A Graft-Based Chemotherapy Method for Screening Effective Molecules and Rescuing Huanglongbing-Affected Citrus Plants

Abstract

Huanglongbing (HLB) is the most devastating disease of citrus. The global citrus industry is in urgent need of effective chemical treatments for HLB control because of its rapid spreading worldwide. Due to the fastidious nature of the pathogens, and the poor permissibility of citrus leaf surfaces, effective screening of chemicals for the HLB control can be challenging. In this study, we developed a graft-based chemotherapy method to rapidly screen potential HLB-controlling chemical compounds. In addition, we improved transmission efficiency by using the best HLB-affected scion-rootstock combination, and demonstrated the HLB bacterial titer was the critical factor in transmission. The HLB-affected lemon scions had a high titer of HLB bacterium, survival rate (83.3%), and pathogen transmission rate (59.9%). Trifoliate, a widely used commercial rootstock, had the highest survival rate (>70.0%) compared with grapefruit (52.6%) and sour orange (50.4%). Using this method, we confirmed a mixture of penicillin and streptomycin was the most effective compounds in eliminating the HLB bacterium from the HLB-affected scions, and in successfully rescuing severely HLB-affected citrus germplasms. These findings are useful not only for chemical treatments but also for graft-based transmission studies in HLB and other Liberibacter diseases.

Effective Antibiotics against ‘Candidatus Liberibacter asiaticus’ in HLB-Affected Citrus Plants Identified via the Graft-Based Evaluation

“…To identify the effectiveness of a chemical against Las and its phytotoxicity to citrus, antibiotics were evaluated using the previously published graft-based chemotherapy [9]. Briefly, HLB-affected budsticks were sampled from severely HLB-affected rough lemons ( Citrus limonum ‘Lemon #76’) at the USDA-ARS-USHRL farm in Fort Pierce, FL and confirmed positive for Las by real-time qPCR [9], [25].…”

“…The recommended management strategy for HLB includes chemical control to reduce psyllid populations, removal of infected trees to eliminate new sources of bacterial inoculum and production of pathogen-free nursery plants [8]. Although 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]–[15], vitrification-cryopreservation [16] and antibiotics [13] have been successfully used for HLB therapy in the greenhouse settings.…”

“…Several researchers reported initial success in reversing the symptoms of HLB [22]–[24]. In our previous studies, penicillin applied alone or in combination with streptomycin (PS) was shown to be effective in eliminating Las, and PS, when compared to administering either antibiotic separately, provided a therapeutically effective level of control for a greater period of time [9].…”

“…Screening and development of a bactericide or other curative product is one of the most promising approaches for the control of HLB. In this study, we have evaluated 31 antibiotics for their effectiveness against Las and their phytotoxicity to citrus using the optimized graft-based chemotherapy method [9]. A number of effective antibiotics, especially agricultural antibiotics have been identified for further evaluation in field trials.…”

Bacterial Citrus Diseases: Major Threats and Recent Progress

“…The effectiveness of several bactericides against CaLas has been demonstrated in greenhouse studies [102,103]. However, foliar application of bactericides suffers from several limitations, including rapid degradation of the compound under sunlight and risk of environmental contamination.…”