Figure 1: Histochemical localisation of esterase activity in infectious structures of Erysiphe necator by optical microscopy after deposition on artificial medium containing indoxyl acetate. (A) Heat-killed conidium displaying no indigo blue crystals. (B) Ungerminated conidium showing indigo blue crystals (arrow). (C) Conidium and germ tube displaying indigo blue crystals (arrows). (D) Conidium and appressorium displaying indigo blue crystals (arrows); the blue colour is present in the overall appressorium. a: appressorium, c: conidium, and h: hypha. Scale bars represent 30 μm.