Abstract
Huanglongbing (HLB), the most destructive citrus disease, is caused by three species of phloem-limited Candidatus Liberibacter. Chemical control is a critical short-term strategy against Candidatus Liberibacter asiaticus (Las). Currently, application of antibiotics in agricultural practices is limited due to public concerns regarding emergence of antibiotic-resistant bacteria and potential side effects in humans. The present study screened 39 antimicrobials (non-antibiotics) for effectiveness against Las using an optimized graft-based screening system. Results of principal component, hierarchical clustering and membership function analyses demonstrated that 39 antimicrobials were clustered into three groups: “effective” (Group I), “partly effective” (Group II), and “ineffective” (Group III). Despite different modes of action, 8 antimicrobials (aluminum hydroxide, D,L-buthionine sulfoximine, nicotine, surfactin from Bacillus subtilis, SilverDYNE, colloidal silver, EBI-601, and EBI-602), were all as highly effective at eliminating or suppressing Las, showing both the lowest Las infection rates and titers in treated scions and inoculated rootstock. The ineffective group, which included 21 antimicrobials, did not eliminate or suppress Las, resulting in plants with increased titers of Candidatus Liberibacter. The other 10 antimicrobials partly eliminated/suppressed Las in treated and graft-inoculated plants. These effective antimicrobials are potential candidates for HLB control either via rescuing infected citrus germplasms or restricted field application.
Highlights
Citrus Huanglongbing (HLB) is an serious citrus disease and has caused enormous economic losses to citrus industry in the world[1,2]
HLB is caused by three species of uncultured, phloem-restricted proteobacteria in the Candidatus Liberibacter genus, L. asiaticus (Las), L. americanus, and L. africanus[1,6,7], and is transmitted by either Diaphorina citri or Trioza erytreae[8]
Screening of non-antibiotic or other chemical compounds that have already been registered for fruit tree production and can reduce the emergence on antibiotic-resistant bacteria is urgently needed for the survival of the Florida citrus industry
Summary
Citrus Huanglongbing (HLB) is an serious citrus disease and has caused enormous economic losses to citrus industry in the world[1,2]. Effective strategies against Las bacterium in citrus production are still limited, and breeding resistant citrus varieties is considered to be the most efficient and sustainable strategy against HLB. Streptomycin sulfate and oxytetracycline can suppress Las titer in greenhouse and field[17,18,19,20], tetracycline and streptomycin were only bacteriostatic rather than bactericidal[11,19] It is necessary for continuous application of these two antibiotics to suppress the disease, frequent applications are high cost and may result in the emergence of antibiotic-resistant bacteria. 39 antimicrobial (nonantibiotic) compounds (including natural product, antimicrobial metals, and commercial product), which can reduce risk of emergence of antibiotic resistant bacteria and potential side effects in humans, were evaluated for effectiveness against HLB and phytotoxicity via an optimized graft-based assay
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