More than two years into a quarantine on citrus trees in much of Southern California, the Asian citrus psyllid continues to spread. This spring researchers discovered the tiny insects on the 140-year-old Eliza Tibbets tree in Riverside, known as the parent of navel orange trees the world over.
To control the insects’ spread, researchers already have introduced a parasitic wasp that preys on the psyllids and their larvae. Southern California growers also are using a rotating regimen of pesticides to protect the state’s $2 billion citrus crop. But protecting the Eliza Tibbets tree will require special measures, and friends of the tree are raising money to build a specialized mesh enclosure around the canopy.
Riverside citrus historian Vince Moses says the seedless navel oranges we know so well today are “a mutant of a Brazilian variety called the Selecta.” Eliza Tibbets, one of Riverside’s founders, introduced two of the Selecta's mutant offspring to California in the 1870s. Beside her house in Riverside, the trees yielded America's first seedless fruit: large, brightly colored and easy to peel.
One tree died in 1921, and the lone survivor now stands nearby at an ordinary intersection ringed with small apartment buildings and a strip mall. But in the late 19th century, the area was transformed by Tibbets’ introduction. “There were thousands of acres of navel orange groves, with streetcar lines, with irrigation canals,” Moses says.
Tibbets' neighbors used cuttings from her two original trees to establish the first navel orange orchards in California. Over the years, mutations of their offspring provided new varieties to farmers from South Africa to Pakistan. California became a global hub for citrus, and by the turn of the 20th century, Riverside was the wealthiest city per capita in all the United States.
But today the tree that made it all possible is at risk of contracting citrus greening disease, caused by a bacterium called huang long bing. In Chinese, Moses says, huang long bing translates roughly as “the yellow shit disease. If the psyllid bites this parent tree, and injects huang long bing, they’re gone. There’s no known cure.”
Citrus greening curls the leaves of new growth on orange trees and causes the fruit to have a bitter metallic taste. The psyllids in California aren’t yet infected with huang long bing, and growers here have not experienced any losses. But the disease already has spread throughout all 32 citrus-growing counties in Florida and much of Texas.
Tracy Kahn, a botanist who curates UC Riverside’s Citrus Variety Collection, explains that most infected trees die within a few years. “They're losing trees in Florida left and right,” she says, “and it's really hard to keep an industry going because trees have a very short life.” The Citrus Variety Collection is the largest in the world, with more than 1,000 kinds of fruit, many of them descendants of the Tibbets tree. To guard against citrus greening, clones of every variety in the collection are now being kept in a nearby greenhouse, too, as a botanical backup.
Giorgios Vidalakis, a citrus virologist with the university’s Citrus Clonal Protection Program (CCCP), says it’s only a matter of time before citrus greening spreads to orchards in California.
“We know it’s not a question of if, it’s a question of when,” he explains. Inside a large greenhouse at the CCCP’s quarantine site in Riverside, researchers use cuttings to propagate new varieties for California citrus growers in a pathogen-free environment.
Vidalakis says that citrus greening is an example of a pathogen getting around the quarantine system. “We believe that a single tree, brought into Miami, Fla. — right now, that one tree is destroying the $10 billion Florida citrus industry," he says.
To protect the Tibbets tree, Vidalakis says, “We have created a buffer zone, removing citrus relatives and ornamental plants. For huang long bing, we don't have the solution yet. The best solution now, to buy us time until science finds a more permanent solution, is to build a protective structure” around the tree. Such a structure would keep infected psyllids from feeding on the tree's sap and could cost as much as $50,000. The city has pledged to cover part of the cost, but additional donations are welcome.
“Right now, it really keeps me up at night,” Vidalakis says. “We don't want to be the generation that loses that tree. But if the mesh plan works, Vidalakis thinks they can keep the tree alive indefinitely: “I don’t see any reason we can’t go on forever.”