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Breeding a solution to citrus greening

Talking to citrus tree researcher Jude Grosser, you get a sense of the scientific challenge he and his team at the University of Florida’s Citrus Research and Education Center in central Florida face in saving the state’s citrus industry.

Grosser’s usual job has been breeding new varieties of oranges, tangerines, grapefruits, lemons, limes and rootstocks for Florida grove owners to plant. The center, for instance, recently came out with several new seedless tangerines.

But he and his colleagues have been spending less time on those endeavors and a lot more on breeding a solution to the greening plague, which started in 1998, when an aphid-sized bug, the Asian citrus psyllid, showed up in south Florida, likely tagging along on an imported product.

The discovery of the small insect sent shudders through Florida’s agriculture industry. The psyllid is the carrier — the “vector,” in epidemiological lingo — of citrus greening, also known as Huanglongbing (HLB), an incurable citrus tree disease from Asia. Citrus greening was ranked by a panel of scientists as the greatest threat to citrus in Florida among the 18 citrus diseases in the world, recalls Harold Browning, chief operating officer of the Citrus Research and Development Foundation.

Today, Florida is in the advanced stage of the epidemic. Every commercial grove in the state is assumed to be infected. Losses have been in the hundreds of millions. Entire groves have been ripped out or abandoned. Grapefruit production is at its lowest level since the Great Depression, excluding hurricane seasons. Orange production hasn’t been this low in 40 years. The state’s total citrus crop is less than half what it was in 2003-04.

Growers, the state and the federal Agriculture Department have combined to spend $263 million on research and prevention. (Florida growers agreed to tax themselves to raise $75 million so far.) Hundreds of scientists are looking for answers to the disease. Indeed, one way or another, every scientist at the University of Florida’s Citrus Research and Education Center in Lake Alfred is working on greening, says center director Michael Rogers.

Grosser, one of them, traces his interest in plants to his grandmother’s garden in his home state of Kentucky. In 1984, he joined the University of Florida, where he’s a professor of citrus breeding and genetics in UF’s Institute of Food and Agricultural Sciences. There, he and his IFAS partner Fred Gmitter — “we joke we’re the orange brothers” — emeritus professor Bill Castle and the rest of their colleagues have developed many new varieties of citrus.

Citrus trees are grown in nurseries by grafting rootstocks with special properties — they thrive in a particular soil or bear fruit quickly, for example — to budding material called scion.

The researchers found some rootstocks took longer to get sick or didn’t get as sick when exposed to greening. The researchers essentially cloned those stocks and grafted infected budding material to them to see whether the rootstocks mitigated the disease. Those that did were put on a fast track to release to growers. UF has released 17 stocks that are doing “really well” against greening, and researchers at the U.S. Department of Agriculture and elsewhere have released several more.

Trees grafted on the stocks aren’t immune to the disease but may tolerate it well enough that growers can earn a living. What’s more, as those stocks fared better just by genetic chance, it opened the possibility of selectively breed- RESEARCH FLORIDA ing to find a genetic stock that thrives in spite of greening.

“We’ll probably find tolerance first,” Grosser says. “If we get lucky, we might get resistance.”

All told, Grosser and his colleagues have experimented with hundreds of stocks and combinations of stocks and grafted-on budding material. Most, Grosser says, “end up in the garbage can.” But the most promising have been planted at the USDA’s Horticultural Research farm in Fort Pierce.

“Dr. Grosser is doing very important work for the citrus industry,” says Ty Strode, vice president and director of marketing for AgriStarts, an Apopka company that takes the UF stocks and does tissue cultures of them to make starter trees.

Growers move cautiously. When they plant a tree, they know it takes several years to bear a profitable amount of fruit, and they want trees to last decades. The latest rootstocks from UF are too new to generate the data on yield and fruit quality growers want to see. Nor has there been time for the trees to show how disease- tolerant they’ll be over their economic lifetimes.

One of the state’s largest citrus processors and a grove owner, Peace River Citrus based in Vero Beach, is planting newer rootstocks on an experimental basis only. “We are very supportive of the work Dr. Grosser is doing in this area, but new varieties are too new at this juncture for largescale commercial planting,” says Andrew Taylor, the company’s senior vice president and CFO.

Nevertheless, Grosser says, demand for the new rootstocks exceeds supply at present. His lab is constantly screening and experimenting as it hunts for the answer to citrus greening. Says Grosser, “we’re trying to save an industry.”

Citrus Greening: Background

The Asian citrus psyllid, a small flying insect that feeds on all citrus varieties and related ornamentals, carries the bacterium that causes citrus greening. The disease, which is progressive, debilitating and incurable, is also known as Huanglongbing (HLB). The disease disrupts the flow of nutrients through the tree’s vascular system. It takes a few years for an infected tree to show symptoms but then, as the tree’s leaves yellow, fruit shrinks, becomes bitter and unmarketable or remains unripe — hence the name, greening — and drops prematurely. Commercial growers typically remove a tree when it ceases being sufficiently productive.

The disease originated in Asia and has spread around the world. The first psyllid was found in Florida in 1998 in Palm Beach County on an Orange Jessamine plant, an ornamental citrus relative used for hedges. Today, the bug, likely riding along on nursery plants, is in all 30-plus commercially producing citrus counties in Florida. (Florida’s top five citrus counties are Hardee, Hendry, Polk, Highlands and DeSoto.) It’s also spread to Louisiana, Georgia, South Carolina and Texas.

Given the disease’s presence in Mexico and the psyllid’s presence in California, researchers say it’s only a matter of time before it gets to California’s citrusproducing areas.

The Impact of Greening

  • 244 million
    Number of 90-pound boxes of oranges produced in the peak year of 1997-98
  • 97 million
    Number of 90-pound boxes the federal Agriculture Department estimates will be produced this year, a drop of 60% from the peak and the state’s lowest output since 1965-66.

Trying Everything

The massive effort and spending to find a scientific solution for greening here in Florida will pay dividends for the industry the world over.

The Citrus Research and Development Foundation, an industry-funded body, has given money to research programs in 22 states, says Chief Operating Officer Harold Browning. “We’re reaching out to the best science and technology wherever it resides,” he says. “We’re working really hard to give growers solutions right now.”

Growers who have maximized pest control, nutrition and irrigation — and have been blessed with neighboring groves similarly managed — are surviving. “There are people retaining their yields,” Browning says.

Most, however, are seeing yearly declines in the health of their groves. Growers whose trees produce get high prices for their citrus, but the aggressive management to forestall the greening decline is costly. Browning says that before greening hit Florida, it cost $700 to $1,000 per acre to manage a grove; now it’s more than $2,000. “There are plenty of growers who have the attitude, I’m going to go down swinging,” Browning says.

An Overview

Near-term tactics buy growers time, though they don’t eliminate the disease:

  • Bactericides: Two companies have federal approval to use bactericides on other crops. The Citrus Research and Development Foundation in Florida is trying to get approval to use the chemicals against citrus greening.
  • Nutrition and irrigation: Growers can keep sick trees productive by feeding and watering them carefully.
  • Anti-vector: Growers and scientists have deployed pesticides and parasites — a parasitic wasp has gotten lots of attention — against the Asian citrus psyllid that carries the disease. Researchers are also experimenting with introducing genetic changes into the psyllids so that they can’t fly or reproduce or carry the disease. Growers have been organized by geography into Citrus Health Management Areas so that they can coordinate efforts. Killing pysllids in your grove isn’t as beneficial if your neighbor’s grove harbors them. “If they all do these things at the same time, they get a better result,” Browning says. Michael Rogers, director of UF’s citrus research center in Lake Alfred, is a key player in the management area program.
  • Thermo treatments: Yongping Duan, of the USDA’s research laboratory in Fort Pierce, proved that treating trees with heat — he put a plastic tent around some to drive up their temperature — prolongs their productivity. UF’s Reza Ehsani treated trees with steam to drive up their temperature. Neither cures the tree since treatments don’t reach the roots.

Long-Term Efforts

  • Breed trees that can tolerate the disease: Key players are Jude Grosser and Fred Gmitter at UF and Kim Bowman at the USDA in Fort Pierce.
  • Engineer trees for resistance: In May, the federal Environmental Protection Agency approved an experimental permit that will allow field testing of trees containing a spinach gene. U.S. Sugar subsidiary Southern Gardens will begin testing the spinach gene variation developed by Texas A&M plant pathologist Erik Mirkov to combat greening. Meanwhile, Manjul Dutt, a UF scientist, is experimenting with a gene from a mustard plant to fight greening. The greening bacterium tricks a tree’s immune system into not recognizing it. Inserting a gene from a mustard plant turns on citrus trees’ immunity system to fight the disease. A tangerine tree with the mustard gene has been disease-free for four years. Scientists still must find a different gene with a different mechanism to “stack” with the mustard plant gene to give redundancy to the tree’s defenses. Growers otherwise would worry that if the bacteria mutates to get around one gene, every tree would be at risk.

While they’re doing all that science, researchers have to worry about how the market, especially export markets, will receive fruit and juice coming from a genetically modified organism. They hope to alleviate public concerns by using genes from plants people already consume.