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  • Author or Editor: Arnold W. Schumann x
  • HortTechnology x
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Fertilizer spreaders capable of variable rate application are increasingly important for enhancing nutrient management in horticultural crops because they improve placement and increase nutrient uptake efficiency. Matching applied fertilizer to fertilizer requirements represents a significant input cost saving for the grower and a reduction in potential pollutant loading to ground and surface water. Variable rate fertilization (VRF) is a precision agriculture technology made possible by embedded high-speed computers, accurate Global Positioning System (GPS) receivers, Geographic Information Systems (GIS), remote sensing, yield or soil maps, actuators, and electronic sensors capable of measuring and even forecasting crop properties in real time. For tree crops like Florida citrus (Citrus spp.), the most important function of the VRF spreader is to detect and avoid fertilizing spaces of the orchard not occupied by trees. Treeless spaces are becoming more common in Florida as diseases such as citrus greening (Candidatus Liberibacter asiaticus) and canker (Xanthomonas axonopodis) cause the removal of thousands of trees every year. VRF works best under those conditions. Because VRF exploits crop and soil variability, it has no value in a perfectly uniform field. VRF enables smaller trees including resets to be fertilized at lower, most appropriate rates, thus minimizing any excess application. This article examines the existing knowledge on using precision agriculture and variable rate technology to keep water and nutrients in the root zone of horticultural crops, thus facilitating maximum uptake efficiency.

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Citrus (Citrus sp.) root length density (RLD) can help in understanding and predicting nutrient and water uptake dynamics. A study was conducted at two sites in Florida to investigate root and water distribution patterns among different irrigation and fertigation systems. The results over the 2 years showed that RLD was highest in the 0- to 15-cm soil depth and decreased with depth for all treatments at both sites. About 64% to 82% of the fibrous roots (<1 mm diameter) were concentrated in the irrigated zones of drip- and microsprinkler-irrigated trees and 18% to 36% were found in the nonirrigated zones at the Spodosol site (SS). At the Entisol site (ES), the RLD (<0.5 mm diameter) in the 0- to 15-cm depth soil for intensive microsprinkler or drip irrigation was 3- to 4-fold (nonirrigated zone) and 4- to 7-fold (irrigated zone) greater at the 0- to 15-cm soil depth than that for conventional irrigation system. The trees at SS were symptomatic for Huanglongbing (HLB; Candidatus Liberibacter asiaticus) in the second year, while those at ES were asymptomatic throughout the study. This might have limited the density and extent of root distribution at SS. The water contents remained either close to or slightly above the field capacity. The results showed higher RLD for intensive irrigation and fertigation practices in irrigated zones compared with conventional grower applications suggesting greater water and nutrient uptake potential for the former.

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Completely enclosed screen houses can physically exclude contact between the asian citrus psyllid [ACP (Diaphorina citri)] and young, healthy citrus (Citrus sp.) trees and prevent huanglongbing (HLB) disease development. The current study investigated the use of antipsyllid screen houses on plant growth and physiological parameters of young ‘Ray Ruby’ grapefruit (Citrus ×paradisi) trees. We tested two coverings [enclosed screen house and open-air (control)] and two planting systems (in-ground and container-grown), with four replications arranged in a split-plot experimental design. Trees grown inside screen houses developed larger canopy surface area, canopy surface area water use efficiency (CWUE), leaf area index (LAI) and LAI water use efficiency (LAIWUE) relative to trees grown in open-air plots (P < 0.01). Leaf water transpiration increased and leaf vapor pressure deficit (VPD) decreased in trees grown inside screen houses compared with trees grown in the open-air plots. CWUE was negatively related to leaf VPD (P < 0.01). Monthly leaf nitrogen concentration was consistently greater in container-grown trees in the open-air compared with trees grown in-ground and inside the screen houses. However, trees grown in-ground and inside the screen houses did not experience any severe leaf N deficiencies and were the largest trees, presenting the highest canopy surface area and LAI at the end of the study. The screen houses described here provided a better growing environment for in-ground grapefruit because the protective structures accelerated young tree growth compared with open-air plantings while protecting trees from HLB infection.

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Huanglongbing (HLB) disease is a threat to most citrus (Citrus sp.) producing areas and is associated with the bacterium Candidatus Liberibacter asiaticus. The disease is transmitted by the vector asian citrus psyllid [ACP (Diaphoria citri)]. Antipsyllid screen houses can potentially reduce and eliminate HLB development in young citrus plantings by excluding the insect vector. These structures are also anticipated to represent a new environmental platform to cultivate high-valued fresh citrus. The purpose of this investigation was to evaluate the effect of screen houses on excluding infective ACP from inoculating grapefruit (Citrus ×paradisi) trees and determine changes on environmental conditions caused by the screen cloth. We tested two coverings [enclosed screen house and open-air (control)] and two planting systems (in-ground and container-grown), with four replications arranged in a split-plot experimental design. Psyllid counting and HLB diagnosis were performed monthly, and the antipsyllid screen excluded the HLB vector from the houses. ACP and HLB-positive trees were found only at the open-air plots. Weather monitoring was performed every 30 minutes from 22 Feb. to 31 July 2014. Solar radiation accumulation averaged 6.7 W·m−2·minute−1 inside the screen houses and 8.6 W·m−2·minute−1 in the open-air. Air temperature was greater inside the screen houses whereas wind gusts were higher in the open-air. Reference evapotranspiration accumulation averaged 3.2 mm·day−1 inside the screen houses and 4.2 mm·day−1 in the open-air. There was no difference in cumulative rainfall between screen houses and open-air. The antipsyllid screen houses reduced solar radiation, maximum wind gust, and reference evapotranspiration (ETo). The environmental conditions inside the protective screen houses are suitable for grapefruit production.

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