Cucurbit downy mildew (CDM), is an important disease of cucurbits in worldwide. Under conductive condition, the disease is highly destructive and the pathogen spreads rapidly with in a field and on a regional scale. Pseudoperonospora cubensis, causal of cucurbit downy mildew, is an obligate biotroph that infects by wind-blown sporangia that land on the leaf surface. Our studies in fact doen on cucumber leaves, because the fungus is an obligate pathogen. A new method was designed, deu to lack of growth chamber and the problems causing disease in greenhouse. For this regards, 7-day-old seedling (Superdaminos cultivar) were transferred to potting soil, kept at 25 °C, 16 hours photoperiod and at a relative humidity (RH) of 70%. Seedling at the three to four leaves stage were used for experiment. Then three Leaves of each plant were detached and inoculated with pathogen. For preparing of P. cubensis inoculum leaves with symptoms of disease were collected from disease nursery and kept in the dark place at 100% RH overnight for producing of sporangia. Sporangia were collected from the lower side of leaves using a brush and suspended in sterilized water. The concentration of sporangia adjusted to 2×104 sporangia per milliliter by a hemacytometer. The inoculated leaves were kept in plastic box with 21°C and >90 percent relative humidity. Non petiolule leaves, inoculated with sporangia suspension of P. cubensis, in plastic box, for 24h in the dark. The plastic box containing wet cotton, moist whatman paper and cucumber leaves (lower surface uppermost), respectively. The plastic box transferred to the various locations such as, laboratory, sunshine and under lath house conditions, at a 16h-8h (light-dark) photoperiod at a temperature of 18-21°C, respectively. In these conditions, sporulation occurred at 6-10 days post inoculation (dpi). Results showed they were only placed under lath house conditions occurred oil spot symptoms and leaves rooting
