A study on the identification and spread of harmful flora on lemon balm (Melissa officinalis) in the region of Dobroudzha was conducted during the period 2015 – 2016. The research established that the percentage of the diseased plants (in a plantation of 1 ha) was 9.42%, while the index of the infected leaves was 19.25%. Small, irregular, light brown to necrotic spots with a darker edge were formed on the leaves. The spots were randomly scattered over the whole lamina. On the branches were found elongated light brown spots, resulting in the formation of shells as well as partial or complete drying of the plants. A pycnidia fungus was isolated from the leaves and branches with typical symptoms. The pathogenicity of the isolates was proven. The morphological and cultural characteristics were tested. On this basis the pathogen Phyllosticta decidua Ellis et Kellermann was identified. The fungicidal activity of 6 products with a different active base as well as of two biological extracts was tested in vitro.