The end of the orange juice / Anna Kushment

A deadly disease is killing citrus trees from Florida to California

One day in 2005, a few days before Hurricane Katrina passed through Florida and destroyed New Orleans, Susan Helbert stood in front of a pomelo tree in a yard near Miami. Something about this tree seemed off, like it was malnourished. The tree had few leaves and the fruits, which were the size of melons, were deformed. But the other trees in that garden thrived, and the woman who took care of them made sure to fertilize the pomelo trees with a new layer of fertilizer. "She definitely knew how to take care of plants," says Halbert, an entomologist at the Florida Department of Agriculture and Consumer Affairs (FDACS).

Like a detective at a murder scene, Halbert examined the tree from all sides as she tried to systematically rule out all possible reasons for its condition. She dismissed the possibility that it was root rot, a disease caused by fungi, because she did not notice the typical symptoms. Then she considered a viral disease, tristeza (tristeza) of citrus, whose name originates in Spanish and Portuguese and means heart, and is typical of trees that have been assembled (mostly farmers grow citrus trees by mounting a branch from one tree on the bark of another tree, and not directly from the seed). However, this pomelo tree was not assembled. Finally Halbert reached the bottom of her list, the deadliest citrus tree disease of all: huanglongbing., whose name originates from a Chinese word meaning yellow dragon disease.

Huanglongbing, also known as HLB or Greening of citrus, slowly spread from India through China, Indonesia and South Africa. Just one year earlier she was diagnosed in Brazil. The disease kills citrus trees by blocking the transport system of the tree, and causes the formation of deformed and bitter fruits. Its cause is a bacterium that hides in the salivary glands of a small, winged insect called the Asian citrus aphid. The aphid injects the bacteria into the tree while it sucks the sap on it. There is no cure for the disease, there is no pesticide that destroys enough aphids and in fact there is no way to treat it.

With great trepidation, Halbert collected some branches and took them to be tested in her laboratory. Within a few days it turned out that she was right. The deadly disease found its way into the heart of the American orange industry.

Halbert set off all the alarm bells. Following her warnings, scientists and citrus growers did everything they could to stop the spread of the disease. They uprooted infected trees, sprayed liberal amounts of pesticides, wrapped entire nurseries in protective covers and imported wasps from Asia to fight the aphids. Other researchers injected antibiotics into tree trunks and looked for genes that would make the orange trees resistant. But the Huanglongbing continued to spread. Over the past eight years, it has damaged more than half of Florida's orange trees, costing the state $2006 billion in 2011-4.54 and cutting more than 8,200 jobs in the orange industry. "In about five years there will be no more orange juice from Florida," says G. Glenn Morris, director of the Center for Emerging Diseases at the University of Florida.

Since Halbert discovered it, the disease has spread to Georgia, South Carolina, Louisiana and Texas. In the spring of 2012 she was discovered in Los Angeles. In November 2012, aphids were found for the first time in the citrus groves of the California orange industry, which apparently shows that the disease has also reached these districts. If scientists don't find a way to stop the disease, it could wipe out the entire US citrus industry.

Meet the aphid

To get to Halbert's office in Gainesville, Florida, the visitor must pass through a reception area where glass cabinets containing colonies of predatory wasps and live tarantulas are located. Across the hall is Florida's collection of arthropods: about nine million dried insects organized in thin wooden drawers arranged in tall metal cabinets that fill several rooms. As an entomologist in the Department of Agricultural Industry at FDACS, Halbert uses the collection of arthropods to identify the dozens of insects that Florida inspectors collect in fields and nurseries across the state, transfer to glass test tubes filled with alcohol and send to her in yellow business envelopes piled on her desk.

Halbert, a woman in her 60s with her hair pulled back in a braided ponytail, comes across as a lovable librarian, but when it comes to fighting the germs that threaten Florida's multibillion-dollar agriculture industry, she's a tough fighter. Halbert is one of Florida's eight entomologists and one of the state's two experts on plant-sucking bugs, a family that includes aphids, cycads and psyllids. Its main responsibility is to liaise between the supervising authorities and the growers and to inform the authorities about the invasion of new pests. "My job is to constantly check what is on the radar screen and know who the potential enemies are out there," says Halbert.

Greening's disease has been on Halbert's radar screen since the mid-90s when she first heard about it from her colleagues in South Africa. In June 1998, she was the first to locate the aphid in the USA after recognizing its characteristic position, when it raises its rear upwards at an angle of 45 degrees.

"Looking back, I should have been better at being on guard," she says. "But aphids have lived in Brazil since the 40s, and in 1998 not a single case of greening had been recorded in the country." Therefore, Halbert and her colleagues decided to just wait for the meantime and continue to follow the aphids. She returned with a group of inspectors to Palm Beach, where the inspectors deployed to check how far the pests had spread. Armed with sticks and plastic trays, the group of inspectors went through and beat the bushes in search of the aphids. They looked for them wherever there were citrus trees and also in the bushes around them. When they found aphids, they knocked them off the leaves with the help of the plastic sticks and then counted how many aphids accumulated in the trays. As they moved away from Palm Beach, the number of aphids decreased, until eventually they disappeared completely. The invasion appeared to be limited to an area of ​​about a hundred kilometers along the coast, and none of the aphids tested carried Huanglongbing.

This was an encouraging sign, but Halbert and her colleagues remained vigilant and continued to look regularly for the appearance of Greening's symptoms. In 2005, Halbert decided to carry out an extensive in-depth survey of citrus trees and to concentrate especially on neighborhoods where a population that comes from countries where the disease is widespread lives. That's how she found the sick pomelo tree in Miami. He grew up in the yard of a woman of Taiwanese origin. "It was another particularly bad day," Halbert says.

However, by the time Halbert verified her fears, the disease had already spread at record speed. The main reason for the rapid rate of spread is the high reproductive capacity of the aphids. In a one-month life cycle, each female lays up to 800 eggs. It turns out that the population of aphids on one orange tree can reach the unimaginable number of 40,000 insects. When the number of pests moving from place to place is so great, even spraying that eliminates 99% of them leaves enough survivors to continue the spread of the plague. But even worse, it turned out that one of the shrubs that the Asian aphid especially likes to eat is the orange jasmine, a very popular shrub that grows in Miami and is sold at discount stores and nurseries all over the country. This is how the aphids managed to spread easily.

Another risk factor for the spread of the disease is Florida's hurricane season. Winds like those brought by Katrina and other tropical storms probably blew the aphids a greater distance than they could have reached on their own. It can be assumed that the storms also weakened the trees and made them even more susceptible to the disease.

Finally, it must be emphasized, the fact that the Asian aphid is a species whose source of origin is not in the USA means that it has no natural enemies and therefore it reproduces freely and quickly. To try to deal with this, the entomologists looked for insects that could wreak havoc among the aphid population, similar to the destruction that aphids wreak in orchards.

Release the wasps

One hot day in the summer of 2012, Mark and Kristina Hodel loaded up their rented Ford sedan and made the hour-long drive from Riverside to Los Angeles. A blue plastic cooler was placed on the back seat. Inside it, packed in ice, were stored a dozen test tubes containing wasps feeding on drops of honey. While Mark drove, Katrina looked through the stack of documents that included data on all the research sites they should visit.

California is the second largest producer of oranges in the US. But while most of the oranges grown in Florida are squeezed to make juice, most of those grown in California are sold for consumption all over the United States. When the Asian citrus aphid was first discovered in the San Diego area, the top priority in California was to prevent the aphids from reaching the north, into the central valleys of California. Although inspectors began spraying pesticides in the San Diego area, the aphids quickly reached the Los Angeles area as well and continued to spread north along the coast. A new plan of struggle was therefore required.

The Hodels are entomologists at the University of California, Riverside, and specialists in invasive species. Shortly after the Asian citrus aphid was discovered in the country, Mark read an article written by an Indian scientist from the Punjab in 1927. The author described the results of Greening's disease in harsh terms ("It is not a rare situation to see a once valuable orchard become a barren plantation from which only the dry skeletons of the trees remain"), but in the same article he also reported on a local species of parasitic wasps that are capable of destroying up to 95 % of the Asian citrus aphids. "Can these wasps survive and reproduce in California?" Mark wondered.

It makes perfect sense that the natural enemy of the Asian citrus aphid would come from South Asia, the presumed place of origin of the aphid. Where, when and how the huanglongbing bacteria met, the aphid and the citrus trees are still open questions. For a long time, researchers believed that the citrus family developed in China, but a new study by Andrew Beatty from the University of Western Sydney in Australia claims that it first appeared in Australia about 35 million years ago and from there spread throughout Asia. From the epidemiology of the disease, it is possible to assume that the species of bacteria suspected of being responsible for huanglongbing, Candidatus liberibacter (the term "Candidatus" indicates that the scientists believe that it is responsible for the disease but cannot prove it because it is impossible to grow the bacteria under laboratory conditions), probably developed in Africa and moved from its natural habitat - A plant that is a relative of the citrus trees - these trees only about 500 years ago (if the citrus trees and the Huanglongbing had met earlier, the citrus trees would have developed resistance to the bacteria or would have become extinct).

Beatty suspects that the transition occurred in Africa when the citrus aphid transferred the bacteria to an orange or tangerine tree that was then sent by sea to India as part of the colonial trade. Agriculture also played a central role in the spread of the disease. The aphids lay their eggs on newly germinated young thrips. The pods serve as a convenient and easy-to-process food source for the young nymphs. As a result of the irrigation and the use of fertilizers, citrus trees grow and bud quickly, which creates an ideal substrate for aphids of all ages.

In order for Tamarixia radiata wasps to survive in California's Central Valley region, where most of the state's citrus trees grow, the climate in the region needs to be similar to the climate in their place of origin. Mark entered the weather data into software that checks the suitability of climate zones and discovered that both California and Punjab have hot and dry summers and cold and foggy winters - an excellent match. He then discovered, perhaps by fate, that the vice-chancellor of the Punjab Agricultural University was a graduate of the University of Riverside. "Suddenly all the doors that I previously feared would be so difficult to walk through opened," says Mark, who is from New Zealand. In early 2011 Mark and Kristina made their way to Pakistan to learn all they could about the T radiata wasps.

Importing wasps from Pakistan in a post-11/XNUMX world was no easy task at all. Mark, in collaboration with the California Department of Food and Agriculture, was able to obtain permission from the United States Department of Agriculture (USDA) to organize the operation to import the wasps, but only on the condition that they be kept in quarantine to ensure that they do not carry diseases. Together with his post-doctoral student, he spent months examining the variety of T. radiata hosts and crossing them against native aphids that live on native plants and against beneficial insects that attack weeds and pests. He did all this to ensure that the wasps would not violate the biological balance between the plants and the animals in California and would not cause damage to the biological control system of the weeds. Finally, he and Kristina placed a series of special cages inside a set of isolated laboratories at UC Riverside that allowed them to study how the wasps reproduce in an ecosystem that includes both the Asian citrus aphids and the trees they attack.

Since December 2011, Mark and Christina have released thousands of T. radiata wasps at over XNUMX sites in the Los Angeles area, Riverside area, Orange County and San Bernardino County. The summer day I joined them they were visiting the wasp release sites in the Los Angeles area to check how they were being picked up. "This is a built-up area of ​​warfare," says Mark from the driver's seat. Despite the heat, he and Kristina wore long shirts and long pants to protect themselves from the scorching sun in the field.

Although the Asian citrus aphid first appeared in the San Diego area, it quickly spread to the Los Angeles area. In these areas it is very common to grow lemon and lime trees in private yards, and many residents bring the trees from Mexico or smuggle branches of citrus trees in their luggage when they return from Asia. You can easily assemble infected branches of the lemon tree on a pomelo tree or on a lime tree and the composite tree will grow both types of fruit. And so, from the moment the aphids arrived in the Los Angeles area, they quickly multiplied on the trees in private yards, just as they did in Florida.

To try and control the spread of aphids and their distribution, the California Department of Agriculture and Food launched an extensive pest control operation in the Los Angeles area. However, it soon became clear that the effort was unsuccessful. A simple examination of the resident register can show what went wrong. In 2010, 40% of the three million residents of the Los Angeles area had at least one lemon tree. In other words, about 1.2 million houses had to be treated. The effectiveness of the spraying is valid between a week and a few months, then the place must be sprayed again. As of October 2012, the state managed to spray 46,941 private yards at a cost of $100 per yard. This is therefore only about 4% of all the private houses that need to be treated, and the total cost of the operation has already reached 4.7 million dollars. "It is easy to understand why such an operation quickly became impractical," says Mark. Once the state halted the spraying operation in the Los Angeles area, the Hodels could release their wasps without fear.

The couple parked their car in the parking lot of a hotel in the Azusa neighborhood, one of the Mexican working-class neighborhoods in the Los Angeles area, and took out their cooler. They decided on the site after looking at the data collected by the state on the number of aphids caught using yellow sticky tapes spread all over Los Angeles. A particularly high number of aphids were captured at this site. Kristina walked over to the Maccabean blood bush growing in the corner of the parking lot. Blood of the Maccabees (Latin Helichrysum) is a close relative of citrus. She examined its shiny green leaves and immediately noticed large groups of Asian citrus aphids in each of their life cycle stages. At the ends of the branches, near the newly hatched buds were orange-yellow eggs, wingless white nymphs, and mottled brown adult aphids moving about and around in a jumble. Three weeks earlier, the Hodel couple released several dozen wasps on some lemon trees on the other side of the parking lot. Now they wanted to see if the hornets had found their way across the parking lot, to the blood of the Maccabees bush. If indeed they crossed the field, this is proof that they began to reproduce independently in the wild.

The news was good. Although the wasps, which are several millimeters in size, are very difficult to locate, they have sown a path of death and destruction among the aphids. The wasp causes the death of the aphid by laying its eggs under the aphid's abdomen. Once the eggs hatch, the larvae, which resemble plump maggots, begin to devour the lower abdomen. They eat its contents and within a week all that is left of the aphid is a hollow shell. After a period of time, the larvae spin silk threads that cause the aphid shell to adhere tightly to the leaf and create a protected area, and then they molt. When the adult wasps hatch, they gnaw a small hole in the end of the aphid shell and go free. Mark and Christina examined some hollow aphid husks left on the blood of the Maccabees.

The wasps also destroy the aphids in another way which Mark calls "one plus one operation". The female wasp stands on top of the aphid, stabs it repeatedly with her oviposition tube and drinks its blood. "The trauma caused to the nymph from the repeated bites and drinking blood is enough to kill it," he says. By combining these killing techniques, a single wasp can destroy hundreds of aphids. Before they leave the place, the Hodel couple hang several more test tubes containing wasps on the Maccabee blood bush and open the lid. Now the wasps can start their hunting trip.

The Hodel couple estimate that as of December 2012, the wasps were able to establish themselves in about 40% of the sites where they were released and began to spread to other areas infested with aphids, sometimes up to a distance of several kilometers from the release site. However, the wasps cannot solve the problem. "It's not a magic solution," says Mark. "I estimate that if we manage to reduce the aphid population by 30%, the population pressure in the urban areas will be eased, and the rate of spread of the aphids will slow down."

the way forward

Florida's biological control attempts preceded those of California. The first shipment of parasitic wasps was released in Florida as early as 1999, and later this year millions of wasps are planned to be released from Pakistan, Vietnam and China in urban areas where the country has stopped spraying (the climate in Florida, unlike that of the central valleys of California, is more similar to the climate of Vietnam than the climate of Pakistan ).

Some of the citrus growers in Florida have chosen a different and controversial way to deal with the disease: allowing diseased trees to continue to live with the disease. As mentioned, C. Liberibacter invades the transport system of the plant and blocks the passage of sugars and other nutrients from the leaves to the roots. "When the roots suffer, they are unable to absorb the nutrients and the necessary additives from the soil and send them upwards to the leaves. This creates a double effect," says Philip Stansley, an entomologist at the Institute of Food and Agricultural Sciences at the University of Florida.

In response, many citrus growers in Florida decided to provide the trees with enriched nutrition that is sprayed directly onto the leaves. "You can compare it to AIDS," says Tim Willis, a third-generation citrus grower and director of the McKinnon Company in Winter Garden, Florida, which owns Orchards. "If it is possible to extend the life of AIDS carriers indefinitely, why should it be impossible to achieve similar results in orange trees?" Even before the aphids arrived in Florida, Willis and Murray Boyd, president of the McKinnon Company, began providing their trees with what Stansley calls "Cadillac" nutrition, a special diet containing as much manganese, zinc and boron as possible. When HLB arrived, agricultural experts advised citrus growers to uproot any infected trees. However, by the time the disease was definitely identified in Florida, it had spread so far that uprooting any infected tree would have bankrupted Willis and Boyd. "These trees took care of my livelihood all my life," says Willis, "thanks to them my son went to college. You can't just give up."

Therefore, the McKinnon company decided to further enrich the composition of the food that the trees receive, and to join together with other citrus growers to implement a regulated, organized and permanent program of chemical spraying. When Willis and Lloyd refused to remove the infected trees, the experts determined that within five years all their trees would die. But today, seven years after they started their treatment plan, the trees are still standing, and their productivity has not been harmed.

In November 2012, Willis drove his van through the orchards surveying their condition. The trees were full of ripe yellow Hamlin oranges, a type of orange that ripens towards the end of autumn or the beginning of winter. Although most of the trees were infected with Huanglongbing, only a few trees and a few leaves showed the typical symptoms of the disease: mottled leaves and unripe, green fruits accumulating on the ground. But, no one knows how long this situation can last. "Once upon a time when we planted a tree we used to think it would last many generations of people," says Willis "Today many growers think it would be good if it produced fruit for 10 or 15 years."

To date, the published studies have failed to show that an enriched diet like Boyd's provides any advantage to the trees. "You can't out-fertilize the problem," says Tim R. Gottwald, an epidemiologist at the USDA. He and several colleagues published the results of controlled experiments that showed that the enriched nutrition regime had no effect on the health of the trees, the quality of the fruits or the yield. In fact, Gottwald argues, such a regime can have a detrimental effect because it hides the symptoms of the disease and turns the trees into latent carriers that continue to spread the disease in secret, much like the first American carrier of intestinal typhoid, Mary Malone.

Other citrus growers, such as the Southern Gardens Citrus Company ("Orchards of the South"), which supplies oranges to the main distributors of orange juice in the US, chose to adopt a different strategy. Rick Kress, the company's president, says that they are replacing more than 650,000 infected trees, about a quarter of the amount of trees in their possession, with healthy trees grown in special nurseries covered with an aphid-resistant sheath. Apart from that, the company's employees are constantly looking for signs of greening. But the attempt to eradicate the disease in this way encountered many difficulties, since trees can be carriers of C. liberibacter months or even many years before the bacteria can be detected in laboratory tests or before the symptoms of the disease appear. Kress did manage to reduce the infection rate of his trees, but his production costs increased by 40% to 50%.

Scientists are desperately searching for new solutions. Some studies have shown that supplying infected orange trees with penicillin, either through the roots or by direct injection into the trunk, can help them overcome the symptoms of the disease and grow stronger roots. In 2011, Jim Graham of the University of Florida showed that copper sulfate, a substance known to kill bacteria, produces similar results. Apparently, the US Environmental Protection Agency (EPA) will approve the use of copper sulfate more readily than the use of penicillin because copper sulfate is not used for human treatment. The problem with such an approach is that the preventive treatment will have to be continued throughout the life of the tree.

The long-term solution probably lies in genetic engineering. Eric Mirkov, a plant pathologist at Florida State University AM (Agriculture and Mechanics), inserted two spinach genes into citrus trees and made them resistant to huanglongbing. Researchers at Cornell University are developing a species of citrus that will repel Asian citrus aphids. It is understood that it will be possible to combine these two technologies. Both projects are financed by the Southern Orchards Company, which has already spent six million dollars on research aimed at stopping this citrus disease. But genetically modified plants have to go through a long bureaucratic process before they get the necessary approvals to start and use them commercially in the field. At the same time, there is still a long way to go before the general public agrees to accept them. Many fear that by the time this happens, the US citrus industry will disappear. Halbert says: "We need to find a solution that we haven't thought of yet."

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in brief

Asian citrus aphid, An insect the size of a mosquito spreads a deadly disease in citrus groves across America. As of today, efforts to stop the spread of the disease, known as Huanglongbing, have failed and it poses an existential threat to the US citrus industry.

Huanglongbing It is caused by a bacterium carried in the aphid's salivary glands. The bacterium penetrates the transport system of the tree and causes it to be blocked and damage the ability to transfer nutrients between the leaves and roots.

Scientists have tried Different approaches to deal with the disease. One of them is the import of wasps from Asia that serve as natural enemies of the aphids. The long-term solution probably lies in the genetic engineering of the trees, but there is still a long and expensive way to go until this solution is approved and accepted by the general public.

And more on the subject

Oranges. John McPhee. Farrar, Straus and Giroux, 1967.

Current Epidemiological Understanding of Citrus Huanglongbing. Tim R. Gottwald in Annual Review of Phytopathology, Vol. 48, pages 119-139; September 2010.

Photo and video slideshow of Kristina and Mark Hodel releasing parasitic wasps

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