[Editor's Note: We wrote this piece prior to the spread of COVID-19 in recent months. At the time, we could never have guessed how prescient our topic for this month would turn out to be. Questions that seemed philosophical only a little while ago feel ever more vital today.]

We take that refreshing glass of orange juice — okay, maybe it’s a mimosa — for granted at brunch time, but citrus fruits are a relatively recent arrival in the 200,000-year history of anatomically modern humans.

First domesticated in Asia about 3,300 years ago, citrus fruits have from the moment of their discovery been highly valued as medicinal and even holy objects.¹ The earliest citrus fruits were almost inedible, due to their thick rinds, but people throughout China and India cultivated them for their restorative properties. After all, those thick rinds contained flavonoids — a kind of plant pigment — with effective anti-inflammatory and pain-relieving properties.²

The Hebrew diaspora is thought to be one of the prime agents behind the citrus fruit’s spread beyond Asia. The citron plays an important ceremonial role in the Jewish festival of Sukkot, and some traditions of Jewish folk medicine ascribe a salutary effect to the fruit as well. For example, placing a citron under your pillow was said to ease the pain of childbirth.³

Today, you’re more likely to find citrus fruits at mealtime than in the medicine cabinet, but that’s thanks to years of crossbreeding — and some lucky genetic mutations — to make the earliest citrus species much more edible. In fact, the fabulous array of citrus fruits we enjoy today — from yuzu and grapefruit to lemons, limes, and all the rest — all derive from only three “true” citrus varieties: pomelos, mandarins, and the citron.⁴

A vast network of delightful fruits from just a few humble parents — it’s quite marvelous, when you think about it. But the biological world is a complex one, and for all the splendors it generates, it also contains forces working in the opposite direction. Such is the case for citrus fruits: They face a quiet but growing threat, and if we don’t take action in the coming years, we may lose them altogether.

Small Pest, Big Problem

It may sound alarmist to suggest citrus fruits are on the brink of extinction, but take a look at Florida. Once dappled with thousands of citrus groves, Florida is now haunted by a growing number of “ghost groves,” or abandoned citrus fields. The number of Florida citrus farmers dropped from 7,000 to 2,000 between 2004 and 2019, all thanks to the rapid spread of citrus greening disease.⁵

First documented in China’s Guangdong province in 1919, citrus greening disease — also known as huanglongbing or “yellow dragon disease” — destroys a tree’s phloem. Roughly analogous to human veins, phloem transports nutrients through a plant. As greening disease progresses, it becomes harder and harder for the plant to nourish itself. Its leaves become mottled and yellow; its fruits grow misshapen and green, and they fall from the tree before reaching full maturity. Within a few years, a once healthy tree can become totally unproductive.⁶

Scientists don’t know much about citrus greening disease, because they haven’t yet figured out how to grow the bacteria responsible for the disease in a lab. But they do know how it spreads: the Asian citrus psyllid. This small flying insect sucks up the greening disease bacteria when it feeds on infected trees, and it passes the bacteria to healthy trees through further feasting.⁷

The citrus greening disease outbreak has decimated citrus groves in the U.S., Asia, and South American. Fresh citrus fruit production in the U.S. has fallen by 21 percent. China’s Jiangxi Province, a top citrus producer, has lost 25 percent of its groves. In Brazil, 52.6 million sweet orange trees — 31 percent of the country’s total — have been wiped out.⁸

Some (Psyllids) Like It Hot

It’s easy to blame the bug and the bacteria for all the damage done, but in a sense, citrus greening disease is a case of human beings reaping what they’ve sown. This pandemic is, in part, a manifestation of climate change.

Temperatures between 60 and 91 degrees Fahrenheit are most favorable to the reproduction and transmission of the bacteria behind citrus greening disease. As global temperatures rise, it becomes easier for the bacteria to thrive in new locations.⁹

The same goes for the psyllid. Perhaps unsurprisingly, it thrives in the same temperature range as the bacteria. While native to Asia, the psyllid has become an invasive species in places like Florida, California, and Brazil — and it can extend its range further and further north as temperatures continue to climb.¹⁰

It also doesn’t help that commercially cultivated plants are more prone to disease than their wild counterparts. As we covered in last year’s Ignite series, industrial farms tend to plant large numbers of the same species of plant close together. The genetic similarity of these plants means there are no biological buffers against the spread of infection, and close quarters allow psyllids to easily flit from tree to tree.¹¹

The spread of citrus greening disease serves as a useful metaphor for the unpredictable ways in which what seem like innocent human activities can ramify throughout the entire ecosystem. When we built our modern societies on the basis of fossil fuels, who could have foreseen that, 150 years down the line, it would drive an unwittingly infected insect into new territories, threatening the very existence of citrus fruit in the process?

Researchers working to solve the citrus greening crisis are acutely aware of the delicately intertwined nature of the Earth’s environment, which is why so many of them are looking beyond pesticides to find a solution that won’t upset the balance further.

Farmers and scientists are embracing the tools nature already has at its disposal to beat back the advance of citrus greening disease — for example, by growing more naturally resistant citrus trees.

The Sugar Belle, a crossbreed of sweet clementine and Minneola tangelo, initially came about because breeders were simply searching for a new citrus fruit. But something curious happened: A Vero Beach, Florida, farmer named Barney Greene found that his grove of Sugar Belles was resistant to citrus greening disease. As it turns out, Sugar Belle trees are extremely effective at growing new phloem, giving the trees a natural defense system against infections like greening disease.¹²

Meanwhile, organic chemist Katherine Maloney found another possible weapon against greening disease while studying grapevine infections. Maloney noticed the plants that survived infection had a particular kind of fungus growing on them. This fungus produced a compound called radicinin, which prohibits bacterial growth. If isolated, the compound could be a natural alternative to antibiotics. In a similar vein, researchers in Brazil have developed a bioinsecticide that uses another fungus to prey on the psyllids.¹³

Of course, even these more natural disease management efforts are not a guaranteed fix. While conscientious scientists aim to tackle greening disease without doing further harm, they know how complexly interconnected the natural ecosystem is. One minor move could throw the whole thing into disarray — much like how the slightest genetic mutations transformed citrus fruits from inedible plants into a worldwide culinary staple.

Curing in Context

Researchers are increasingly calling for a more holistic approach to the problems of plant epidemics. Climate change affects a broad spectrum of natural factors in the spread of plant disease, from temperature and humidity to soil quality and drought frequency.

Furthermore, the impact of climate change on a given disease can often have contradictory implications. For example, one study found that while higher temperatures might promote increased growth of a fungal infection called “wheat rust,” increased drought frequency might also mitigate the spread of that rust. This puts us in the unenviable position of trying to guess how these factors will balance out, if at all.¹⁴

An additional problem is that most studies of plant disease focus on major crops like rice and wheat. That’s understandable, but these studies overlook staple crops in developing regions like yams, sorghum, and cassava. That can leave us ill-prepared for what the future might hold for these crops and the people who rely on them.¹⁵

Worth the Squeeze

As plant pathologist Dr. Diane Saunders writes, it is only through a “holistic perspective of plant-pathogen interactions” that we can “design truly sustainable solutions.”¹⁶

As the rise of the citrus greening disease due to modern agricultural practices shows, a sick plant doesn’t exist in a vacuum, and neither can our response to the problem of any single plant illness. Protecting our biosphere from further harm — and hopefully undoing some of the harm we’ve already caused — will require a concerted interdisciplinary effort on the part of researchers, farmers, governments, and, really, all of us.

— The Keap Team