About 15 years ago, Rex Clonts got out of the vegetable growing business after many years to “retire to the easy life” of a citrus grower. But, the onslaught of HLB has turned the easy life on its ear and requires management more akin to row crops these days.
Years as a vegetable grower taught Clonts mechanical skills, which he hopes will serve him well trying to grow citrus in the presence of HLB. He is putting those skills to work to create a viable and systematic approach to utilize thermotherapy for treating HLB-infected trees in his grove — literally heating HLB-infected trees up to 130°F.
Dr. Yongping Duan, a researcher at Ft. Pierce’s USDA Horticultural Lab, is among those studying thermotherapy as a means of killing the HLB bacteria in citrus trees.
“I went down to visit Dr. Duan to learn about his research and then went back to my experience in making machinery that we used to grow and harvest vegetables,” says Clonts. “There are growers out there trying heat therapy, but nobody is trying to design a system that can be used industry wide on all-sized trees.”
Clonts says his goal is to build a system capable of treating 100 of his acres per summer with a crew of no more than four people. The setup would include a string of tent structures capable of treating an eighth of a mile (600 to 650 feet depending on grove layout). In Clonts’ grove, that would take 26 modules covering 52 trees.
“If this is ever going to be adopted by the industry, it must be simple and systematic,” says Clonts. “It needs to be predictable enough that when the four-man crew shows up in the morning, they know exactly what their duties are for the day and can go perform them efficiently. And, everything I am using to build these tents is commercially available materials sold for greenhouse production.
“Using this very systematic approach, you would operate enough of these units to treat a third of your acres per year. The theory is the heat treatments provide positive benefits to the tree for two to three years. If you are treating a third of your acres per year, by the fourth year, you would be starting back over as the benefits begin to wear off.”
Another key to the system is the ability to treat large trees, which require much larger tents than what many growers have seen in action to date with smaller trees. Clonts has built two prototypes and a third is planned, which will be slightly smaller and easier to manage. Clonts gable-cut tops his trees at 12 to 14 feet, so the tents must be sized large enough to accommodate the height.
Time And Temperature
Clonts began actively treating his grove in June. The goal is to capture solar energy in the tents and maintain temperatures at 105°F to 130°F, which research shows will kill the HLB bacteria.
“Right now our rule of thumb is we keep the structure over the trees for three days, seeking 10 hours per day of temperatures more than 105°F,” says Clonts. “One of the big things we’ve learned is the heating is very different over these mature trees than if you just put the tent out over a grass field.”
The canopy and moist soil surface have a moderating effect on the temperatures inside the tent, making air circulation critical to the process. Clonts fixed this challenge by installing circulating fans in the tops of the tents to move air.
“In these big structures, if you don’t move the air with the fans, you are going to only heat the top of the tree and possibly damage it, and the bottom canopy is never going to get heated,” he says. “We are finding that one fan per tree in the structure takes care of this problem.”
To ensure temperatures are monitored correctly, three standard digital thermometers are placed inside the tent at two feet, six feet, and 12 feet in the canopy. When temperatures get too hot, a thermostat in the structure engages an automatic venting system.
“The automatic venting system is counter-intuitive because we have a motor that keeps the vents sealed until we hit the critical temperature of 130°F,” Clonts says. “When it hits the temperature, the motor shuts off and the vents open. We do this because we want the tents to self-vent if we lose power. When the temperature drops back to 125°F, the motor starts back up and closes the vent.”