower at or (209) 571-0414.