Cassava is an economically important food crop in Thailand. Leaf spot is important disease on cassava, could cause yield loss up to 30%. The fungicides play an important role for control plant disease but it showed high potential for residue problem, phytopathogens resistance and environmental pollution. The nanoparticles (NPs) are an ecologically promising way to manage plant disease. The objectives of this study were to evaluate the efectiveness of chitosan nanoparticle (CS-NP) formulations on reducing (1) cassava leaf spot by applying methods (pre-inoculation and post-inoculation) at net-house and (2) potential virulence of Alternaria alternata in detached leaves. The results showed that the control efcacy of the mean both applying methods was 39.79–43.62% in CS-NP formulations, which similar with futriafol (39.66%) and signifcantly higher than other solutions. Moreover, the CS-NP loaded salicylic acid 400 ppm (T8) and CS-NP loaded silver 200 ppm (T9) could inhibit conidial germination by 32.51 and 23.25%, respectively. The soaking of A. alternata H-Vi 7 plugs into these two solutions could inhibit mycelia growth and their potential virulence causing rot lesions in detached leaves by 42.26 and 51.11%. While spraying T8 and T9 inhibited rot diameter by 44.95 and 42.79%, respectively. However, the futriafol fungicide was the highest inhibition, with 70.02% in soaking, 58.41% in spraying. Furthermore, these solutions caused higher cell death/live rate than the zinc oxide NP but lower than futriafol fungicide. These results indicated CS-NPs loaded salicylic acid and silver can be used as promising solution in cassava leaf spot management.