Florida's citrus industry is exploring a new tool to combat greening: exposing infected trees to high temperatures.
Thermotherapy mimics for plants the human body's fever response. In this case, it kills or suppresses the bacteria that causes greening, also known as huanglongbing or HLB, says Reza Ehsani, associate professor of agricultural and biological engineering at the University of Florida's Citrus Research and Education Center in Lake Alfred.
The bacteria are susceptible to heat, and populations decrease significantly over time starting at about 107 degrees, Ehsani says. Citrus trees can handle higher temperatures for short periods, although thermotherapy may cause some leaf drop and sunburn damage.
Researchers and growers are experimenting with different approaches to reach temperatures between 115 and 140 degrees, including tents that focus solar heat over a few days and short steam blasts from an external source.
Optimum time, temperatures
Determining the optimum time and temperature for treatment is a key goal, but in general lower temperatures require longer exposure.
While thermotherapy produces promising results in infected trees—a healthier appearance and a halt, at least temporarily, to declines—it's not a cure.
"You can't compare it to an uninfected tree," says Yongping Duan, research plant pathologist at the U.S. Agriculture Department's Agricultural Research Service lab in Fort Pierce, Fla. "But compared to an untreated infected tree, it's in better condition for at least two years."
Duan also sees hints that thermotherapy may reduce fruit drop, but says a definite answer to that question requires more data.
Heat treatment tackles HLB bacteria in the upper part of the tree, but a reservoir may remain in the roots, he says.
Ehsani hopes to extend steam treatment to the roots as a way to increase overall effectiveness.
But the growth flush that trees produce after any form of thermotherapy will attract Asian citrus psyllids that vector the disease, Ehsani says. Maintaining a strong psyllid control program is crucial to preventing reinfections.
That vigorous growth response means you can't slack off on nutrition, either, Duan says.
Weeds and grass within the tented area also may flourish, calling for extra attention afterward, says Parker Platts, University of Florida's fruit crops Extension agent for Indian River and St. Lucie counties.
The earlier, the better
Thermotherapy produces better results at early stages of infection, Duan says.
"It's not a miracle," says David McKenzie, grove manager at Edsall Groves Inc. in Vero Beach, Fla. "It may not work if the tree is too far gone."
So far experiments have focused mainly on young trees small enough to fit easily into the structures. Depending on your operation, that may be a significant hurdle.
"If we're going to stay in business, we have to get the 2- to 4-year-old trees into the productive zone," McKenzie says. "I'm seeing 1 to 4 percent of my trees showing signs (of HLB) even with everything I'm doing. I can't turn my back on them. I've got too much money invested."
McKenzie has used thermotherapy with solar tents since 2011, starting with 40 homemade tents. The low cost—$2 to $3 per tree—makes the approach "a no-brainer," he says.
"It's the only way we're keeping these trees in the game," McKenzie says. Without it, "within a year you'd be taking them out anyway."
A focus on mature trees
But Rex Clonts, president of Clonts Groves Inc. in Oviedo, Fla., says focusing on young trees isn't the answer for his operation, where mature trees dominate.
"If we can't get a good positive response to therapy on big trees, I don't know how heat therapy is going to have much of a future," he says.
Clonts is experimenting with modules large enough to cover two 14-foot trees and that incorporate fans to maintain a uniform temperature. The power source for the fans also controls vents to hold a programmed temperature range, he says.
If needed, the module can stay in place unattended for a longer period without burning the tree.
His prototype can be linked in a string of up to 25, allowing users to cover 50 or more trees at once, after which they can be unlinked and moved to a new row, he says.
They add flexibility to cover all size trees, but they're harder to move than the smaller solar tents now in use. Clonts is in the early stages of testing, and won't have results -- including how they affect production and returns—until next year, after a full season.
Ultimately, combining larger structures like his with steam heat may turn out to be the most efficient solution, he says.
Efficiency is vital
Only further experiments and additional data will tell. Efficiency is vital.
"We're trying to cover vast areas and many trees," says Daniel Scott, vice president of Scott Citrus Management.
Fine-tuning time and temperature guidelines means growers won't waste time tenting trees twice as long as needed, for instance. But adding more automation such as an external heat source "would be a huge step forward," Scott says.
Steam treatment provides that, if field trials can match results from solar tents, he says. Steam would allow treating trees even at night or during winter's shorter days—and produce more uniform heat distribution.
Data loggers in his solar tents show that the upper sections tend to run 10 to 15 degrees hotter than the lower sections, Scott says. Without extra attention to vents, users risk damage to higher branches in order to get the desired effect in the lower half.
A simple tent
Tents have an advantage in simplicity. "They're cheap and something you can do today," Platts says.
He recommends using 3/4-inch PVC pipe and a high-grade greenhouse plastic to construct tents that hold up to typical conditions, at a cost of less than $50. Thinner plastic, such as Visqueen, isn't durable enough.
Use an opaque plastic for the top piece to block UV light and reduce burning, McKenzie says. He also suggests poking holes in the top not only for ventilation but also to keep rain from pooling.
"Wind is your enemy," Platts says. Cinder blocks are ideal for weighing down tent edges.
Once built, tent costs are limited to labor to move and maintain them, while steam also needs a generator and fuel in addition to a containment structure. That's still less than the cost to remove dead trees, replant and absorb lost production, Ehsani says.
But solar tents are also more dependent on weather conditions. "The heat that we get in April may be different than the heat we get in June or August," McKenzie says.
The longer you leave trees tented, the better, he says. But you also have to balance that with practical considerations: How many tents do you have, how many trees need treatment and what other tasks will tenting conflict with.
Duan encourages growers to experiment with thermotherapy and share their results as a way for the industry to keep going until researchers develop a lasting solution to greening.
Heat treatment is "a tool to get those trees another year or two down the road," Scott says.