Evidence for an osmotic mechanism in the control of powdery mildew diseaseof wheat by foliar-applied potassium chloride

The mechanism by which foliar application of potassium chloride solution re duces symptoms of powdery mildew disease (Erysiphe graminis f.sp. tritici) of wheat was investigated. The hypothesis that potassium chloride reduces m ildew by an osmotic effect on spore germination was tested in three glassho use experiments. Either potassium chloride solution or the osmoticum polyet hylene glycol 200 was sprayed on wheat at the three- or four-leaf stage. Th e plants were inoculated with spores and spore germination and leaf area af fected by mildew were assessed. Leaf water potential was determined as a me asure of the osmotic effect of the treatments. Spore germination and leaf a rea affected by mildew were related to leaf water potential using regressio n analysis in groups on the data averaged over the three experiments. Both spore germination on the leaf and leaf area affected by mildew were reduced as leaf water potential decreased. There was no difference between potassi um chloride or polyethylene glycol in the relationship between spore germin ation and leaf water potential, but polyethylene glycol was slightly less e ffective at reducing mildew symptoms at any given leaf water potential. The results are compatible with the hypothesis that potassium chloride reduces symptoms of powdery mildew by an osmotic effect on spore germination.