Evaluation of the control effect of SAR inducers against citrus Huanglongbing applied by foliar spray, soil drench or trunk injection

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.

Copper oxide nanostructures as a potential method for control of zucchini yellow mosaic virus in squash

“…Ryu and colleagues found no difference in CMV titer in Arabidopsis plants at 3 weeks after infection across all treatments, but the npr1 plants showed much‐decreased disease severity 38 . In this study, the result could be due to the effect of salicylic acid on virus movement and/or symptom expression, which is highly correlated with the application method 39 . The relative uptake of salicylic acid applied by foliar spray could be higher than when applied as a soil drench.…”

“…The relative uptake of salicylic acid applied by foliar spray could be higher than when applied as a soil drench. Multiple factors might be responsible for the varying efficacy between soil drench and foliar spray 39 . Foliar sprays could be more effective than soil drench.…”

“…In addition, many inducers such as Acibenzolar‐S‐methyl and salicylic acid are highly photodegradable and biodegradable, and are vulnerable to other environmental conditions 40, 41 . Soil particles might absorb inducers and reduce root uptake 39 …”

“…38 In this study, the result could be due to the effect of salicylic acid on virus movement and/or symptom expression, which is highly correlated with the application method. 39 The relative uptake of salicylic acid applied by foliar spray could be higher than when applied as a soil drench. Multiple factors might be responsible for the varying efficacy between soil drench and foliar spray.…”

An Overview of the Mechanisms Against “Candidatus Liberibacter asiaticus”: Virulence Targets, Citrus Defenses, and Microbiome

“…Currently, many strategies have been developed for HLB mitigation, including application of antimicrobials ( Zhang et al, 2011a , 2012 , 2014 , 2021 ; Hu and Wang, 2016 ; Hu et al, 2018 ; Yang et al, 2018 ), thermotherapy ( Hoffman et al, 2013 ; Fan et al, 2016 ; Yang et al, 2016b ; Doud et al, 2017 ; Ghatrehsamani et al, 2019 ; Vincent et al, 2019 ), macro-and micronutrients ( Spann and Schumann, 2009 ; Gottwald et al, 2012 ; Rouse et al, 2017 ; Mattos-Jr et al, 2020 ; Dong et al, 2021 ; Zhou et al, 2021 ), plant defense inducers ( Canales et al, 2016 ; Li et al, 2016 , 2019 , 2021a ; Hu et al, 2018 ; Wang, 2021 ), control of the insect vector ( Grafton-Cardwell et al, 2013 ; Boina and Bloomquist, 2015 ; Cocuzza et al, 2017 ; Pierre et al, 2021 ), biocontrol ( Trivedi et al, 2011 ; Hopkins and Wall, 2021 ; Nan et al, 2021 ; Poveda et al, 2021 ), and eradication of HLB symptomatic citrus trees ( Bassanezi et al, 2013 ; Yuan et al, 2020 ). However, these strategies have shown limited success in field applications and effective HLB management remains a challenge.…”