Abstract

Citrus production under enclosed structures can exclude the Asian citrus psyllid (ACP, Diaphorina citri) and eliminate the negative effects of citrus greening or huanglongbing (HLB) disease caused by Candidatus Liberibacter asiaticus to the grapefruit (Citrus paradisi) fresh fruit industry. Physically impeding the insect vector from accessing trees is a logical method to have disease-free groves. Our objectives were to assess the ability of enclosed screenhouses to exclude the ACP, stop HLB inoculation and dissemination, and improve fruit yield of in-ground and container-grown 6-year-old “Ray Ruby” grapefruit at super-high planting densities relative to open-air trees. We built a large structure to allow commercial-scale trials and tested two production systems (screenhouse and open-air), two planting systems (in-ground and potted), and two rootstocks (“Sour Orange” [Citrus × aurantium] and “US-897” [Citrus reticulata × Poncirus trifoliate]). The experimental design was a randomized complete block design split-split-plot with four replications. Four passively ventilated 1,080-m2 completely enclosed screenhouses were constructed using a 50-mesh monofilament high-density polyethylene screen. The main support for each enclosed, covered structure consisted of pressure-treated, wooden utility poles. Trees were planted in Sept/2013 on a density of 1,957 trees/ha. Irrigation was performed on-demand using two 7.6-LPH drip emitters per tree, and fertigation was applied three times/week using 15N-2.6P-22.4K water-soluble fertilizer at 180 kg N/ha. Psyllids were monitored using sticky cards and detected inside the screenhouses post-Hurricane Irma, which damaged the screen structures in Sept/2017, leaving openings until repairs were completed in Apr/2018. Screen aging and a tropical storm in April/2019 caused another major screen opening fixed in Oct/2019. Despite the weather-related damages to the screens, only trees cultivated in open-air tested positive for Candidatus Liberibacter asiaticus after 6 years. There was fast disease progression for all outside treatments, with 100% infection. Covered, in-ground trees exhibited the highest trunk diameter and canopy volume (P < 0.0001). Trees grown inside screenhouses exhibited higher fruit yield than outside trees, with the highest yield observed for in-ground trees on “US-897” (51,081 kg/ha) (P < 0.0001). Several open-air treatments particularly in containers did not produce any fruit. On the other hand, potted grapefruit trees cultivated inside the enclosures had the highest soluble solids content (P < 0.001). The screenhouses provided disease exclusion, increased fruit yield, and fruit quality, representing an alternative for growers interested in producing high-quality fruit for the fresh market. Production cost and economic viability still need to be evaluated for large-scale implementation.

Highlights

  • Citrus is directly linked with the “Sunshine State” agricultural identity

  • Our objectives were to assess the ability of enclosed screenhouses to exclude the ACP, stop HLB inoculation and dissemination, and improve fruit yield of in-ground and container-grown 6-year-old “Ray Ruby” grapefruit at super-high planting densities relative to open-air trees

  • The first single psyllid inside the UF/IFAS Indian River Research and Education Center (IRREC) citrus under protective screens (CUPS) was detected in May 2017, 7 months after Hurricane Matthew caused minor damages to the screen in October 2016 (Figure 3, top)

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Summary

Introduction

Citrus is directly linked with the “Sunshine State” agricultural identity. Florida’s total citrus production declined 83% from 291.8 million boxes in 2003/04 to 49.58 million boxes in 2017/18, indicating a severe crisis in the fruit market (U.S Department of Agriculture, 2019). Citrus bearing acreage declined drastically from 679,000 acres in 2003/ 04 to 400,900 acres in 2017/18. On-tree citrus values dropped 47% from $1.046B in 2008/09 to $0.551B in 2017/18 (U.S Department of Agriculture, 2019). Florida grapefruit production for the fresh market experienced an even steeper decline, dropping by 90% from 1.738 million tons in 2003/04 to 0.165 million tons in 2017/18. Grapefruit (Citrus paradisi) bearing acreage reduced from 82,300 acres in 2003/04 to 29,800 acres in 2017/18 (U.S Department of Agriculture, 2019)

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