At the end of the day, I’m not worried about the Citrus Greening disease, also called HLB or Huanglongbing, because it fails the eyeball test. Where are all the dying trees? I don’t see them.

What is this HLB/Citrus Greening disease?

It’s a condition where, I’m told, a citrus tree has leaves that are mottled yellow and green, the fruit are misshapen and partly green and sour, and the vascular system of the tree (where the sap flows, like the tree’s blood vessels) are clogged. This HLB disease is said to be caused by a bacterium called Candidatus Liberibacter asiaticus (CLas for short) that is carried around from tree to tree by a tiny flying insect called an Asian citrus psyllid (ACP for short).

Walk with me down memory lane to look at a handful of pivotal experiences I’ve had over the years which inform my current perspective on the California citrus HLB story.

Back in February of 2012, I trained to be a Master Gardener volunteer in San Diego County. During my group’s class on “Issues in Plant Pathology,” the county’s plant pathologist opened her lecture with what she considered the most dangerous issue: Citrus HLB. I had never heard of it. In my notes, I wrote:.

“No cure; tree must be removed or serves as innoculum to infect other trees.”

My Master Gardener classmates and I were told that HLB was currently devastating citrus groves in Florida. Luckily, however, the deadly disease had not arrived in California yet. The ACP insect had been detected here since 2008, but the CLas bacteria had yet to be detected.

Only a couple weeks later, BOOM! We were informed that a tree in a backyard in Hacienda Heights in Los Angeles County had tested positive for HLB. This “poses a threat to all California citrus,” we were told.

Local news headlines were apocalyptic: “California braces for a deadly stalker of citrus.”

The California Department of Food and Agriculture descended upon the neighborhood and destroyed the lemon/pomelo tree that had the positive test, sprayed insecticides on all nearby citrus trees, and established a quarantine in the area in order to prevent the movement of citrus fruit or plant parts.

Through the next few years, my classmates and I were given many informational handouts to share during speaking events and Master Gardener booths about the now-present and imminent threat of HLB. We were told to ask gardeners to inspect their trees for signs of this existential threat. The website that we were advised to direct gardeners to was CaliforniaCitrusThreat.org which stated, “HLB threatens to erase this tradition [of citrus trees] from our state’s history and put thousands out of work.”

In 2018, at the Festival of Fruit held by the California Rare Fruit Growers in Arcadia, I attended a presentation made by Tracy Kahn, Curator of the Citrus Variety Collection at U.C. Riverside, where she stated that 1,547 citrus trees in Southern California had tested positive for HLB at that point, and she recommended that if we wanted to save the citrus trees in our yards we should apply pesticides during each of their flushes of new growth in order to kill the insects that carried the CLas bacteria, and/or we could cover our trees with nets.

In Riverside, not far from the university, is the oldest Washington navel orange tree, and around this time it was covered with a protective net structure.

(My post, “Saving the Parent Washington navel orange tree.”)

California citrus seemed doomed.

But it had been six years since the first detection, and I was starting to secretly doubt the dire predictions. I had learned to spot the insect, ACP, and I had been seeing them all over the new flush on my trees for years. They were on everyone’s trees as far as I saw.

Much testing had been employed on trees throughout Southern California so increased detections occurred, as would be expected, but I wasn’t seeing sick trees.

So I chose not to follow Kahn’s advice about spraying or netting.

In 2022, during a presentation for citrus farmers in San Diego County, Neil McRoberts, Associate Professor of Plant Pathology at U.C. Davis, said, “I’m an epidemiologist . . . but personally, I’d be more worried about water than HLB if I were growing citrus in California.”

What? This was a new tone. This was the first change in tone regarding HLB that I’d heard from a U.C. researcher in ten years.

McRoberts went on to explain why he had become less worried about HLB. He had observed that both the CLas bacteria as well as the ACP insect find it harder to live in California than in Florida. McRoberts said that he could see after ten years of studying and testing that both were not proliferating here as they had in Florida, and he thought it was mainly due to climate. California’s colder winters and hotter summers slow down the reproduction of the insects and sometimes kill them.

After this event, I wrote the post, “Citrus, ants, and HLB: the latest research.”

I felt relief, but I also started to feel suspicion.

Thereafter, I began to do some homework. What are the foundations for the claims about this HLB disease, the CLas bacteria, and the ACP insects?

I contacted the most informed people I knew and asked for foundational papers to read. I was sent to “Huanglongbing: a destructive, newly emerging, century-old disease of citrus” by J.M. Bove from 2006 in the Journal of Plant Pathology. It contained no research of its own but was a comprehensive overview of the disease.

A couple of things stood out after reading it. One was that HLB is apparently a disease that was identified in China at least as far back as the 1870’s. In other words, it’s an old disease. But it took 150 years to cross the Pacific? So many people and so much plant material, including citrus plant material, have been coming to California (and the U.S. generally) from China since 1870 (the Meyer Lemon in 1908, for example). It’s hard to believe that this insect-bacterium-disease took so long to arrive.

My hunch became that the recent arrival was actually the HLB testing.

So how exactly is a citrus tree determined “HLB positive”? According to page 33 of the California Department of Food and Agriculture’s Action Plan for Asian Citrus Psyllid and Huanglongbing, “testing of plant samples for the presence of CLas [is done] using two USDA-validated multiplex TaqMan Real-time Polymerase Chain Reaction (qPCR) tests.” 

The important thing to note is that even if you grant that these PCR tests are accurate, then all you are acknowledging is that they can detect whether DNA unique to the CLas bacteria is present within a tree. Only. PCR tests do not diagnose illness, sickness, disease.

The CLas bacterium is not the HLB disease. But maybe the presence of CLas within a tree means that is also has the HLB disease?

Now I wanted to back up further and ask this question: How do we know that the CLas bacteria causes the HLB disease? That’s what I’d always been told: The ACP insects carry the CLas bacteria inside their bodies and inject the bacteria into citrus trees when they feed on them, thereby infecting the citrus trees.

But then I went to U.C. Riverside’s page on Huanglongbing and discovered totally different language. It never claims that HLB is caused by CLas. In fact, it states explicitly that “it is improper to refer to the HLB-associated bacterium [CLas] as the “causal agent”, the “agent” or the “pathogen” of HLB.” This is because “in spite of many attempts,” the necessary scientific criteria to claim that CLas causes the HLB disease have not been met. (Specifically, Koch’s postulates have not been met.) CLas can only be said to be “associated” with HLB.

So to be clear, authorities have been destroying citrus trees when they test positive for a bacterium that has not been shown to cause any disease.

Although the Bove paper from 2006 that I mentioned earlier didn’t contain any research of its own, it did help me find original research on HLB to read. I especially wanted to read about experiments that showed transmissibility of the HLB disease. In other words, has HLB been shown to be passed from tree to tree in any way (whether or not through the CLas bacterium)?

I found that South Africans have studied this disease for many decades and performed numerous experiments aiming to figure out the cause. One 1965 paper by McClean and Oberholzer reported on and summarized many of these experiments.

The authors write: “Often [HLB disease] spreads destructively: this is the case in the orchard in Pretoria and in three small commercial areas. Sometimes it is present but fails to spread to new trees: this is the case in the orchard below the Hartebeespoort irrigation dam. Or it may be present in sectors of some trees where it remains confined without dangerous spread. So too with experiments in propagating from diseased trees. One bud or twig tip gives a healthy tree, the next a diseased one. For example, less than ten per cent of the progeny in one group of experiments developed greening; in a second series more than half the progeny were positive. And attempts to transmit the infection by grafts have been equally inconsistent. Many inoculations fail entirely, others succeed easily.”

In summary, they state: “[HLB] as a transmissible disease is full of contradictions.”

A few months ago, I called up a citrus farmer friend. By this time, it had become clear to me that HLB was not what I was told it was, or was going to become, in 2012. I didn’t know much about it, but I knew it wasn’t that. But this farmer is involved in the highest levels of research discussion about HLB in California and maybe he knew secret stuff that I didn’t. Maybe he could put the worry back in me?

I asked him about many aspects of the citrus HLB situation, but none of his answers put the worry back in me, especially his answer to my last question.

“Have you personally seen, with your own eyes, a tree infected with HLB?” I asked him.

“No,” he said.