Tomato (Lycopersicum esculentum Mill), which is cultivated extensively throughout the world, is one of the most valuable crops in the Philippines. However, its production is affected by pest and diseases, particularly early blight disease caused by Alternaria solani which significantly affects crop growth and yield. Citronella oil (CO) have promising inhibitory effect for controlling plant diseases and are seen as safer alternative measure from the harmful effect of fungicides.  Hence, this study was conducted to evaluate the antifungal effects of CO at different concentrations on the growth of A. solani in vitro, and to determine the phytotoxic response and growth effects of CO at different concentrations on tomato plants. Moreover, the study also evaluated the efficacy of CO in controlling the incidence, severity and progress of early blight infection on tomato caused by A. solani under nursery condition. The antifungal potential of CO significantly inhibited the mycelial growth of the A. solani with 100% inhibition up to 10 days of observation. The phytotoxic response of CO showed that at 0.1% and 0.3% concentrations has no significant immediate negative effect in tomato plant. At the same rate, CO showed significant increase on plant height. However, CO at all concentrations, was not able to delay disease development as indicated by a 100% disease incidence, but the development of the infection on tomato tissues brought by A. solani inoculation was partially controlled by CO application at the rate of 0.3%. There is a potential promise from citronella oil application as an effective alternative to chemical control against tomato early blight disease.