Florida citrus growers who face the most severe citrus disease in history notice how citrus trees under oak tree hammocks appear to tolerate the disease. Lukas Hallman believes oak trees may hold a compound that boosts the citrus trees’ ability to tolerate the disease.
Hallman is a graduate student at the University of Florida Institute of Food and Agricultural Sciences Indian River Research and Education Center (UF/IFAS IRREC) in Fort Pierce, in the heart of the world’s premier grapefruit production region. The disease, Huanglongbing, or HLB, is caused by a bacterium and vectored by the invasive insect, the Asian citrus psyllid. Once infected, trees have a reduced fine root mass, yellowing of leaves, and smaller and bitter tasting fruit. In the U.S., the disease’s common name is citrus greening, said Hallman.
“Anecdotal reports from Florida growers claim that citrus trees growing within the drip line of large oak trees have minimal HLB symptoms, while trees nearby, but not under the oak drip line, show severe symptoms,” said Hallman.
Oak extract was medicinal during American Civil War
In his literature review of scientific journal articles, Hallman found that compounds from white oak tree bark were used as antimicrobials during the American Civil War. Marco Pitino, a former UF postdoctoral researcher, published the first research study for oak tree extract used in the greenhouse against the bacterium. The work took place at IRREC. Pitino found oak extract would improve citrus trees’ ability to tolerate HLB in the greenhouse.
“The literature review and Pitino’s greenhouse study were enough to form a viable hypothesis for a field study with oak mulch beds under citrus trees,” Hallman said. “Pitino’s work took place in a greenhouse. His findings need field tests, and we need an answer to help the local industry, which has seen their crops drop by 90% in the last 15 years,” said Hallman.
Lorenzo Rossi, assistant professor of plant root biology at IRREC, is Hallman’s graduate research advisor. Rossi persuaded Hallman to apply for a Southern Sustainable Agriculture Research and Education (SSARE) graduate student grant to fund research of his hypothesis.
Southern Sustainable Agriculture Research and Education Grant
When Hallman began to write the SSARE grant, a large oak tree fell on the IRREC property in a 2019 hurricane. With funds from a UF/IFAS Horticultural Sciences Department A.H. Krezdorn Memorial Fund, and collaboration with Robert Shatters, a research molecular biologist with the U.S. Department of Agriculture, Rossi and Hallman prepared the tree for mulch. The researchers used it for a bed under citrus trees in a research grove. With the research infrastructure in place, Hallman began to take monthly data from the root rhizosphere under the oak-mulched citrus tree beds.
Rossi said Hallman’s grant application was successful, and in the fall of 2020, he began work to fulfill the project’s objectives. The SSARE grant provides more than $12,000 for the 2-year study to determine if oak mulch will suppress citrus greening in the open field.
“The oak mulch is easier to apply to trees than the oak extract,” said Rossi. “Through the research, we may find oak mulch soil amendments improve the soil and that the compounds in the mulch help citrus trees tolerate HLB.”
The project, “Deploying oak mulch to contain and suppress HLB disease in citrus,” has three objectives: to determine the capability of oak mulch to contain and suppress citrus greening, to measure the effect of oak mulch on HLB-affected citrus physiology, root growth and development, and to study the effect of oak mulch on microbial life biodiversity within the rhizosphere. Hallman carries out daily data collection for the project. Those tasks include soil samples, soil respiration, photosynthesis measurement, and nutrition studies.
“With the SSARE project, we are able to expand the research,” said Rossi. “In the future, we will need to identify which compounds are beneficial, where those compounds are in the trees, which oak species hold the specific compounds, and how much of the right compounds will control the disease.”
One year into the project, Hallman said he found more nutrients in the root rhizosphere. Also, preliminary findings show that as the mulch breaks down, soil biodiversity increases.
“More nutrients are available to the trees as a result of the mulch breaking down into the soil,” said Hallman. “The nutrients are potassium and phosphorus. We have also found that mulch improves soil texture. Improved soil holds more moisture and requires less irrigation.” Soil that holds more moisture enhances plant root health and the trees’ nutrient uptake, resulting in more fruit and a longer life for the trees, Rossi said.
“The research is a collaboration with the USDA,” said Rossi. “It confirms that UF/IFAS and the USDA are committed to the development of ‘an out-of-the-box’ cure for HLB.”
Long term outcomes for the research are to improve economic profitability for growers, to improve environmental health by reducing chemical inputs, and to support the surrounding community. In the years from 2006 until 2011, HLB took more than 6,500 jobs from Floridians. Oak mulch could help restore some of those positions, said Hallman.
Following is a movie in which Lukas Hallman carries out his daily routine as a graduate student:
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