Influence of rootstocks on the resistance of grafted cucumber (Cucumis sativus L.) scions to powdery mildew (Podosphaera xanthii U.!Braun & N.!Shishkoff)

Whether or not powdery mildew resistant and susceptible rootstock cultivars would impart these characteristics to the grafted cucumber scions was investigated. Two newly released pumpkin rootstock cultivars used for bloomless cucumber production, 'Tokiwa Power Z' and 'White Power'. were found to be highly resistant to powdery mildew from the seedling stage (tested parts: cotyledons) to the adult stage (tenth true leaves) at cool to moderate temperatures. The accession PPMR-1, a blooming rootstock, showed moderate to high levels of resistance, especially during the adult stage. The conventional bloomless rootstock 'Hikari PowerGold' and the conventional blooming rootstock 'Shin-tosa' were found to be susceptible. The rootstocks had little to no influence on the resistance to powdery mildew of the grafted scions at younger stages, even though the levels of resistance of the rootstock cultivars differed considerably. Scions, grafted onto PPMR-1 or 'Shin-tosa' rootstocks, showed increasing powdery-mildew resistance or tolerance with growth (tested parts: fifth and seventeenth true leaves). The powdery-mildew-resistant 'Tokiwa PowerZ' and 'White Power' did not impart high levels of resistance to scions, whereas 'Hikari Power Gold', which is used for bloomless cucumber production, reduced the resistance of scions to powdery mildew. These results indicate that rootstock cultivars, including certain pumpkin rootstocks, e.g., PPMR-1, can alter the powdery-mildew resistance or tolerance of cucumber scions at the adult stage.