<em>Powdery mildew</em> caused by<em> Podosphaera xanthii</em> and<em> Golovinomyces cichoracearum</em> is one of the principal diseases on cucurbit crops in temperate climates. In order to control this disease, various biological methods, including induced resistance by the use of extracts from<em> Fallopia sachalinensis</em>, have been identified. This study was conducted to characterise the diversity of susceptibility to this plant extract among 52 isolates of <em>P. xanthii</em> and 5 isolates of <em>G. cichoracearum</em> collected from various cucurbit species in different production areas. To this end, disks excised from melon leaves were soaked in a preparation of<em> F. sachalinensis</em> extract (1% w/v) or in a control solution, inoculated with fresh conidia of powdery mildew 24 hours after treatment and placed in a growth chamber. Ten days after inoculation, symptoms were rated in classes based on a visual estimation of the leaf area infested by powdery mildew. Additionally, spore production on the leaf disks was assessed. The plant extract significantly decreased the severity of disease for all the powdery mildew isolates tested. On average, spore production was only ca. 20% in presence of the plant extract relative to the untreated control. However, the extent of the reduction in spore production variedwidely among isolates.