Resistance(R) genes against plant pathogens often have age-related resistance(ARR) effects. However, the mechanism involved in this phenomenon remains unknown. In this paper, Solanum lycopersicum ‘CLN2037B’ and S. pimpinellifolium ‘L3708’ harboring the Ph-3 gene, as well as S. habrochaites ‘LA2099’, ‘LA1777’ and ‘LA1033’ harboring quantitative trait loci(QTLs), were tested to investigate age-related resistance against late blight(LB; caused by Phytophthora infestans) in the three-leaf stage of the plants. The results demonstrated that the QTL-related LB resistance showed the same age-related resistance as the Ph-3-mediated resistance at the six- and nine-leaf stages compared with the three-leaf stage. This indicated that there is a common defense mechanism in tomatoes against P. infestans via ARR. In addition, we combined ethylene(ET), salicylic acid(SA) and jasmonic acid(JA) mutants with virus-induced gene silencing(VIGS) to study the Ph-3-dependent resistance signaling pathway. The results showed that ethylene and salicylic acid, but not jasmonic acid, are involved in the LB resistance mediated by the Ph-3 gene. However, the mechanism involved in this phenomenon remains unknown. In this paper, Solanum lycopersicum ’CLN2037B’ and S. pimpinellifolium ’L3708’ harboring the Ph- 3 gene, as well as S. habrochaites ’LA2099’, ’LA1777’ and ’LA1033’ harboring quantitative trait loci (QTLs), were tested to investigate age-related resistance against late blight (LB; caused by Phytophthora infestans) in the three -leaf stage of the plants. The results demonstrated that the QTL-related LB resistance showed the same age-related resistance as the Ph-3-mediated resistance at the six- and nine-leaf stages compared with the three-leaf stage. indicated that there is a common defense mechanism in tomatoes against P. infestans via ARR. In addition, we combined ethylene (ET), salicylic acid (SA) and jasmonic acid (JA) mutants with virus- induced gene silencing (VIGS) to study the Ph-3-dependent resistance signaling path way. The results showed that ethylene and salicylic acid, but not jasmonic acid, are involved in the LB resistance mediated by the Ph-3 gene.