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Hypertrophy sorosis scleroteniosis caused by <i>Ciboria shiraiana</i> is the most devastating disease of mulberry fruit. However, few mulberry lines show any resistance to <i>C. shiraiana</i>. An increasing amount of research has shown that host-induced gene silencing (HIGS) is an effective strategy for enhancing plant tolerance to pathogens by silencing genes required for their pathogenicity. In this study, two G protein α subunit genes, <i>CsGPA1</i> and <i>CsGPA2</i>, were identified from <i>C. shiraiana.</i> Silencing <i>CsGPA1</i> and <i>CsGPA2</i> had no effect on hyphal growth but reduced the number of sclerotia and increased the single sclerotium weight. Moreover, silencing <i>CsGpa1</i> resulted in increased fungal resistance to osmotic and oxidative stresses. Compared with wild-type and empty vector strains, the number of appressoria was clearly lower in <i>CsGPA1-silenced</i> strains. Importantly, infection assays revealed that the virulence of <i>CsGPA1-silenced</i> strains was significantly reduced, which was accompanied by formation of fewer appressoria and decreased expression of several cAMP/PKA- or mitogen-activated protein-kinase-related genes. Additionally, transgenic <i>Nicotiana benthamiana</i> expressing double-stranded RNA targeted to <i>CsGpa1</i> through the HIGS method significantly improved resistance to <i>C. shiraiana</i>. Our results indicate that <i>CsGpa1</i> is an important regulator in appressoria formation and the pathogenicity of <i>C. shiraiana</i>. <i>CsGpa1</i> is an efficient target to improve tolerance to <i>C. shiraiana</i> using HIGS technology.