Abstract: To determine outbreak occurrence, economic infection level and economic threshold of downy mildew (Peronospora astragalina) on Astragalus membranaceus in Gansu Province, the disease dynamics and the correlation between disease and weather conditions were analysed based on data from fixed on-site field investigation and systematic field surveys. A curve fitting technique was used to define the relationship between disease index and yield loss for A. membranaceus. Fungicides to control the disease were tested. It was found that the peak period for downy mildew occurrence was from early August to mid-September, the disease risk index was low after mid-September. Statistical analysis revealed that the disease risk index increased gradually with the increasing mean night relative humidity and disease cardinality. The logarithmic regression model (y=33.81 ln x-97.15) adequately described the relationship between disease index (x) and yield loss (y) of A. membranaceus. The economic injury levels for downy mildew on A. membranaceus treated with 5% Pyraclostrobin·55% Metiram diluted 800 times were 5.65%, and the economic threshold was about 17.73. Sensitivity testing for fungicides showed that 22.5% Famoxadone·30% Cymoxanil diluted 1500 times, 5% Pyraclostrobin·55% Metiram diluted 1000 times, 8% Cymoxanil·64% Mancozeb diluted 400 times, 75% Chlorothalonil diluted 350 times and 65% Zineb diluted 500 times could be used in rotation to control powdery mildew on A. membranaceus.