Septoria leaf spot of lemon balm

Septoria leaf spot of lemon balm is an important disease of lemon balm caused by Septoria melissae. The fungus can be seed-borne and may also survive in plant debris and on volunteer plants. Under favourable environmental conditions, the disease can spread rapidly, affecting both yield and quality. Because of its seed-borne nature, the disease can potentially develop on crops under any production system, although those that are overhead watered are more at risk.

Symptoms
Septoria leaf spots are dark brown/black, up to 3mm (⅛in) diameter and angular in shape (being constricted between leaf veins). Spore cases are sometimes visible within leaf spots on the underside of the leaf.

Prevention
As Septoria species can be seed-borne, use of clean seed is important for disease avoidance. Thiram fungicide can be used as a warm water soak for parsley seed to reduce Septoria infection to acceptable levels. However, under EC regulations this is not permitted for seed being used for organic production. There is potential for reducing levels of Septoria species on seed using hot water treatment, but treatment conditions are critical and are not well defined for parsley seed. In Sweden, research is ongoing to adapt a seed treatment technique originally developed for cereals, for use on vegetable. The technique involves treatment of seed with hot humid air and precise control of temperature, air humidity and treatment duration.