Auxin is throughout the entire life process of plants and is involved in the crosstalk with other hormones, yet its role in apple disease resistance remains unclear. In this study, we investigated the function of auxin/indole-3-acetic acid (IAA) gene MdIAA24 overexpression in enhancing apple resistance to Glomerella leaf spot (GLS) caused by Colletotrichum fructicola (Cf). Analysis revealed that, upon Cf infection, 35S::MdIAA24 plants exhibited enhanced superoxide dismutase (SOD) and peroxidase (POD) activity, as well as a greater amount of glutathione (reduced form) and ascorbic acid accumulation, resulting in less H₂O₂ and superoxide anion (O₂^.−) in apple leaves. Furthermore, 35S::MdIAA24 plants produced more protocatechuic acid, proanthocyanidins B1, proanthocyanidins B2 and chlorogenic acid when infected with Cf. Following Cf infection, 35S::MdIAA24 plants presented lower levels of IAA and jasmonic acid (JA), but higher levels of salicylic acid (SA), along with the expression of related genes. The overexpression of MdIAA24 was observed to enhance the activity of chitinase and β-1,3-glucanase in Cf-infected leaves. The results indicated the ability of MdIAA24 to regulate the crosstalk between IAA, JA and SA, and to improve reactive oxygen species (ROS) scavenging and defense-related enzymes activity. This jointly contributed to GLS resistance in apple.