Abstract

Currently, oxytetracycline and streptomycin are being applied to citrus groves in Florida for the control of citrus greening disease caused by the unculturable bacterium 'Candidatus Liberibacter asiaticus'.Here, the closest cultured relative, L. crescens, was used to estimate the frequency of spontaneous antimicrobial resistance of, as yet, uncultured 'Ca.Liberibacter asiaticus'.Results yielded thirteen streptomycin and zero oxytetracycline mutants after exposing 13 billion cells to the antimicrobials.These low rates, alongside the restrictive habitats of the vector and pathogen, suggest resistance may develop very slowly, if at all.Thus, the treatments will likely remain useful long enough before pathogen-resistant or -tolerant genotypes are deployed. IntroductionOnly three antimicrobials are registered by the Environmental Protection Agency (EPA) for use in plant agriculture: oxytetracycline (OTC), streptomycin, and kasugamycin.Prior to 2016, streptomycin comprised 90% of the antimicrobials used, largely to control plant diseases on temperate fruit trees such as apples, pears, and peaches (Stockwell and Duffy 2012).Nowadays, the use of antimicrobials in plant agriculture has increased dramatically with streptomycin and OTC registered by the EPA and Food and Drug Administration (FDA) for use on citrus canker disease (FDA 2013).Recently, the EPA's emergency exemption for use of streptomycin sprays for citrus greening disease was vacated (9th Cir 2023).Meanwhile, under FIFRA Section 24(c), OTC trunk injections were approved for citrus in Florida in 2022 (Archer et al. 2023).Currently, field tests have shown OTC trunk injections are more effective than sprays on 'Candidatus Liberibacter asiaticus' (CLas)-infected trees because of the systemic nature of CLas.(Hu and