The susceptibility of Pseudoperonospora cubensis (causal agent of cucumber downy mildew) to cucumber was conducted using leaf discs on the effect of cucumber leaf ages, effect of different inoculums, and effect of surfac tants (methanol, Tween 20, and the mixture of them).Results showed that the leaves from position 4 to 6 in a plant was more susceptive, greener and fresher for disease incidence evaluation; 1 × 104 sporangia/ml inoculum of P.cubensis was more expediency with consistency of results and easy to get the mounts of sporangia; 0.005% Tween 20 with or without 0.02% methanol had no effect on the disease incidence.Thus, they were chosen for next baseline sensitivity and resistance monitoring studies.In 2007 growing seasons, 90 single-lesion isolates of P.cubensis col lected from Liaoning and Jiangsu provinces, which had never treated with CAA fungicides, were used to test basen line sensitivities to dimethomorph.Result exhibited the baseline sensitivities was distributed as unimodal curve, with individual EC50 values ranged from 0.0176mg/L to 0.5572 mg/L (mean 0.1879 (± 0.1251) mg/L).Based on this data, a total of 535 single-lesion isolates of P.cubensis were obtained from infected cucumbers in plastic green houses or fields in 2007 and 2008 in Shanxi, Shandong, Anhui, Jiangsu, Henan, Hainan and Hulei provinces in China, where CAA fungicides had been extensively applied ; and detedted for the sensitivities to dimethomorph at dis criminatory concentrations of0, 4, 8 and 16 mg L.Results showed that all the isolates presented highly sensitivity or low-resistance to the fungicide, and no resistant isolates were detected.Therefore, dimethomorph could be remain as the major fungicide for controlling cucurbit downy mildew in China.