Studies on the effect of manuka oil (0.1%) and patchouli oil (0.1%) on growth of fungal colonies of Trichoderma harzianum were carried out in 34-day cycles. The preparation Sporgon 50 WP (0.3%) was used as a reference. Observations were conducted using the traditional method and by computer image analysis. A comparative analysis was performed on the course of variation in the inhibition of T. harzianum colony growth. A highly significant variation was found in the course of the investigated process when using selected essential oils and the preparation Sporgon 50 WP. The efficacy of oils was similar; however, they differed in terms of the course of variation in their effect on T. harzianum growth.