Mature citrus trees affected by the bacterial disease huanglongbing (HLB) typically need about 25 percent less irrigation than their healthy counterparts However, that doesn’t necessarily mean young trees with the disease will benefit from water deficits, according to a preliminary greenhouse study by researchers with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS).
Experiments on 1-year-old orange trees showed similar responses to irrigation in healthy trees and those affected by HLB. Diseased or not, the trees grew faster when they received daily irrigation that kept pace with losses from evapotranspiration, a measurement that combines two types of water loss — evaporation from soil, trunk, stem and leaf surfaces, and losses from transpiration, the process by which water vapor exits the tree through its leaf pores during the day.
This result suggests that young, HLB-affected trees benefit most from a “full ration” of water, a finding that surprised researchers, said soil and water science expert Davie Kadyampakeni, an assistant professor with the UF/IFAS Citrus Research and Education Center in Lake Alfred. The study was funded by the UF/IFAS Citrus Research Initiative.
“Over the past few years, there has been interest in the possibility that mature trees with HLB might benefit from temporary irrigation deficits, because when we impose those conditions it can force a tree to improve its water-use efficiency,” Kadyampakeni said. “But our findings indicate that young trees with HLB grew faster when they received what growers would consider a full ration of water.”
In recent years, UF/FAS researchers have determined that mature HLB-affected citrus trees require about 25 percent less water than their healthy counterparts, he said. The exact reasons are unclear, but reductions in leaf area and root function have been cited as possible causes.
Because mature HLB-affected trees often continue to produce marketable fruit for years, one of the top priorities for UF/IFAS citrus experts is to find methods of supporting tree health throughout the early stages of the HLB disease process.
Results from the current study suggest that the team may have uncovered an important new aspect of the disease process, and that the state’s growers might want to consider providing full irrigation to all young trees, regardless of their HLB status, he said.
“These findings are intriguing, but we’re not ready to issue a recommendation yet,” Kadyampakeni said. “We need to follow up with field trials, probably over a period of time close to three years, to assess the irrigation needs of young trees under typical industry conditions. You have to remember that this was a greenhouse study, and the vast majority of Florida citrus is grown outdoors.”
The team conducted the irrigation study as part of a larger project aimed at improving management of citrus trees affected by HLB, he said.
In the study, scientists used 144 individually potted 1-year-old Hamlin sweet orange trees on Swingle citrumelo rootstock. Half the trees were irrigated each day with an amount of water equal to 100 percent of the trees’ daily evapotranspiration. The other half were irrigated with water equaling 75 percent of their evapotranspiration.
The scientists assessed the effects of both irrigation rates by measuring the stem water potential of individual trees. Roughly analogous to the blood pressure measurements that doctors use to assess human health, stem water potential measures the degree to which water is present in a specific piece of tree tissue –typically a single leaf — and treats the result as an indicator of the tree’s overall hydration status, due to the fact that water circulates freely throughout the tree via water-conducting tissue known as the xylem. Consequently, stem water potential is often employed to determine whether citrus trees are experiencing water stress.
Results showed that HLB-positive trees showed the same degree of water stress as their healthy counterparts after six months on the same regimen. More importantly, HLB-positive trees that received full irrigation — equal to 100 percent of evapotranspiration — grew taller, developed thicker trunks and achieved greater root length density, compared with trees receiving irrigation equal to 75 percent of their evapotranspiration rate. Similar results were found in healthy trees. Also, healthy trees grew faster than their HLB-positive counterparts.
The overall project, which included experiments concerning nitrogen fertilization and soil amendments, was conducted by Kadyampakeni and colleagues Arnold Schumann, a soil and water sciences professor at the Lake Alfred center, and soil microbiologist Sarah Strauss, an assistant professor with the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
Findings from the study will be published in the near future, Kadyampakeni said, and he plans to pursue further research on the possibility that soil amendments including compost and biochar might be used to increase the water-holding and nutrient-retention capacity of sandy grove soils.
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