Abstract
Citrus huanglongbing (HLB), caused by three species of fastidious, phloem-limited ‘Candidatus Liberibacter’, is one of the most destructive diseases of citrus worldwide. To date, there is no established cure for this century-old and yet, newly emerging disease. As a potential control strategy for citrus HLB, 31 antibiotics were screened for effectiveness and phytotoxicity using the optimized graft-based screening system with ‘Candidatus Liberibacter asiaticus’ (Las)-infected citrus scions. Actidione and Oxytetracycline were the most phytotoxic to citrus with less than 10% of scions surviving and growing; therefore, this data was not used in additional analyses. Results of principal component (PCA) and hierarchical clustering analyses (HCA) demonstrated that 29 antibiotics were clustered into 3 groups: highly effective, partly effective, and not effective. In spite of different modes of actions, a number of antibiotics such as, Ampicillin, Carbenicillin, Penicillin, Cefalexin, Rifampicin and Sulfadimethoxine were all highly effective in eliminating or suppressing Candidatus Liberibacter asiaticus indicated by both the lowest Las infection rate and titers of the treated scions and inoculated rootstock. The non-effective group, including 11 antibiotics alone with three controls, such as Amikacin, Cinoxacin, Gentamicin, Kasugamycin, Lincomycin, Neomycin, Polymixin B and Tobramycin, did not eliminate or suppress Las in the tested concentrations, resulting in plants with increased titers of Las. The other 12 antibiotics partly eliminated or suppressed Las in the treated and graft-inoculated plants. The effective and non-phytotoxic antibiotics could be potential candidates for control of citrus HLB, either for the rescue of infected citrus germplasm or for restricted field application.
Highlights
Three species of the fastidious, phloem-residing, gram-negative bacteria, ‘Candidatus Liberibacter asiaticus’ (Las) [1], ‘Ca. L. africanus’ (Laf) [2] and ‘Ca. L. americanus’ (Lam) [3] are the causal agents of huanglongbing (HLB, known as greening), one of the most devastating diseases of citrus
Less than 13.6% and 6.3% of scions treated, respectively survived; whereas, a higher percentage of the scions treated with the remaining antibiotics and control solvents survived and produced more flushes and/or new leaves
Transmission efficiency of Las along with scion survival (%) and scion infection (%) and the cycle threshold (Ct) value in the scion and rootstock in the inoculated plants when grafted with Las-infected scions (Ct value = 25.762.1) treated with antibiotics. ySDA: Stepwise discriminant analysis; The group is classified by stepwise discriminant analysis (SDA). doi:10.1371/journal.pone.0111032.t002
Summary
Three species of the fastidious, phloem-residing, gram-negative bacteria, ‘Candidatus Liberibacter asiaticus’ (Las) [1], ‘Ca. L. africanus’ (Laf) [2] and ‘Ca. L. americanus’ (Lam) [3] are the causal agents of huanglongbing (HLB, known as greening), one of the most devastating diseases of citrus. Both Las and Lam are transmitted by Diaphorina citri Kuwayama, while Laf is transmitted by Trioza erytrea (Del Guercio) [4]. There are no practical methods for the control of HLB in commercial groves [1], graft-based chemotherapy [9], shoot tip grafting [10,11], thermotherapy [12,13,14,15], vitrification-cryopreservation [16] and antibiotics [13] have been successfully used for HLB therapy in the greenhouse settings
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