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<i>Candidatus</i> Liberibacter asiaticus’ (Las) is the globally prevalent species of Liberibacter pathogens that cause huanglongbing (HLB), a worldwide destructive disease of citrus.  Like other bacterial pathogens, Las encodes different effectors to regulate host responses. In this study, we characterize several putative Las effector genes via agrobacterium-mediated transient expression and/or stable transformation since Las is not culturable. Using N-terminal and/or C-terminal GFP tagging, we were able to visualize two putative effectors targeting the mitochondria and two putative effectors targeting the chloroplasts of inoculated plants or transgenic plants. In addition, the expression of these effector genes was confirmed with polyclonal antibodies against their respective antigens. The association of the effectors with pathogenesis was determined in transgenic plants and through transient expression at cellular and molecular levels. Transgenic citrus plants expressing one of the chloroplast-targeting effector genes displayed HLB-like symptoms with yellow shoots and retarded growth. A dramatic proliferation of root hairs was observed in transgenic Arabidopsis expressing one of the mitochondrion-targeting effector genes<i>.</i> To identify molecules that interfere with the function of these effectors and disrupt Las infection, we have developed a novel <i>in vitro</i> screening system that can evaluate small molecules effective against Las effectors in less than two weeks.