click image to zoomReza Ehsani, University of FloridaThe trees on the left were not treated. The trees on the right received thermotherapy, or heat treatments.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.
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