In the greenhouse experiments, Duan and his ARS colleagues Michele Hoffman, Melissa Doud, David Hall, and Ed Stover, along with FDACS scientists, exposed 30 HLB-infected citrus seedlings to different levels of heat in growth chambers for periods ranging from 2 days to 10 days. The seedlings were about 2.5 years old, about 2 feet in height, and were growing in 1-gallon containers.

They were divided into three groups and heated to 104 ̊F, 107 ̊F, or 113 ̊F. Fluorescent lamps provided light for 12-hour “days” and were turned off for 12-hour “nights.”

For comparison, the researchers also applied the heat treatments to citrus budwood and periwinkle, which is also susceptible to HLB. They used PCR technology, which amplified the pathogen’s DNA, to measure the HLB pathogen levels in the trees, budwood, and periwinkle. Infection levels were measured a week before heat treatments began and again 30, 60, and 270 days after they ended.

The researchers quickly learned that constant exposure to temperatures of 113 ̊F or higher would defoliate citrus seedlings. But if they interrupted the steady onslaught of intense heat by dropping temperatures down to about 80 ̊F for at least 5 hours each day, the leaves stayed alive.

Results from the greenhouse tests, published in the journal Phytopathology, also showed that exposing citrus seedlings to a minimum of 48 hours of temperatures of 104 ̊F to 107 ̊F significantly reduced, and often eliminated, HLB infection. All of the heat treatments were equally effective, regardless of temperatures and exposure times.

The researchers continued to test the seedlings, and after 2 years of heat treatments, they have remained free of HLB.

“This application would be useful for nurseries and greenhouses and rescue of germplasm that’s been infected,” Duan says.

By contrast, exposing newly grafted budwood to alternating temperatures of 102 ̊F and 86 ̊F for up to 4 months did not reliably cure it. The heat controlled the HLB in periwinkle, but the periwinkle plants were more heat tolerant than the citrus trees and required more prolonged heat for infection levels to be reduced.

Another approach to controlling HLB damage may have wider implications by prolonging the productivity of full-grown citrus-producing trees.

Florida citrus grower David McKenzie, working with Duan, has been encasing citrus trees in opaque, plastic PVC “tents” to heat them up for about a week, then removing the tents and trimming off the top 10 or 12 inches of the trees that have been “browned up” by the solar heat.