Impact of Ground Applied Micronutrients on Root Growth and Fruit Yield of Severely Huanglongbing-Affected Grapefruit Trees

Abstract

Citrus greening (huanglongbing, HLB) is a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas) and vectored by the Asian citrus psyllid (Diaphorina citri) (ACP). No cure is yet available, and as a result, increased fertilizer applications continue to be a major management method used to prolong the productive life of affected trees. However, questions remain regarding proper fertilizer rates and in what form these nutrients should be applied to increase tree health and yield. Therefore, the goal of this study was to identify optimum micronutrient application forms and rates to increase root growth, canopy size and fruit yield as well as reduce leaf and root CLas cycle threshold (Ct) value of HLB-affected grapefruit trees (Citrus × paradisi Macfad). A large-scale field study consisting of six-year-old HLB-affected ‘Ruby Red’ grapefruit trees grafted on ‘sour orange’ (Citrus × aurantium) rootstock was conducted in the Florida Indian River District. Eight different treatments consisting of four granular and four liquid fertilizers with 1×, 2×, and 4× the current University of Florida, Institute of Food and Agricultural Sciences recommended rates of boron, zinc, manganese, and iron were applied for three times a year (granular), biweekly (liquid), or weekly (liquid), for three years. Root density, length and diameter, root, and leaf CLas Ct value, canopy volume, visual HLB symptoms, and fruit yield were measured throughout the study. Regardless of treatment, tree health declined over time, and no significant impact on severely HLB-affected grapefruit tree health was detected within the three-year time frame of the study. This was the first study to solely compare the impact of ground applied micronutrient application rates and methods on HLB-affected grapefruit tree health in Florida’s Indian River District. More time would be needed to determine the potential benefits of increased micronutrient performance of HLB-affected grapefruit trees.

Enhancing nutrient uptake and grapefruit (Citrus paradisi) growth through soil application of beneficial bacteria (Bacillus spp.)

“…On the other hand, water and fertilizer play vital roles in the growth and fruit yield of citrus. The quantity of fertilizer used in citrus management is linked to both production costs and environmental concerns (Ma et al, 2022), thus nutrient management has been implemented to promote citrus trees' health in Florida where soils are poorly drained and have low fertility (Hallman et al, 2022). Based on the current disease and nutrient problems the citrus industry in Florida is facing, the objective of this study is to evaluate the role of Bacillus spp.…”

Grapefruit Root and Rhizosphere Responses to Varying Planting Densities, Fertilizer Concentrations and Application Methods

“…Much citrus production worldwide has significantly declined due to a disease known as huanglongbing (HLB, or citrus greening) [1]. In Florida, citrus production has been reduced by more than 70% since HLB was first detected in 2005 [2][3][4]. The disease is associated with Candidatus Liberibacter asiaticus (CLas), bacteria transmitted to tree hosts by a vector called the Asian Citrus Psyllid (Diaphorina citri, ACP) [5][6][7].…”

“…There are several approaches toward dealing with HLB and alleviating symptoms, including the development of disease-tolerant rootstocks [7,11], the implementation of vector control [12] and the use of soil amendments [13] to improve soil health. The effect of different methods and rates of fertilizer application on citrus health and yield have also been studied [4,[14][15][16]. There are several ways of applying nutrients to citrus: foliar applications, ground-applied granular fertilizers, fertigation and banding [14].…”

“…The development of improved nutrient guidelines has proven to be beneficial since most of the established guidelines are based on healthy noninfected citrus trees rather than HLB-affected trees [18]. Several studies have examined the effects of various fertilizer types and rates on citrus health and yield [4,9,[19][20][21][22]. The fruit yield increased in 'Valencia' sweet orange trees (Citrus × sinensis) when evaluating varying rates of manganese (Mn) via foliar application at 3× the recommended rate [23].…”

Root-Shoot Nutrient Dynamics of Huanglongbing-Affected Grapefruit Trees

“…The studies described above clearly demonstrate that HLB changes how nutrients are taken up by and translocated within affected trees. Although much research has been focused on how different nutrient rates impact HLB-affected tree health [ 18 , 19 , 20 , 21 ], there is a lack of understanding regarding where in the trees these nutrients are translocated. For example, a previous study conducted by [ 22 ] analyzed nutrient contents on different components of sweet orange trees, but this was done with non HLB-affected trees.…”