LOS ANGELES >> The worst disease known to the citrus industry may have arrived in California on a bud of friendship.

A graft of pomelo — a symbol of good fortune and prosperity in many Asian cultures — was the likely source of the state’s first documented case of huanglongbing, a citrus disease with no known cure, say researchers involved in the investigation. The suspected plant shoot, or budwood, was passed freely among San Gabriel Valley church friends who loved to garden and experiment with hybridization, according to residents.

Until a month ago, California was the last major citrus-growing region in the world to avoid a scourge that has decimated groves in China, Brazil and Florida. The disease arrived the way experts had long predicted: in a tree in a Southern California yard. Now, agriculture officials are scrambling to slow the disease’s march north and save a $2 billion industry based in the Central Valley.

Authorities launched a massive containment effort involving quarantines, pesticides and public hearings when a lemon-pomelo tree in Mary Wang’s lush Hacienda Heights yard tested positive for the disease on March 30. The sickly looking tree was quickly removed for study.

Wang’s yard was once little more than a scraggly patch of grass. Then she met Qifu Wang at the United Chinese Christian Church. In China he had studied horticulture and worked as a landscape artist. But in this country, he ran a convenience store with his wife, lived in a condo and had no place to grow things.

Mary Wang, 59, invited him to garden on her land: “You’re the expert,” she said. “Do what you want.”

Qifu Wang, who gives his age only as “more than 60,” planted figs and almonds, persimmons and pomegranates. He skillfully grafted buds onto trees, growing hybrid peaches and lemons that surprised Mary Wang with their sweetness. More than a year ago, he added a graft of pomelo grapefruit to a lemon tree.

Qifu Wang said he does not recall who gave him part of a pomelo tree. It was almost two years ago, and the church’s tight-knit group of gardeners enthusiastically shares seeds, cuttings and buds.

“You share good things,” he said. “Especially if it will help other trees grow better fruit.”

He grafted the pomelo to Mary’s lemon tree because he wanted to experiment with combining something large with something small, he said.

Some researchers immediately suspected the pomelo graft as the cause of the infection. There are strains of prized Chinese pomelo not available in the U.S. The fear had long been that a smuggled tree would carry the disease.

“We may never know for sure it’s the graft,” said Ted Batkin, president of the Citrus Research Board. “But … we know from experience that this kind of graft is the most likely cause. Teams will trace it backward and forward. What tree did this budwood come from and where else might it have gone?”

Larry Hawkins, a spokesman for the U.S. Department of Agriculture, said it has not concluded how the state’s first case of huanglongbing, also known as greening disease, arrived.

“The investigation is ongoing. We’ve reached no conclusions,” Hawkins said.

Officials at the California Department of Food and Agriculture have long been bracing for the arrival of huanglongbing. It is spread by the Asian citrus psyllid, a flea-size bug that can feed on an infected tree and transmit the disease to others. Once the first bug arrived in the state in 2008, the clock was ticking.

State workers have monitored more than 10,000 traps in Southern California neighborhoods, testing hundreds of thousands of bugs.

The highest concentration of the bugs was in neighborhoods near Dodger Stadium. Officials also closely monitored the San Gabriel Valley, once the orange capital of California, before the industry moved to Central California. Only one bug so far has made it to the Central Valley, the heart of the state’s commercial citrus industry, and it did not test positive for the disease.

No bugs tested positive until one tiny infected psyllid caused state and federal workers to show up on Mary Wang’s doorstep and tell her she had a sick tree they needed to remove.

It’s a knock on the door that other California residents could soon receive. The state’s plan to isolate huanglongbing includes the mandatory removal and destruction of infected citrus trees and mandatory spraying of insecticides in areas 400 to 800 meters around the infected trees.

Word of the diseased tree has spread rapidly through the San Gabriel Valley’s insular, Chinese-speaking community. “Huanglongbing” translates to “yellow dragon disease,” and many worried what the news meant for their own citrus trees and how much danger it posed.

Others knew exactly what it meant.

Wingtack Wong, who with his siblings owns Temple Garden Center in El Monte, is sadly familiar with huanglongbing. He worked for the agricultural commission in China’s Guangzhou province when the disease arrived there in the 1960s. He remembers the commission tried everything from pesticides to experimental cures to stop it, but by the 1980s and early ’90s, the disease had traveled slowly north, killing all the citrus.

A wiry man who usually bustles around his sunny nursery answering three questions at once, Wong sat down to speak of huanglongbing.

He recalled a farm famous for “fields and fields of mandarins.”

“One yellow leaf tuned into an entire yellow tree, the fruit shriveled up and slowly the rest of the trees turned yellow, and then it was all gone,” he said, slouching in his chair.

He said he respected California’s fevered effort to halt the disease.

“Maybe we’ll be luckier this time.”