Location: Subtropical Plant Pathology ResearchTitle: Development of the Hydro-Solar Thermotherapy (HSTT) for treatment of HLB infected citrus.
Submitted to: International Research Conference on Huanglongbing
Publication Type: Abstract Only
Publication Acceptance Date: 12/20/2016
Publication Date: 5/18/2017
Citation: Gottwald, T.R., Poole, G.H., Taylor, E.L., Kainz, J. 2017. Alternative Method of Thermotherapy Application in Citrus. International Research Conference on Huanglongbing. 4(1):1/45.
Interpretive Summary: In this project, we test a different system of thermotherapy, by using a hot water jacket that wraps around a tree to increase the temperature within the tree. The preliminary tests show that we can significantly increase the temperature in an infected citrus tree. Additional studies have shown that there is a maximum temperature a citrus tree can withstand, but it is unclear if this temperature will be able to adversely affect the bacterial infection.
Technical Abstract: The use of thermotherapy to treat HLB-infected citrus trees has been demonstrated by various systems over the past few years, which basically fall into two primary categories – passive solar systems (tents), and active heated water/steam spray systems. The system envisioned here is a combination of the two – a solar-heated water system, that is active but requires no power or user input, or expendable materials that need to be replenished. Circulating heated water is used to transfer heat to the tree by means of a thermal insolated jacket placed around the trees trunk, or on larger trees, around main scaffold branches. Multiple trees can be linked to the water source in series or in parallel subunits. Preliminary tests to determine the maximum temperature achievable along with feasibility of operation were done with a solar heating system, similar to the envisioned final design. Water temperatures in excess of 60C can be generated, and under bark temperatures reached the low to mid 40’s. A pulsed system has also been tested which runs for only 5 hrs and produced higher temperatures at the trunk, but not in excess of 45C. Once the proof of concept was completed, tests beds were designed to determine the optimum temperature of operation. The first test bed was basic and only provided heated water at a constant temperature, for extended periods of time. Studies showed that the maximum temperature that a diseased tree could tolerate was 48-49C over 48hrs. The second test bed, known as the Mobile Thermotherapy Lab, or MTL, is more versatile and designed to provide heating and cooling of water to simulate what might be achieved through solar heating and supplemental cooling with a 40C differential in temperature. The goal of the MTL is to look more in depth at the parameters, and to test the effect of sequential heating and cooling on CLas bacteria survival in the phloem. Results indicate that a passive solar water heating system can produce higher temperatures under the bark of the trunk, as well as increased temperatures into the canopy. This heat generated is below the thermal tolerance level of the tree, but also may not be hot enough to significantly decrease the bacterial population in situ as determined by pre- and post-PCR assay. We will discuss the question, ‘If thermotherapy does not decrease bacterial populations, why do treated trees display disease remission for multiple months?”. Temperature of 49C would be the maximum usable temperature due to the thermal tolerance of citrus trees, however optimal duration at that temperature has not been determined.