Citrus Water Use and Root Density Patterns as Influenced by Citrus Greening and Regulated Deficit Irrigation under Greenhouse Conditions

Florida citrus production accounts for $8 billion of economic activity and represents about 49% of citrus production in the United States. Current citrus production is at a historical low due to the devastating impact of citrus greening disease (also known as Huanglongbing, HLB) which triggers tree defoliation, root loss, fruit drop and canopy decline. Potential for improving citrus tree performance lies in the ability to develop appropriate water management strategies, good nutrient management options, and the restoration of soil and root health to optimize water and nutrient uptake. A study was conducted in the greenhouse on two-year-old Citrus sinensis 'Hamlin' oranges to: 1) determine plant water use under regulated deficit irrigation (RDI) and full irrigation of HLB-affected and non-affected trees; and 2) determine the effect of irrigation rate on tree growth and root density of HLB-affected trees as a function of soil amendments. Results showed that water use was significantly limited for HLB-affected trees grown on sand only, resulting in elevated water contents in the root zone at full irrigation. However, water use was marginally improved when the HLB-affected trees were grown on sand amended with 5% compost and 3% biochar. No significant differences in root density were observed between irrigation and fertilization rates and soil amendments, but significantly lower root densities were noted on HLB positive trees, as expected. Under RDI, canopy size and leaf area were significantly limited for HLB affected compared to non-affected trees suggesting that it would be important to maintain irrigation at optimal level for young trees affected by HLB to avoid compromising tree performance.