Thyme Essential Oil as an Eco-Friendly Alternative for Managing Powdery Mildew of Cilantro

Powdery mildew, caused by Erysiphe heraclei DC., is a destructive disease of cilantro (Coriandrum sativum L.). The pathogen spreads rapidly through airborne spores and can cause significant yield losses. However, reliance on synthetic fungicides is increasingly constrained by the development of fungicide resistance, as well as growing concerns over food safety and environmental sustainability. Therefore, environmentally friendly alternatives for powdery mildew management are urgently needed. This study evaluated the efficacy of thyme (Thymus vulgaris L.) essential oil (EO) for managing cilantro powdery mildew under greenhouse conditions. Three concentrations of thyme EO (0.03%, 0.06%, and 0.12%) were applied either as a single preventive spray or as a double application consisting of preventive and curative sprays. Disease severity and the first onset of symptoms were assessed following artificial inoculation with conidia using a custom-developed spore dispersal hood. All thyme EO treatments significantly reduced powdery mildew severity compared with the untreated control. The double-application strategy consistently delayed symptom onset until 12 days post-inoculation (DPIs) and provided greater disease suppression (55–66%) than the single preventive application, in which symptoms first appeared at eight DPIs and severity was reduced by 25.5–36.6%. Among the treatments, double applications of 0.12% thyme EO provided the highest level of disease control and significantly delayed symptom development. In addition, a custom-designed powdery mildew spore-dispersal hood enabled uniform conidial inoculation, resulting in consistent disease development in control plants and providing a reliable system for inoculating powdery mildew and evaluating fungicide efficacy under in vitro conditions. To our knowledge, this is the first study to demonstrate the effectiveness of thyme EO for managing cilantro powdery mildew caused by E. heraclei under greenhouse conditions. These findings highlight the potential of thyme EO, particularly when applied using a preventive and curative strategy, as an environmentally friendly alternative to conventional fungicides for sustainable powdery mildew management in cilantro and other leafy vegetable crops.