Citrus greening or HLB, caused by Candidatus Liberibacter asiaticus (CLas), is the most fatal and quickly spreading citrus disease worldwide (Garnier and Bove, 1983; Gonzalez et al., 2012; Jagoueix et al., 1994). Citrus [Citrus ×sinensis (L.) Osbeck] trees affected by this disease decline in productivity and produce deformed and inedible fruit (Hall and Gottwald, 2011). Currently, there is no known cure for HLB (Raithore et al., 2015), and it has spread throughout the entire state of Florida with an estimated 80% of citrus trees affected (Singerman and Useche, 2015).
Due to the threat of reduced production and negative economic impacts, there is an urgent need for strategies to manage HLB without severely depleting our resources including water. Water is a limiting factor in crop production, particularly in Florida with predominantly sandy soils in citrus-producing regions. During the last six decades, water demands for irrigated crops have increased (Maupin et al., 2014). In the United States, irrigated agriculture consumes more than 80% of total water used and contributes to more than 45% of the total agricultural production costs (Schaible and Aillery, 2012). Recently, the Southwest Florida Water Management District has reported that more citrus growers are exceeding their monthly and yearly permitted water allocations (Ferguson, 2015).
In addition to plant characteristics associated with plant age, health, and structure, SF is impacted by environmental factors including air temperature and soil water availability. Soil water availability is mainly affected by a series of factors including soil physical and chemical properties (e.g., soil texture, structure, organic matter content, and topography), climate factors (e.g., temperature, solar radiation, and rainfall), and vegetation cover. Soil water availability may also affect evapotranspiration by reducing water uptake (Morgan et al., 2006). However, studies investigating relationships between soil moisture and SF can be confusing. Various studies have reported a limited effect of soil moisture on crop SF (Granier et al., 2000; O’Brien et al., 2004). Conversely, others have reported a negative relationship between soil moisture and SF (Holscher et al., 2005).
In Florida, citrus groves are typically planted in areas with sandy soils with low water-holding capacities and organic matter contents. Irrigation strategies that allow growers to increase fruit production without depletion of water resources must be documented and practiced. Scientists are facing tremendous challenges for increasing water efficiency in citrus production systems as a consequence of HLB (Kadyampakeni et al., 2014). Improved water-use efficiency may be achieved by using the proper irrigation schedule (Fereres and Soriano, 2007).
Improving irrigation management is a vital factor in developing sustainable agricultural practices (Fernandez et al., 2008). In addition, there are increasing concerns about the availability of water that must be used in agricultural production (Fereres and Gonzalez-Dugo, 2009; Fernandez et al., 2008). During the last few decades, environmental concerns over water scarcity have stimulated research to invest into new irrigation technologies and more efficient scheduling approaches (Fernandez et al., 2008). Various methods for irrigation scheduling under different crops have been tested. Many of them rely mainly on nondestructive SF measurements (Fernandez et al., 2008; Jones, 2004). Thus, SF measurements play an important role in understanding the dynamics of plant water flow in irrigated crops (Smith and Allen, 1996). The use of nondestructive SF and Ψ measurement methods are exceptionally suited for determining crop water status in irrigation scheduling studies (Ballester et al., 2013).
Investigating improved irrigation practices for HLB-affected trees has not been conducted in Florida or other areas where greening occurs. It has been documented that HLB-affected trees have lower root densities than healthy trees (Hamido et al., 2016; Kadyampakeni et al., 2014), reducing the ability of the tree to take up water and nutrients from dryer soils compared with healthy trees. Thus, field scale experimentation is needed to determine if current citrus irrigation practices need to be changed for citrus trees with HLB. The combination of experiments and demonstrations in a wide range of soils and areas of the state would improve the applicability and adoption of improved irrigation management recommendations. Thus, the objective of this study was to determine the impact of irrigation scheduling on growth and water relations of HLB-affected citrus trees.
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