Oomycetes Phytophthora infestans, pathogen of potato and tomato late blight, is one of the most devastating pathogen in the incidence years. The aim of this research was the study of the antifungal effect mushroom water extract of Shiitake (Lentinula edodes) against pathogenic oomycetes Ph. infestans in vitro. Shiitake mushrooms were completely dried after collecting in the hot air. The mushrooms were completely powdered with a greengrocer. Amount of 20 grams of dry powder of shiitake mushrooms mixed with 100 mililiter sterilized distilled water and put in 65°С water bath for 24 hours. Mixtures were placed in refrigerator for 72 hours under 4°С temperature. After transmission of mixtures among filter paper, 100% concentration of water extract was prepared. Concentrations of 75, 50 and 25% were prepared by adding sterile distilled water. Concentration of zero was considered as control and for comparison. Extracts were mixed with PDA (potato-dextrose-agar) culture medium using poisoned food method. In spring 2017, during the several visits on the potato fields of the western regions of Golestan province, samples with blight symptoms were collected and transferred to the laboratory. Isolation and purification of pathogen were conducted on rye B agar culture media by adding tetracycline antibiotics and benomyl fungicide at 500 ppm. Petri dishes were inoculated with a 5mm pill of seven day pathogenic pseudofungus cultivation margin. The experiment was done as completely randomized design with five treatments in Petri dishes with five replications. The average of colony radial growth in treatment and control Petri dishes on vertical coordinate axis was measured daily by milimeter ruler. Until colony completely grew up in control Petri dishes, radial growth measurement countinued. The percent of colony growth inhibitory for each extract was calculated using Abbott's formula. In order to determine the antifungal property, the 5 mm pill was transferred from the treatment Petri dishes with a inhibitory effect of 100% to the control (no extract).The results of analysis of data variance showed that there was a significant difference between experimentet treatments at statistical level (p0.001). Water extract of Shiitake mushroom at concentrations of 100, 75 and 50% had inhibitory effect of 100% and at 25% concentration, inhibitory effect of 38.50% on radial growth of oomycetes colony of Ph. infestans in comparison to the control in vitro. No growth of the pill after 14 days in control Petri dishes showed that the water extracts of Shiitake mushroom at concentrations of 100, 75 and 50% had fungicide (fungitoxic) property.