Abstract
Huanglongbing (HLB or citrus greening) is a disease caused by an insect-transmitted bacterial pathogen Candidatus Liberibacter asiaticus (CLas). Thermotherapy has been successfully used by others to reduce the population of CLas bacteria in HLB-affected citrus trees under greenhouse studies. Thermotherapy is the application of heat as a strategy to reduce the adverse economic impact of certain pests and diseases. CLas is a fastidious, non-cultivable organism. The high variance in CLas titers in canopy samples together with this lack of cultivability makes it impossible to use classical bacteriological techniques to measure the viability either before or after treatments. Therefore, we used the survival rates of a surrogate bacterium, Klebsiella oxytoca, in order to evaluate the effectiveness of a mobile thermotherapy delivery system developed for in-field treatment of HLB-affected trees. K. oxytoca is a Gram-negative, rod-shaped bacterium that was originally isolated from soil and has been used in the development of industrial applications related to ethanol fuel production. It served as a biologically-based sensor of temperature stress (biosensor) in this study. Thermocouples and biosensor packets (plastic cups with suspended small snap-top tubes) containing the K. oxytoca were attached to an HLB-affected citrus tree and their canopy locations mapped. The mobile thermotherapy treatment hood covered the canopy of the HLB-affected tree. Then, steam and hot water were injected through nozzles inside of the hood to increase the temperature of the tree canopy. A standard temperature–time combination of 54 °C for 90 s was chosen based on preliminary studies where heat treatment parameters caused a significant reduction in CLas populations without inflicting permanent damage to the tree. The survival ratio of the K. oxytoca in the biosensor packets was found to range from complete elimination to 5% with treatments of 250 s and a maximum temperature of 54 °C.
Highlights
Effective plant disease control is critical to the reliable production of food and could potentially result in substantial reductions in agricultural use of water, land, fuel, and other inputs
Candidatus Liberibacter asiaticus (CLas) titers can be severely reduced or eliminated by exposure to elevated temperatures without inflicting lasting harm on the plant, we have developed a mobile thermotherapy system based on steam and hot water application for field evaluation and use [26,27]
A mobile heat treatment system was evaluated in the field by using a biologically-based sensor
Summary
Effective plant disease control is critical to the reliable production of food and could potentially result in substantial reductions in agricultural use of water, land, fuel, and other inputs. There is an effort in modern agricultural practices to reduce the use of chemicals as nutritional supplements or in pathogen control, especially for organic products. Thermotherapy approaches utilize either dry or wet heat, including the application of steam, hot water, or both a combination of them. The challenge in the thermotherapy approach is finding the best combination of time and temperature that maximizes reduction in pathogen survival while minimizing the damaging effect to the plant [1,2]. An underlying assumption justifying the thermotherapy approach to pathogen control is that the target pathogen is more sensitive to high temperature stress than the plant
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