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The effect of foliar applied potassium chloride on Septoria tritici on winter wheat was quantified and possible modes of action investigated by in vitro, glasshouse, controlled environment and field experiments between 1996 and 1999. In vitro experiments showed that as the concentration of potassium chloride increased, mycelial growth and germination of conidia decreased (EO sos of 1.36M and 0.7M, respectively). One glasshouse and two field experiments showed a significant reduction in the leaf area affected by S. trttict after foliar application of potassium chloride compared to untreated controls. In general, application of potassium chloride reduced the leaf area affected by S. tritici by 20 -.40%. However, a significant yield increase was not observed. Potassium chloride applied to the lower leaves of winter wheat did not confer systemic acquired resistance against S. tritici on the upper leaves. Inhibition of conidial germination, on leaf surfaces by potassium chloride was observed. Similar inhibition was observed when polyethylene glycol, an inert osmoticum, was applied at the same calculated osmotic potential. During field experiments there was no significant difference in the leaf area affected by S. tritici in plots treated with potassium chloride or polyethylene glycol, although both significantly reduced the leaf area affected compared to untreated control plots. Therefore, the principal mode of action of potassium chloride was proposed to be as a result of adverse osmotic conditions caused by the salt on leaf surfaces. However, the addition of a range of adjuvants to potassium chloride did not increase the efficacy of S. tritici control in glasshouse studies. The results from this study show that potassium chloride, when applied to foliage of winter wheat can reduce the leaf area affected by S. tritici and it is proposed that this reduction was by adverse osmotic conditions caused by the salt on leaf surfaces.