A 2 year field trial was conducted using established English oak (Quercus robur L.) to assess the efficacy of four commercially available systemic-inducing resistance (SIR) compounds (salicylic acid, potassium phosphite, harpin protein, betaine) applied as a single therapeutic spray treatment against the foliar pathogen oak powdery mildew (Microsphaera alphitoides). In addition, a comparative evaluation of a conventional spray program (3 week spray intervals) used within the United Kingdom for powdery mildew control was conducted using the fungicide penconazole. The SIR-inducing compound containing betaine and a single spray treatment of penconazole had no significant influence on disease severity and specific activity of peroxidase and superoxide dismutase in both the 2005 and 2006 trials. Salicylic acid and potassium phosphite had no significant long-term effect on disease severity, although a short-term reduction in disease severity was recorded that was associated with enhanced leaf peroxidase and superoxide dismutase activity. A single therapeutic application of the SIR-inducing agent harpin protein significantly reduced disease severity of powdery mildew in the 2005 trial. No significant effects, however, were recorded in the 2006 trial. Only repeat spray applications of penconazole significantly reduced disease severity of oak powdery mildew in the 2005 and 2006 trials. The fungicide penconazole appears also to posses marginal SIR properties.