HLB Reaches Imperial County: California's Newest Citrus Greening Quarantine Zone

A single backyard tree in Calexico has pulled Imperial County into a fight that began in a Los Angeles County yard fourteen years earlier. On August 18, 2026, the California Department of Food and Agriculture and the USDA confirmed that a residential citrus tree in Calexico had tested positive for Huanglongbing, the incurable bacterial disease known as citrus greening. It was the first HLB detection ever recorded in Imperial County, and it made the county the seventh in California to fall under a state quarantine for the disease.

The finding is small in scale — one infected tree — and large in implication. Imperial County sits on the U.S.-Mexico border in a farming region built around winter vegetables, alfalfa, and desert agriculture, not citrus. Its inclusion on California’s HLB quarantine map says less about the county’s citrus acreage than about how far the disease’s vector, the Asian citrus psyllid, has now traveled.

The Detection and the Boundary

CDFA staff treated and removed the infected tree shortly after confirmation. Because HLB spreads through the psyllid rather than through soil or water, the agency’s response follows a fixed protocol: survey every citrus tree within 250 meters of the find site, and apply psyllid-control treatments within that same radius to knock down any local vector population before it can move the bacterium further.

The quarantine itself is considerably larger than the survey zone. State rules impose a mandatory five-mile radius around any confirmed HLB detection, and for Calexico that boundary is drawn as a rectangle rather than a circle: bounded on the north by McCabe Road, on the south by the U.S.-Mexico border itself, on the west by Rockwood Road, and on the east by Barbara Worth Road. Within that perimeter, citrus nursery stock, budwood, and plant material may not be moved off a property, though CDFA’s standard exception for well-washed, leaf-free fruit still applies.

A Border Complicates the Response

What makes the Calexico case different from the ordinary quarantine expansion is geography. Most of California’s HLB detections have occurred well inside the state, where the five-mile radius closes entirely on domestic ground. In Calexico, the southern edge of the quarantine zone simply stops at the international line, because California and CDFA have no regulatory reach across it.

That has pushed USDA into direct coordination with Mexico’s National Service for Agro-Alimentary Health, Safety and Quality, known by its Spanish acronym SENASICA, to work through the cross-border implications of the quarantine. The Mexicali Valley immediately south of Calexico grows citrus of its own, and HLB does not recognize a fence line. Psyllids move on wind and on infested nursery material regardless of which country’s agricultural agency has jurisdiction, which is precisely why the two countries’ plant-health services need a shared picture of where the bacterium has turned up on either side.

The Insect Arrived First

Imperial County’s HLB history did not begin with the August tree. Industry news coverage of the detection has noted that an Asian citrus psyllid collected in the county tested positive for the HLB-causing bacterium back in November 2025 — roughly nine months before an actual infected tree was confirmed. An insect-only detection like that does not trigger a quarantine under CDFA’s rules, because the agency’s quarantine authority is tied to a positive finding in plant tissue, not simply in the vector that carries it. But it is exactly the kind of early-warning signal California’s citrus-protection program is built to catch, since a CLas-positive psyllid can turn up in an area well before the tree it eventually infects does.

In that sense, Calexico’s August tree was not really the first sign of trouble. It was the moment an already-flagged risk became a confirmed one.

The Statewide Count Imperial County Joins

Imperial County’s quarantine does not stand alone. Since the state’s first confirmed HLB-positive tree was found in a Los Angeles County backyard in March 2012, CDFA and USDA APHIS have documented more than 10,700 HLB-positive trees found and removed across California, concentrated in Los Angeles, Orange, Riverside, San Bernardino, San Diego, and Ventura counties. Imperial is now the seventh county added to that list, and its addition extends the disease’s confirmed footprint from the urban citrus belt of Southern California all the way to the state’s southeastern border.

The pattern across those fourteen years has been consistent: detections cluster in residential and backyard settings rather than commercial groves, because dooryard citrus is planted, shared, and moved between properties with none of the monitoring that a commercial operation applies to its own trees. Imperial County’s single backyard tree fits that pattern exactly.

What Growers and Residents Should Do

For homeowners inside the new Calexico quarantine boundary, the practical rule is straightforward: do not move citrus branches, leaves, potted trees, or budwood off the property, and expect a CDFA survey crew to inspect trees within the 250-meter treatment zone. For citrus growers on the Imperial Valley side of the border and in the Mexicali Valley beyond it, the detection is a reminder that a quarantine line drawn on a map does not track how psyllids actually move. The most useful step available to anyone farming or gardening citrus near the new zone is the same one CDFA has been urging statewide for a decade — inspect new flush regularly for psyllids, report suspect trees to the county agricultural commissioner, and treat host plants before a routine survey turns an early warning into a second confirmed county.