Abstract

Huanglongbing (HLB) or greening disease, associated with the bacterial pathogen Candidatus Liberibacter asiaticus (Las), is currently the most devastating citrus disease worldwide and no cure is available. Inducers of systemic acquired resistance (SAR) are effective and sustainable to combat various plant diseases. In this study, the SAR inducers acibenzolar-S-methyl (ASM), imidacloprid (IMI), 2,6-dichloroisonicotinic acid (INA), and salicylic acid (SA), applied individually by foliar spray, soil drench or trunk injection at various rates and frequencies, were evaluated for control of HLB in a 3-year field trial with mature Hamlin sweet orange trees in central Florida, USA in the 2016, 2017, and 2018 crop seasons. Six foliar sprays, six soil applications, and three trunk injections of ASM, IMI, INA, or SA per year were conducted with the untreated as a negative control. HLB disease severity, Las titers, pre-harvest fruit drop, yield and fruit quality were investigated for the treatments. By the end of the 2018 season, all trunk injection treatments at 0.25 g/tree and foliar sprays of INA or SA (but not ASM or IMI) at 0.5 g/tree significantly reduced disease severity, Las population, and fruit drop, and increased fruit yield; whereas all foliar spray treatments at 0.25 g/tree, trunk injection treatments at 0.125 g/tree, and soil drench treatments at 0.25 or 0.5 g/tree did not provide effective control of HLB. Additionally, all trunk injection treatments at 0.25 g/tree had shown a significant decrease in fruit drop and increase of fruit yield starting from 2016 after 1 year of applications, whereas foliar sprays of INA or SA at 0.5 g/tree exhibited similar effects at 2018 after 3 years of applications. None of the SAR inducer treatments had significant effect on fruit quality. Economic analysis suggested that the trunk injection treatments at 0.25 g/tree might produce financial benefits. Overall, this study presents useful information for management of citrus HLB with SAR inducers.

Highlights

  • Citrus huanglongbing (HLB), called citrus greening disease, is currently the most destructive citrus disease affecting the global citrus industry and causes significant economic losses worldwide (Blaustein et al 2018; Wang2019; Andrade et al 2020)

  • The HLB disease severity over the experiment duration was significantly (P < 0.05) reduced by foliar sprays of INA (0.5 g/tree) and salicylic acid (SA) (0.5 g/tree), and trunk injections of ASM, IMI, INA, or SA at 0.25 g/ tree compared with the untreated control (UTC) (Fig. 2)

  • From the initiation to end of the tests, the mean values of Liberibacter asiaticus (Las) titer in the INA (0.5 g/tree) or SA (0.5 g/tree) spray-treated trees increased from 1.21 × 107 and 0.87 × 107 to 3.05 × 107 and 2.12 × 107 cells/g plant tissue, respectively, whereas that of the UTC increased from 0.75 × 107 to 7.63 × 107 cells/g plant tissue

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Summary

Introduction

Citrus huanglongbing (HLB), called citrus greening disease, is currently the most destructive citrus disease affecting the global citrus industry and causes significant economic losses worldwide (Blaustein et al 2018; Wang2019; Andrade et al 2020). Most commercial citrus varieties are susceptible to Las infection, with few, such as Sugar Belle, showing tolerance to HLB (Clark et al 2018; Wang 2019; Andrade et al 2020). Since it was first identified in East Asia over 100 years ago, HLB has spread to most citrus producing areas in the world, including Asia, Africa, and the Americas, except Australia and the Mediterranean region (Wang 2019). The first Las-positive ACP was found in a commercial citrus grove in California in August 2020 (California Citrus Pest and Disease Prevention Program 2020)

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