Beneficial horticultural responses from the application of solar thermotherapy to mature Huanglongbing-affected citrus trees

Abstract

Abstract The detrimental effects of Huanglongbing (HLB) on citrus are well known and so is the need for effective methods to combat this disease. Solar thermotherapy (ST), one of the management methods to help alleviate some of the negative effects of HLB, is the process of heating trees to therapeutic temperatures by encompassing the tree within a plastic structure (Visqueen) to harness the sun s natural energy. ST was applied to mature ‘Valencia’ citrus trees in three locations. Tree vigor, yield, and Candidatus Liberibacter asiaticus (Las) titer in leaves were monitored for two consecutive seasons post ST treatment, while fruit and juice quality were evaluated at the end of the second season. ST promoted an increase in canopy density in most groves tested and did not have a significant effect on fruit quantity, despite the prolonged exposure of the trees to increased temperatures. Moreover, Las titer was reduced in both leaves and juice processed from affected trees post ST. With respect to fruit quality, the ratio of total soluble solids to titratable acidity was higher post ST compared to controls as was the sucrose content of the juice in most treated groves, whereas limonin was higher in juice from control trees. Principal components analysis of aroma volatiles showed significant differences between juice from treated versus control trees with volatiles imparting top-note and freshness to orange juice, such as acetaldehyde, hexanal, Z-3-hexenol and linalool, being higher in juice from treated trees. Taste panels confirmed that flavor differences existed, ultimately showing that ST resulted in improved juice flavor in well-managed groves. Although the effects of ST on the trees appear temporary, such outcomes emphasize its utility when used as part of an integrated management strategy for HLB-affected citrus, imparting beneficial horticultural responses with minimal to positive effects on subsequent juice flavor.