Preliminary assessment of bacterial antibiotic resistance and Candidatus Liberibacter asiaticus titer in three Florida commercial citrus groves

Abstract

Over the last 16 years, the U.S. citrus industry has been devastated by citrus greening or Huanglongbing (HLB) disease. The causal agent, Candidatus Liberibacter asiaticus (CLas), is an unculturable bacterium transmitted by the invasive Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera: Liviidae), and by grafting. In 2017 and 2019, the Environmental Protection Agency approved foliar applications of the antibiotics streptomycin (STR) and oxytetracycline (OTC) in commercial citrus groves in Florida. This study provides a preliminary assessment of antibiotic spray effects on three commercial citrus groves by quantifying CLas titer from leaves and antibiotic resistant bacterial colony count from leaf, root+rhizosphere, and soil samples. Two groves used antibiotic spray treatment and followed conventional management, one used no antibiotic treatment and was accredited as an organic grove. Samples were plated onto TSA medium containing either STR (100 μg/mL), OTC (15 μg/mL), or no antibiotic (control) then colony-forming units (CFU) per gram of sample were estimated. Among leaf samples, there were no significant differences in CFU or CLas titer based on grove. For soil samples, we found a larger number of resistant CFU in the organic grove. Root+rhizosphere samples from all groves (organic and conventional) showed high prevalence of both STR and OTC-resistant bacteria indicating that plant selective processes during root colonization might favor antibiotic resistant acquisition. The lack of significant difference in CLas titer among the three groves is supported by other studies which suggest that antibiotic spray might not be an effective control for CLas. This research further highlights the need for continued monitoring of antibiotic efficacy to control plant pathogens in commercial settings.