Riverside has played a historic role in citrus production in California, the United States, and in the world. The introduction of the navel orange by the Tibbetts family is a legendary part of Riverside's heritage. The vast wealth created by the navel orange in and around Riverside was responsible for locating the second campus of the University of California, the UC Citrus Experiment Station, in Riverside. The subsequent research leadership by that Station made Riverside the logical location for the very first International Citrus Congress in 1968. The current incarnation of that Experiment Station at the University of California at Riverside remains a global leader in citrus research.
But it's tough to buy a citrus tree in Riverside. You can grow citrus trees (oranges, lemons, limes, grapefruit, mandarins, kumquats, etc.) in Riverside. You can give fruits from those trees to your neighbors or sell them. You can even find nurseries online that you can buy citrus trees from and plant on your property. But very few of those nurseries are in Riverside; the majority do not offer citrus.
The reason for the mystery of the missing citrus is due to quarantines associated with the presence of a difficult and fatal citrus disease in Southern California. Huanglongbing (a.k.a. "HLB" or "citrus greening disease") has devastated citrus production globally. First known in China, it has spread to many, but not all, of the major citrus production areas in Asia, South Africa, South America, and Mesoamerica. In the United States, the disease has spread through the citrus producing areas in the Southeast, notably Florida. Indeed, Florida's citrus industry has been hit so hard that it is no longer the number one citrus-producing state. First identified in California in March 2012, HLB has been found in hundreds of individual trees in residential areas but not in California's citrus production areas. Part of California's success in limiting the spread of the disease has been the creation of quarantines.
To better understand the need for quarantines, we need to appreciate the biology of the disease. The organism responsible for HLB is a bacterium. The bacterium has been particularly difficult to study because it has proven challenging to culture. Another frustrating factor is that the organism spreads slowly through a tree; it takes years for symptoms to occur. The first symptoms are asymmetric, misshapen leaves, then come ill-flavored patchy-colored fruits, later the tree weakens and eventually dies. Despite years of research, no cure or preventative treatment is known.
The HLB bacterium is not capable of moving from tree to tree on its own. There are two known agents (vectors) which are known to spread the disease from tree to tree. Asian citrus psyllid (a.k.a. "ACP") is a sap-feeding insect distantly related to aphids that exclusively feeds on the sap of citrus species and their close relatives. If an ACP feeds on an infected tree, it picks up the disease organism which persists in its gut and mouthparts. Once the insect moves to another tree to feed, it injects the bacterium into the sap of that tree, effectively inoculating it. ACP arrived in Southern California a few decades ago, spreading north from Mexico, years before the arrival of the bacterium. In high densities, the psyllid is a pest in its own right. Pesticides and parasitic wasps are capable of reducing ACP populations, but the insect is so abundant and widespread in Southern California that it cannot be extirpated.
The other vector is humans. Humans are capable of grafting a bud or buds of one variety of citrus onto another. If the grafted material comes from a diseased tree, the offending organism can move into the recipient tree and spread through it. Some scientists have suggested that the introduction of the HLB organism in Southern California may have occurred via human vectors who smuggled budwood from Asia to graft to their own tree or trees.
In summary, at early stages, HLB disease is difficult to detect. If ACP is not present and an infected tree is not used for budwood, the disease cannot spread. But an infected tree behaves normally for years such that if there is a vector, spread will likely occur long before symptoms. So, a quarantine is necessary for limiting the spread of the disease.
In California, if a single infected tree is discovered, it is destroyed and its location becomes the center of a quarantine zone with a five-mile radius. The California Department of Food and Agriculture (CDFA) manages such quarantines and provides an online map (2026 Huanglongbing (HLB) Quarantine Boundaries). If you take a look at that map, you'll see that Riverside appears to be totally included within the 2026 boundaries.
I contacted Joshua Bingham, Public Affairs Officer for the CDFA, for verification. He confirmed that the entire city of Riverside falls within the HLB quarantine. I also asked him, given the quarantine, whether any Riverside nurseries were still selling citrus. He explained that only nurseries that built and maintained insect-resistant structures were eligible for certification in Riverside, and these are inspected every 30 days by USDA (United States Department of Agriculture) or CDFA personnel. Only Louie's Nursery, Cal West Nursery and Y and M Nursery offer certified clean ("disease free") trees for sale to the public. I called each of these nurseries to confirm that they were actively selling citrus, and indeed they are. A good variety of trees are available at each location. There are only three other nurseries in Riverside County that have regulated structures.
That's the good news. But it begs the question: How long will this HLB quarantine last?
Unlike the quarantines that come and go when a rare insect pest appears in a single location and is eventually eradicated, the HLB-ACP situation is quite different. The insect vector is well-established in California and the disease undoubtedly persists yet undiscovered in normal-looking trees. With regard to the quarantine, it seems like we are in for the long haul.
CDFA's Joshua Bingham offers this analysis of the situation. "At this time, there is no exit strategy to remove an area from ACP or HLB quarantine. California must, at minimum, parallel federal requirements, and USDA does not have a pathway to lift a quarantine area. Due to the nature of HLB, especially considering its latency and difficulty to detect, lifting a quarantine is biologically unfeasible. A tree may become infected today but may not test positive for several months ... or years as the disease progresses through the tree."
In other words, until research yields a way for ACP extirpation, for HLB prevention, or for curing disease, this quarantine should be considered more or less permanent. Beyond ongoing research, the best efforts are spent preventing the spread of the disease. As Joshua Bingham concluded in his note to me, "CDFA continues to place significant resources in early detection and removal of trees to protect California citrus."
Portions of this article are based on my prior RG article "A Tale of Two Quarantines."
