Abstract: Previous research works by our group have demonstrated that hexanoic acid(priming agent, Hx) induces resistance in Solanum lycopersicum and Arabidopsisthaliana plants against Botrytis cinerea. Alternaria alternata is the cause of abandonmentof the hybrid mandarin Fortune (C. tangerina x C. clementina) due to Alternaria brownspot damage. In this work, induced resistance against citrus plants has been assessed. Inorder to study the mechanisms implied in IR-Hx against A. alternata, a histochemicalanalysis was carried out to assess the effect of reactive oxygen species ROS and callosedeposition on production. We previously demonstrated that the number of infected leavesand the diameter of infection diminished when treating citrus plants with hexanoic acid1mM, and that the physiological parameters improved. A. alternata infection quicklyincreased H2O2 and superoxide ions levels in tissues. However, the Hx priming agentreduced the H2O2 level in leaves by at least four times on day 5 post-infection ifcompared with control plants. Likewise, superoxide lowered in treated plants.Furthermore, the priming agent seemed to considerably accumulate callose around thenecrotic area, which could delay the development of infection and induce a faster andstronger accumulation of OPDA, JA-Ile and ABA. Hence, the obtained results supportthe idea that oxygen species ROS, callose and the JA pathway could be involved in Hx-IR in citrus plants against A. alternata.