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Miracle cure or wishful thinking? Proposed treatment for citrus greening gets mixed reviews from growers

 Citrus fruit hangs on one of Ben Bateman's trees. (Courtesy of Ben Bateman)
Ben Bateman
Citrus fruit hangs on one of Ben Bateman's trees.

Research using oak leaves on citrus trees to combat citrus greening has been fruitful so far. Some growers say it’s been a lifesaver, but others say it’s not enough.

When Ben Bateman first started growing citrus, he did it to remember his childhood. Bateman grew up in Brandon, Florida, and he said his neighborhood back then was covered in citrus trees.

“You could go anywhere in Brandon and be walking around, and there'd be a citrus tree, like, hanging over the sidewalk,” he said. “And if you were hungry, you could just grab some fruit because it was everywhere and it was falling on the ground.”

He would go running and grab tangelos and oranges when he got hungry. But his favorite thing was the smell.

“It seemed like the entire town smelled like orange blossoms,” he said. “I think it's one of the most amazing flowers in the world.”

 A cut citrus fruit. (Courtesy of Ben Bateman)
Ben Bateman
A cut citrus fruit.

As he grew older, more and more of his hometown started being developed into neighborhoods, knocking down the old citrus trees. Bateman, now 44, considers himself a “recreational citrus grower.” He started a small grove with a few dozen citrus trees to bring back the childhood memories.

But in the mid-2000s, a blight swept over the state.

Huanglongbing, more commonly known as citrus greening, is a bacterial disease that infects citrus trees. It’s spread by an insect called the Asian citrus psyllid.

Lorenzo Rossi is an assistant professor in the University of Florida Horticultural Sciences department. He explained that when an infected psyllid bites a citrus leaf, the tree forms starchy calluses to block the bacteria from spreading, but in doing so it also blocks the leaves and roots from receiving vital nutrients.

“Instead of actually killing the bacteria, the plant is killing itself,” Rossi said.

Rossi began his work with UF studying citrus greening and how soil health affects tree health. He said farmers noticed citrus trees planted under big oak trees were doing better at fighting off greening than those that weren’t. Rossi began to speculate on why that is.

In 2020, Rossi and his team published a paper in the Journal of Plant Physiology and Biochemistry. They purposely infected a few citrus trees with greening, then sprayed a “tea” made with fresh oak leaves and water on the citrus leaves and roots. This was designed to mimic the effect of a citrus tree standing under an oak canopy in the rain.

“When it rains, these compounds kind of get released from the leaves and go both into the canopy of the citrus trees and to the soil. And so it's basically a natural spray and drench application of this compound from the leaves,” he said.

 Citrus trees line a field in Lorenzo Rossi's oak leaf extract experiment. (Courtesy of Lorenzo Rossi).
Lorenzo Rossi
Citrus trees line a field in Lorenzo Rossi's oak leaf extract experiment.

Compared to the control group, they found the tea significantly improved a tree’s chances of surviving greening disease. The treatment reduced the yellowing of the leaves caused by callus buildup and “significantly increased root nutrient uptake.”

Bateman said he stumbled upon a similar solution by accident.

“I was having no luck with the trees down here in Brandon, and they were just declining and declining and declining,” he said.

He noticed his neighbor was cutting off some large laurel oak branches from a tree in their yard and making mulch. Normally, farmers don’t recommend putting mulch on citrus trees as they need lots of drainage. But Bateman was desperate.

“I thought, ‘You know what, I'm just going to build the soil.’ And if the trees survive great, if they die, that's the direction they're headed,” he said.

As the mulch began to break down, his trees started doing better.

“I noticed the trees flushed out in the spring, they looked a lot better. There was a lot of mushrooms growing in the little grove that I had set up, all sorts of different colors and sizes, they were popping up everywhere,” he said.

Oak mulch covers the base of a citrus tree in Ben Bateman's grove. (Courtesy of Ben Bateman)
Ben Bateman
Oak mulch covers the base of a citrus tree in Ben Bateman's grove.

But not all farmers had the same experience.

Steve Crump is the owner and manager of Vo-LaSalle Farms in DeLand, and a fourth-generation citrus grower. He said as greening began to take hold, it devastated the industry.

“My personal production in our orange groves is down 80% from 15 years ago when greening started. Quality of the fruit's down. It's really sad and depressing and discouraging,” he said.

According to Crump, 100% of the trees in his grove are infected, and he’s tried everything. Currently, he’s experimenting with growing trees inside massive screen houses and injecting them with a bactericide called oxytetracycline. The houses, he said, are effective but also expensive, and they don’t hold up well during hurricanes. The injections are a relatively new development, so it’s unclear how long the effects will last before the citrus greening bacteria becomes resistant to it.

Crump tried oak mulch a few years ago but said it didn’t work.

“Everyone who's observed it is right to say [citrus] trees do better growing under oak trees. And they do better, they do even better growing in an oak hammock. I agree with them 100%,” he said. “But I think it has more to do with soil organic matter, soil fertility and water.”

He said he doesn’t think there’s anything specific about oak trees that helps the citrus trees. Rossi says otherwise.

His study focused specifically on the tea made of oak leaves and water, while Bateman and Crump only used oak tree mulch. Rossi said his team has also been looking into the use of mulch to combat greening, but the results are inconclusive.

“It was extremely beneficial for the soil. It didn't translate in a response of the trees yet. So we are still collecting data,” he said.

Rossi is currently working in conjunction with the U.S. Department of Agriculture trying to isolate what it was about the oak leaves that had a positive effect.

“We don't know if it's just one compound, if it's multiple compounds or if it's a pool of compounds that only work in a specific solution when they are with oak,” he said.

 Rows of citrus trees line Lorenzo Rossi's experiment. (Courtesy of Lorenzo Rossi)
Lorenzo Rossi
Rows of citrus trees line Lorenzo Rossi's experiment.

Rossi’s 2020 study also had some limitations. It took place in a greenhouse, an enclosed environment, so it was unclear if it would work as well in the groves. Additionally, he said this solution requires a near-constant supply of fresh oak leaves, which if applied on a large scale would affect the oak trees in the state.

“We cannot tell the producers in the state to go and cut down oak leaves and make teas,” he said. “Now the next question is how can this be translated to the bigger picture? How producers, growers can benefit, can utilize this discovery in a way that is sustainable?”

Bateman said he has always been “pessimistic” about whether Florida can return to the booming citrus haven of his childhood.

“But our food has to come from somewhere. So, you know, I'm hoping that there'll be some people that will kind of carry on the tradition,” he said.

Crump is more hopeful that Florida citrus will bounce back.

“I'm also really holding on for a real solution, an orange tree that's resistant to greening,” he said. “And I'll be 58 next month. If we can get a resistant tree before my career ends, then I can plant it for the next generation.”
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