Screening for Resistance to Downy Mildew among Major Commercial Cucumber Varieties

belonging to 2 wild species and 20 varieties of the genus Cucumis, originating mainly from the Asian Vegetable Research and Development Center (AVRDC) Gene Centre, was evaluated for resistance to Pseudoperonospora cubensis, causal agent of cucumber downy mildew. The youngest fully expanded true leaves were found suitable for in vitro screening. Both leaf discs and full leaves could be kept fresh longer when applying 0.2 µg/ml of gibberellin acid (GA). The incubation temperature of 20oC was found to be the most suitable temperature for symptom development comparing with 15 and 25oC. Symptom development was faster when contact diseased leaf discs (2 weeks after inoculation) on to fresh leaf samples comparing with using conidia suspension (105 spores/ml). The numbers of spots in ‘C-19 ’ were lower than other varieties. ‘C-19 ’ variety was also showed the highest level of downy mildew resistant at 20oC chamber in 6 days after inoculating with pathogen and displayed 0.90 (under 10%) of the infected rate. However, other varieties displayed susceptible in the pathogen sprayed plots. ‘C-19 ’ was the most resistant variety and no lesion wa