New Research Shows How HLB Damages Trees

Josh McGill HLB Management, Research

Recent research describes in detail how HLB causes damage to citrus trees and presents the case that HLB is a pathogen-triggered immune disease. A pathogen-triggered immune disease results from the activation of an organism’s immune cells fighting a pathogen. Nian Wang, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) professor, took part in the research.

This is the first time that this explanation of HLB symptoms as pathogen-triggered immune responses has been presented and defended. Pathogen-triggered immune diseases have not been reported in the world of plants, but they are common in humans.

A: Systemic and chronic reactive oxygen species (ROS) production triggered by Candidatus Liberibacter asiaticus kills citrus cells (such as phloem cells), which can be mitigated by gibberellic acid (GA) and antioxidants. Green spots indicate live citrus protoplast cells. B: HLB-positive Valencia trees were treated with GA and an antioxidant (uric acid) via foliar spray weekly. Both GA and the antioxidant reduced ROS levels and HLB symptoms.
(Photos courtesy of Nian Wang)

With this new evidence that supports a premise that HLB is an immune-mediated disease, researchers can see new light on how to manage HLB.

Wang has shown that HLB infection stimulates systemic and chronic immune responses in phloem tissue, especially overproduction of reactive oxygen species (ROS), which are part of the plant’s immune response. Chronic and excessive ROS production is responsible for systemic cell death of phloem tissues, which in turn causes HLB symptoms. This supports the hypothesis that HLB is an immune-mediated disease.

Antioxidants and immunoregulators are commonly used to treat human immune-mediated diseases. They halt or reduce the process that results in cell death. In citrus, the researchers tested whether growth hormones like gibberellic acid (GA) and antioxidants (uric acid and rutin) could impact cell death triggered by the infection and, therefore, block or reduce HLB symptoms. 

The GA and uric acid had an encouraging positive impact on infected trees. This has also been supported by other research in process at the UF/IFAS Citrus Research and Education Center.

Wang’s research also tested existing grove management practices that integrate antioxidants, micronutrients, gibberellins and optimized fertilization and irrigation that are available in Florida. All the products tested, except the antioxidants, have already been registered for citrus production. Thus, citrus growers can easily adopt them. 

Read more about the research, published in Nature Communications, here.

Source: University of Florida Institute of Food and Agricultural Sciences

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