Abstract
The decrease in the rate of inflow and outflow of water—and thereby the uptake of plant nutrients as the result of Huanglongbing (HLB or citrus greening)—leads to a decline in overall tree growth and the development of nutrient deficiencies in HLB-affected citrus trees. This study was conducted at the University of Florida, Southwest Florida Research and Education Center (SWFREC) near Immokalee, FL from January 2017 through December 2019. The objective of the study was to determine the effect of rootstocks, nutrient type, rate, and frequency of applications on leaf area index (LAI), water relations (stomatal conductance [gs], stem water potential [Ψw], and sap flow), soil nutrient accumulation, and dynamics under HLB-affected citrus trees. The experiment was arranged in a split-split plot design that consisted of two types of rootstocks, three nitrogen (N) rates, three soil-applied secondary macronutrients, and an untreated control replicated four times. LAI significantly increased in response to the secondary macronutrients compared with uncontrolled trees. A significantly greater gs, and thus a decline in Ψw, was a manifestation of higher sap flow per unit LA (leaf area) and moisture stress for trees budded on Swingle (Swc) than Cleopatra (Cleo) rootstocks, respectively. The hourly sap flow showed significantly less water consumption per unit LA for trees that received a full dose of Ca or Mg nutrition than Ca + Mg treated and untreated control trees. The soil nutrient concentrations were consistently higher in the topmost soil depth (0–15 cm) than the two lower soil depths (15–30 cm, 30–45 cm). Mobile nutrients: soil nitrate–nitrogen (NO3-N) and Mg2+ Mg2+, Mn2+, Zn2+, and B leached to the lower soil (15–30 cm) depth during the summer season. However, the multiple split applications of N as Best Management Practices (BMPs) and optimum irrigation scheduling based on reference evapotranspiration (ETo) maintained soil available N (ammonium nitrogen [NH4-N] and NO3-N) below 4.0 mg kg−1, which was a magnitude 2.0–4.0× less than the conventional N applications. Soil NH4-N and NO3-N leached to the two lower soil depths during the rainy summer season only when trees received the highest N rate (280 kg ha−1), suggesting a lower citrus N requirement. Therefore, 224 kg ha−1 N coupled with a full Ca or Mg dose could be the recommended rate for HLB-affected citrus trees.
Highlights
Florida is the second-largest producer of U.S citrus fruit, accounting for 41% in the 2016–2017 and2017–2018 growing season [1]
During spring 2018, leaf area index (LAI) was significantly greater for trees that received secondary macronutrients than the untreated control trees for trees budded on Cleo rootstocks as compared with Swc rootstocks (Table 1)
In summer 2019, LAI showed an increase of 3.1×, 3.2×, and 3.5× and 4.4×, 2.6×, and 2.7× more under the secondary macronutrient relative to the untreated control trees for trees budded on Cleo and Swc rootstocks, respectively
Summary
Florida is the second-largest producer of U.S citrus fruit, accounting for 41% in the 2016–2017 and2017–2018 growing season [1]. During the last 15 years, the Florida citrus industry has experienced significant citrus production decline because of Huanglongbing (HLB) disease outbreaks. HLB is a destructive disease, caused by the phloem-limited bacterium ‘Candidatus Liberibacter asiaticus’, and it was first identified in Florida in 2005 where currently dispersed throughout the commercial citrus groves [2]. After a citrus tree is infected with Candidatus Liberibacter asiaticus, photosynthesis, xylem sap flow, phloem movement, fine root length density (FRLD), and water and nutrient uptake are severely affected [2,3,4]. The availability of water is one of the most significant restrictions on crop production in Florida. Higher irrigation amounts could result in a substantial runoff of nutrients and herbicides from the citrus root zone through deep percolation.
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