Over three days each autumn, mandarin lovers descend on the Gold Country Fairgrounds in Auburn for an annual festival celebrating this signature fruit of the Sierra-Nevada foothills. The Mountain Mandarin Festival is a well-attended affair, where the region’s mostly small-scale  citrus growers hawk their fresh fruit and mandarin-flavored goodies. This November’s event will be the 23rd annual.

Mandarins dominate commercial citrus production in the foothills, where oranges, lemons, limes, grapefruit and kumquats also flourish. Last month, citrus growers in Placer County and surrounding regions were given a dire warning to safeguard their industry: Do not move outside citrus into this county — no matter where it is from.

That’s because the U.S. is battling citrus greening, a fast-spreading bacterial disease. Experts regard this disease, also called by its Chinese name of huanglongbing (yellow dragon disease), as the greatest threat faced by the citrus industry around the world. No cure exists. An infected tree dies. The presence of citrus greening anywhere spells potential disaster for a country’s whole industry.

In the U.S., only four states produce our nation’s multibillion-dollar citrus crop: Florida and California grow the bulk, with Texas and Arizona combined producing the remaining 3 percent. California’s citrus production was valued at $1.9 billion in 2014. The industry also directly employs more than 10,000 people statewide, according to California Citrus Mutual, a nonprofit trade association. Most of the production occurs in the south: in Tulare, Kern, Fresno and Ventura counties. Placer County citrus — mostly mandarins — is relatively small, with a value of $1.8 million in 2014. But the local industry still matters.

“One thing I like to remind people is that there is also non-monetary value associated with citrus [here], like people having access to locally grown fresh produce, or events like the Mountain Mandarin Festival bringing people together as a community and educating folks on the importance of agriculture in their local community,” says Victoria Hornbaker, who manages the Citrus Pest and Disease Prevention Program of the California Department of Food and Agriculture.

Citrus greening hasn’t been detected in Northern California — yet — and the Mountain Mandarin Growers Association hopes to keep it that way. The Placer County-based association, which represents about 35 members, participated in an event in March hosted by the California Department of Food and Agriculture to learn how to proactively keep the disease at bay. It won’t be easy.

Being Vigilant

The culprit behind huanglongbing (HLB) is a tiny insect called the Asian citrus psyllid, which feeds on citrus leaves and spreads the disease through its saliva. Once infected, a plant’s roots decay, its leaves get blotchy and its fruit turns sour — and green, hence the name. Since its discovery nearly a century ago in China, the disease has been confirmed in several countries, including South Africa, India, Iran, Brazil — the world’s largest citrus producer — and U.S. (in Florida, in 2005). In 2012, HLB surfaced in California, on a tree in Los Angeles County — likely as a result of an infected tree brought in from Mexico. The insect has been inching its way up north ever since.

Rich Colwell, of Colwell Thundering Herd Mandarin Ranch in Placer County, says commercial growers in our region have undergone training over the past few years on how to prevent citrus greening. They’ve learned to identify an infected plant and the process for immediately notifying government officials. Quarantines have been instituted elsewhere in the state where the insect has been found. Commercial growers are given sticky pads that catch random insects flying by, which are later analyzed.

The Asian citrus psyllid has been identified in more than a dozen counties in California. The pest flies over backyard fences, putting all citrus plants at risk if certain precautions aren’t taken. For example, transporting citrus fruits with stems and leaves still attached can inadvertently spread the pest. Colwell says it’s critical that residents recognize their role in stopping HLB: People traveling into or out of California should not transport citrus fruit, leaves or whole plants. Nor should people bring trees or fruit from Southern California up north. Residents should regularly inspect trees when watering, pruning or picking for signs of the insect or disease.

Colwell’s mandarin ranch started 30 years ago. He and his wife grow mandarins, oranges, lemons and permissions on eight acres. They ship their fresh fruit all over the country to retail establishments and markets, and the ranch is known for its mandarin olive oils. Even with drought and other pests already on the minds of citrus growers, HLB worries Colwell most of all. “It takes five to seven years to get a tree into production and about 10 years for full maturity,” he says. “So it’s not like you can tear down an orchard and put up a new one. Most growers would go out of business.”

A Collective Effort

Commercial growers aren’t the only group that should care about citrus greening. Consumers have a vested interest, too: If HLB ravages the industry, the availability of citrus will significantly drop and the cost will skyrocket.

Two scientists from UC Davis are on the case. They are collaborating with researchers from other universities on a U.S. Department of Agriculture grant to figure out how to address citrus greening through genetic engineering, according to Dr. Bryce Falk, a plant pathologist in the College of Biological Sciences at UC Davis. Once the technologies are developed, the next step will be their implementation.

All parties working to stop HLB agree that the solution won’t boil down to any one thing; it will take a collective approach among commercial growers, homeowners and scientists with citrus plants in their yards.

“We have to figure out the psyllid’s weakness,” says John Miller, president of the Mountain Mandarin Growers Association and owner of Miller Honey Farms in Newcastle. “We have to meaningfully push back with natural [parasites]. We have to identify resistant rootstock. We have to stop leaf and stem tissue from being hauled in grandma’s car. We’re challenged.”