Huanglongbing (HLB) is considered the most destructive citrus disease and occurs in the major citrus producing areas around the world. In Brazil, HLB is associated with the bacteria 'Candidatus' Liberibacter asiaticus (CLas) and 'Candidatus' Liberibacter americanus (CLam). CLas is the most aggressive and widespread species of the HLB bacteria. Currently, there is no curative control measure for this disease, and the eradication of diseased trees and control of the insect vector are the basic management measures for HLB. The search for other measures with higher efficiency and lower economic and environmental impacts for control of HLB has become a research priority. The use of chemicals that activate plant resistance mechanisms is a novel method for disease control. Thus, the objective of this study was to investigate the use of systemic acquired resistance (SAR) inducers for control of HLB. Nursery trees of Valencia sweet orange (Citrus sinensis (L.) Osbeck) grafted on Rangpur lime (Citrus limonia Osbeck) were inoculated with CLas by bud grafting technique using diseased material from HLB citrus trees, seven days after the first treatment with the SAR inducers, under greenhouse conditions. The SAR inducers imidacloprid (IMI), thiamethoxan (TMX), or acibenzolar-S-methyl (ASM), as well as the combination of ASM with IMI or TMX, were applied as soil drench. The nursery trees were evaluated according to the presence of HLB symptoms and the bacterium in leaf tissue. The presence of CLas was determined by using the PCR technique, at 120, 180, 240, and 300 days after the HLB bacterium inoculation. The first symptoms of HLB were observed on mature leaves of the check citrus trees 60 days before these symptoms were observed on trees treated with the SAR inducers. Two years after inoculation, 67% fewer citrus trees with HLB symptoms were observed for the ASM + TMX treated trees as compared to the check ones. Citrus trees treated with IMI, TMX, or ASM also provided lower levels of PCR-detected HLB bacterium compared to the untreated trees. Key words: Acibenzolar-S-methyl. 'Candidatus' Liberibacter asiaticus. Citrus spp. Imidacloprid. Neonicotinoids. Thiamethoxan. ResumoHuanglongbing (HLB) é considerada a mais destrutiva doença para a citricultura, e ocorre nos principais países produtores de citros ao redor do mundo. No Brasil, o HLB está associado às bactérias 'Candidatus' Liberibacter asiaticus (CLas) e 'Candidatus' Liberibacter americanus (Clam), sendo a primeira espécie a mais agressiva e disseminada nos pomares brasileiros. Atualmente, não existe controle curativo para esta doença, sendo a erradicação de plantas doentes e o controle do inseto vetor as medidas básicas para o manejo do HLB. A busca por outras medidas de maior eficiência e de menor impacto econômico e ambiental para o controle do HLB tem se tornado prioridade para diversos grupos de pesquisa. A utilização de produtos que ativam os mecanismos de resistência das plantas é uma das principais linhas de investigação. Assim, o objetivo deste estudo fo...

Huanglongbing (HLB) caused by 'Candidatus Liberibacter spp.' is the most destructive disease for citrus production. These bacteria are disseminated naturally by insect vectors, such as the psyllid Diaphorina citri. However, the transmission of the bacteria vertically from seeds to seedlings has been controversial. Therefore, the objective of this study was to re-investigate the vertical transmission of the bacterium 'Ca. Liberibacter asiaticus' (CLas) from seeds to seedlings. Seeds of 'Rangpur' lime, 'Swingle' citrumelo, 'Ponkan' mandarin and the sweet oranges 'Folha Murcha', 'IAPAR-73', 'Pera Rio', 'Shamouti' and 'Valencia' were included in the study. Besides the seeds, 3493 citrus seedlings were also examined for the presence of the bacterium and for the development of HLB symptoms. The presence of the bacterium in different parts of the seeds and in citrus seedlings was examined by standard PCR. CLas was detected in all parts of the seeds, including seed coat or testa, tegmen, and endosperm plus embryos of all citrus cultivars, except for 'Shamouti' sweet orange. Although transient, the presence of the HLB bacterium was also detected in seedlings of 'Ponkan' mandarin and 'Folha Murcha' sweet orange obtained from seeds of HLBinfected trees up to 18 months after seedlings transplanting. However, no typical symptoms of HLB were observed in the citrus seedlings. Although CLas may infect the citrus seeds, the results of this study do not support the vertical transmission of viable cells of CLas from seeds to seedlings.

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