February 21, 2024

Editor's Page

Fruitless: Chasing the yellow dragon

Mark R. Howard | 12/26/2014

An enjoyable and mildly exotic aspect of living in Florida for me has been the ability to go into my back yard and pick a piece of fruit off a citrus tree. In addition to an orange tree, I have a small tangerine tree, a key lime tree and a Meyer lemon tree that has typically yielded beautiful, softball-size fruit. The trees have been hardy: I once overzealously pruned the lemon tree back to a couple of branches with about a dozen leaves and within a year it was again yielding dozens of Meyers.

No more. Over the last several years, I noticed a gradual, then rapid, decline in the production of all the trees, starting with the orange. I flagged down a state Department of Agriculture truck that happened to be in the neighborhood recently; the ag agent took a look at the trees and just shook her head. "Yep," she said, "citrus greening." Four out of every five citrus trees here in Pinellas County have it, she estimates.

My trees likely will be dead within a year.

The greening disease is a bacterium carried by Asian plant lice, called psyllids, that are about an eighth-of-an-inch long. When a psyllid siphons the juices from a citrus leaf, the bacteria in the insect's digestive juices flow into the leaf, then down into the roots and back up to uninfected parts of the tree.

It may take a year before any symptoms appear. Fruit begins to fall off the tree before it matures. Mottled, blotchy patterns of light and dark green appear on the leaves. The citrus that remains may appear misshapen, as half of each individual fruit ripens but the other half remains green — hence the name. Greening also goes by the initials HLB, an acronym for the Chinese word for the disease, which translates as "yellow dragon."

As the disease progresses, the bacteria clog the tree's vascular system, with nutrients unable to get down to the roots or back up to the fruit. Tim Schubert, a plant pathologist at the state Department of Agriculture, compares the process that kills the trees to that of clogged arteries in humans.

The decline of my trees brings home, literally, the disease that's whittled away a big chunk of Florida's citrus industry since it was found in south Florida in 2005 and began spreading northward. Today, more than half of the 80 million citrus trees in Florida are infected with greening, which is present in all 32 citrus-growing counties in the state, says Andrew Meadows, director of communications for Florida Citrus Mutual, a growers organization. Between 2004 and 2014, citrus production fell by more than 50%, to levels not seen since a period of hard freezes in the 1980s.

Most media accounts of the greening story take a "is-it-theend- of-oranges-in-Florida?" approach, but the story is more complex, with lots of subplots.

In response to greening, the citrus industry, state and federal government are funding millions of dollars worth of Research down every conceivable avenue — from more effective insecticides to kill the psyllids; to the release of sterile psyllids to breed the bugs out of existence; to specialized nutrients; to anti-bacterial drugs; to cultivating greening-resistant root stock. A bit of gallows humor in the industry these days is that if you're a citrus researcher and haven't gotten a grant, you need to be in a different field.

Among the more than 100 research projects under way in 18 countries is a promising bit of research from a scientist in Texas who has found a way to introduce into citrus a gene that's present in spinach. The gene has strong anti-microbial properties that appear to be effective in blocking the greening bacteria. Sadly, that research might not get past opposition from anti-GMO zealots even if it's as promising as it seems.

Meanwhile, many commercial growers have adopted an approach to controlling the disease that combines regimens of pesticides to kill the psyllids along with regular applications of micro-nutrients sprayed directly onto a tree's leaves.

That strategy increases growers' cost of production up to three-fold, but appears to have kept many groves viable. Market forces have provided a bit of silver lining: Since the dwindling supply of citrus has driven prices up, growers have been able to recoup their increased costs and still make a living, at least for the short term. Prices are high enough that some are trying to expand. Meadows says he knows of more than 5,000 acres of new groves planted in the past year.

Greening is still a big-time problem but appears to be on a trajectory toward becoming a long-term, ongoing management issue rather than the death of the citrus industry. "It's going to be a matter of managing the bug and uncovering therapies that will allow the trees to live with it," says Meadows.

In that respect, greening may be emblematic of many problems that are emerging as increased trade and travel move more people and organisms around the globe — problems that require, in the short term, an intense, expensive response, followed by long-term, attentive management, with lots of collaboration between industry and government. The citrus industry is already eyeing several new potential biological threats that appear to be in the pipeline.

In the meantime, backyard growers like myself may want to think twice about running down to the garden store to replace dead orange or Meyer lemon trees. Without the same kinds of rigorous, expensive disease control measures as the commercial growers provide, the new tree may be dead before it starts yielding.

I've planted a peach tree, developed at UF for the Florida climate. It doesn't have quite the cachet as citrus and isn't native to Florida. But then neither, originally, was the orange.

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