SummaryThe efficacy of candidate activators of systemic acquired resistance (SAR), Bion® (Syngenta, Basel, Switzerland), Rezist® (Stoller Enterprises, Houston, Texas, USA), and Milsana® (P. J. Margo Pvt. Ltd., Bangalore, India), was evaluated for the control of powdery mildew (Podosphaera xanthii) on cucumber and zucchini. Each candidate activator significantly reduced the incidence of disease on both crops in a glasshouse, where spore inoculum was applied to treated leaves. Both Bion® and Rezist® induced increased chitinase activity in plants, confirming their action as activators of SAR. However, Milsana® did not have this effect, suggesting that its mode of action was as a direct inhibitor of infection by spores. In the field, two or three applications of Bion®, at 14-day intervals, gave consistently better results in protecting cucumber plants from powdery mildew. Several applications of Milsana® alone were less effective than a single application of Bion® alone. However, several applications of Bion® adversely affected plant growth, resulting in lower yields. The application of Milsana® before or after Bion® provided effective control of powdery mildew and minimised the adverse effects of Bion® on yield. Application of all three candidate activators alone, on field-grown zucchini did not control powdery mildew, although a systemic increase in chitinase activity occurred in response to Bion® or Rezist®. A greater understanding of the mode of action of Milsana®, and of the interaction between disease load and the timing of activator application(s) may improve the control of powdery mildew.

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