HLB Cause and Control Explained

University of Florida Institute of Food and Agricultural Sciences researcher Nian Wang reported in a July 20 webinar that HLB is a pathogen-triggered immune disease. After explaining the sequence in which the disease attacks trees, he suggested ways growers can use that knowledge to combat HLB. Wang is a microbiologist and cell scientist at the Citrus Research and Education Center.

Wang said Candidatus Liberibacter asiaticus (CLas), the causal HLB bacterium, triggers systemic and chronic immune response in citrus trees’ phloem tissue. HLB symptoms are caused by systemic cell death of phloem tissues. That response is instigated primarily through excessive and chronic reactive oxygen species (ROS) production, he said. According to Wang, CLas leads to ROS, which leads to cell death of phloem tissue, which leads to HLB symptoms or damage to the tree.

MANAGEMENT METHODS
A take-home message from the presentation suggested that reducing ROS damages caused by HLB, as well as promoting tolerance to ROS, can increase plant growth and productivity. Wang said the following methods can reduce ROS damages or increase tolerance to ROS damages:

• Use balanced nutrition. Nutrient deficiency causes more ROS production.
• Application of the micronutrients boron, iron, zinc, molybdenum and nickel (not to be mixed with copper) increase the activity of antioxidant enzyme activities and promote plant growth.
• Use of gibberellic acid protects cells against ROS damages, inhibits ROS production, promotes plant growth hormone and phloem cell regeneration, and reverses ROS-induced plant growth inhibition.

The researcher also discussed factors that increase ROS damages in addition to damages caused by CLas. These factors include heat stress, salinity stress and drought.

HOPE FOR CRISPR TECHNOLOGY
“We have successfully developed non-transgenic CRISPR genome-editing technology as ready for citrus,” Wang reported. He said HLB-tolerant/resistant varieties are “not quite ready but getting there.” However, he added, “We have generated non-transgenic canker-resistant sweet orange using the CRISPR technology.”

Multi-county citrus Extension agent Mongi Zekri hosted Wang’s virtual presentation.

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