How a Vaccine Could Protect Florida's Orange Trees

A disease called "citrus greening" has devastated and permanently altered citrus production in the United States, but a vaccine that could protect orange trees may be part of a winning strategy to beat the bacteria that is killing the trees.

Citrus greening, also known as Huanglongbing (HLB), was discovered in Florida in 2005. The disease is caused by a bacterium by the name of "Candidatus Liberibacter asiaticus" and has already attacked citrus groves in at least 40 countries around the world.

The bacteria infects the tree, consuming nutrients and sugars used by the tree to sustain itself and bear fruit. It causes the leaves of affected trees to become blotchy, and shoots and leaves begin to yellow and eventually die. Fruit from infected trees are misshapen and remain green, which makes citrus greening an accurate name for the disease. The fruit — and juice made from it — are bitter.

How a Vaccine Could Protect Florida's Orange Trees — HLB on a sweet orange. Note the yellowing of leaves. *Image via J.M. Bove, INRA Centre Des Recherches*

The bacteria, and the small insect that spreads it, have impacted more than 90% of citrus acreage in Florida, in both commercial and residential properties.

In Florida, citrus production has declined significantly for three years in a row: Orange production dropped from about 133.6 billion boxes in 2012–2013 season, to about 81.7 billion in the 2015–2016 season, and a predicted low of 67 billion boxes in the 2017 season.

Florida commercial citrus growers lost a net of 21,275 acres of citrus trees in 2016, an uptick from 2015 (13,752 net change in citrus acreage) and the highest loss since 2008. Abandoned citrus groves worsen the problem by allowing the psyllids and the bacteria to grow unchecked.

Louisiana, Georgia, Texas, Hawaii, and California have also experienced citrus greening, though it hasn't had as significant of an effect there yet. As the bacteria spreads, it could get worse. The impact of HLB in California could have significant economic consequences. In what could be a harbinger of spreading infection, a neighborhood citrus tree in Los Angeles County was removed after being identified as infected with citrus greening in January due to worries its infection could spread.

Researchers have discovered three strains of HLB bacteria that harm citrus trees in the US. The bug Diaphorina citri, also called the Asian citrus psyllid, spreads these bacteria from tree to tree. As the psyllid spreads, so does citrus greening disease.

Growers reduce spread by treating infected trees with insecticides and they avoid planting related-host trees, among other measures.

Even if precautions are taken against the bacteria on commercial and personal properties, the continued spread of abandoned trees can bring the bug back. More than 130,000 acres of citrus trees sat abandoned in Florida in 2016, acting as a breeding ground for the psyllid and citrus greening bacteria.

As they would with a serious disease that infects humans, scientists are developing tools and techniques to battle the bacterium and the bug that carries it.

Significant work continues toward developing trees resistant to citrus greening. The Florida Citrus Research and Education Center developed an HLB-resistant mandarin orange tree through the traditional crossing of cultivars to yield a resistant tree. The center has already received 150,000 orders from commercial growers.

Earlier, the center announced the development of a genetically-modified tree that shows resistance to HLB, and possibly other hard-to-treat diseases like citrus canker. While commercial development of the tree is still years away, others question the viability of an orange juice industry based on genetically-modified trees.

As researchers explore methods to deter the psyllid, others work on countermeasures to the bacteria itself. Plant pathologist Bill Dawson has been developing a modified a virus to vaccinate the citrus trees against the Candidatus bacteria that causes citrus greening. In other words, he created what could be a vaccine for citrus trees.

It wasn't just Dawson's idea to create the vaccine, though: Two research paths merged to create a potential "arboreal (or tree) vaccine" for citrus greening. As plant scientist Dawson was developing a method for a tree to produce its own virus vector, a large commercial juice producer, Southern Gardens, was searching for plant genes to make trees immune to HLB.

A vector within a tree would allow the plant to develop its own proteins to fight infection. According to a recent article in WIRED, Dawson modified a harmless virus that could be used to help trees protect themselves. At the same time, Southern Gardens discovered two genes from the spinach plant that are resistant to citrus greening. Combined with the virus produced by Dawson, the spinach genes create "bacteria-killing spinach proteins" that are effective against citrus greening.

With a vaccine in hand, researchers only needed to develop a way to get the viruses into the citrus trees. This was accomplished by grafting a plant that had been treated with the viral vector onto a citrus tree. The viruses spread from the graft to the rest of the tree, delivering the vaccine without genetically modifying the tree itself. Tim Eyrich, the vice president of research at Southern Gardens, said they're "attacking the bacteria where it lives," and that they "can do it without changing the biology of the tree at all."

Switching gears from developing transgenic trees, Southern Gardens is working with the US Department of Agriculture on a process to begin testing the vaccine on a large scale. A notice to prepare an environmental impact statement on the use of the virus and the vaccine was filed in April. If given approval, the vaccine might be in use by 2020.

If you like reasonably priced orange juice, or fresh citrus fruits, this one's for you — and for those who work and make their living from producing citrus products across the United States.