Epidemiological study of Downey mildew (Peronospora parasitica.) on Cucumis Sativus

Abstract An experimental investigation was carried out to examine the conidial germination of Peronospora parasitica under different temperature and humidity conditions. Results indicated that the highest germination occurred at 30°C under conditions of high relative humidity, although the overall germination rate remained low. No germination was recorded at temperatures below 20°C, above 30°C, or when humidity dropped below 90%. Field studies on pea plants demonstrated that powdery mildew symptoms first appeared as small, white, powder-like patches on the upper surface of leaves. These spots gradually increased in size, eventually covering entire leaves, petioles, stems, and pods. The earliest leaves did not exhibit symptoms until 7 to 8 weeks after germination, whereas newly formed leaves showed symptoms within 2 weeks. Conidial germination was found to be significantly lower on younger leaves (2 weeks old) compared to older ones (6 weeks old), suggesting greater susceptibility with leaf age. Further field observations revealed that the disease became visible approximately one week following extended periods of leaf surface wetness (around 12 hours) in combination with high humidity (close to 90%). Initial infections typically appeared in small, scattered patches before spreading more extensively across the foliage.