How to Identify Soybean Downy Mildew? A Complete Guide to Prevention and Control Techniques
Downy mildew is found in all major soybean-producing regions of China, occurring more frequently in the cold-climate areas of the Northeast and North China during the soybean growing season, with the disease worsening in rainy years, severely causing premature leaf drop or wilting, seed rot, and yield losses of 30%-50%. It mainly damages the leaves, pods, and seeds of seedlings or mature plants.
Symptoms: The pathogen on infected seeds causes systemic infection leading to seedling disease, but the cotyledons show no symptoms. After the first pair of true leaves unfolds, chlorotic patches appear along both sides of the leaf veins, spreading along the main vein and lateral veins, causing the entire leaf to lose chlorophyll, and sometimes the whole leaf turns yellow. In rainy or humid weather, a grayish-white mold layer grows on the underside of the lesions, and the diseased leaves turn yellow, then brown, and dry up. At the mature plant stage, circular or irregular yellow-green spots with indistinct edges appear on the leaf surface, later turning brown, with a grayish-white mold layer also forming on the underside. The lesions often merge into large patches, and the diseased leaves dry up and die. When pods are infected, external symptoms are not obvious, but a yellow mold layer, namely mycelium and oospores, often appears inside the pods. Infected seeds turn white and lose their luster, with a layer of yellowish-white powdery mold attached to the surface.
Transmission routes and disease conditions: The pathogen overwinters as oospores on diseased plant residues or on seeds. Oospores attached to seeds are the most important source of infection, while oospores on diseased plant residues have fewer opportunities for infection. The oospores germinate as the seeds germinate, producing zoospores that invade from the host hypocotyl, enter the growing point, and spread throughout the plant, becoming a systemic infectious disease, and the diseased seedlings then become a source of re-infection.
Prevention and Control Methods
1. Select varieties with strong disease resistance according to local conditions.
2. Based on the characteristics of the pathogen, promote crop rotation of more than three years to reduce the initial infection source.
3. Use disease-free seeds and carry out chemical treatment before sowing, dressing seeds with 0.3% of seed weight of fosetyl-aluminum or 35% metalaxyl powder.
4. Strengthen field management. When hoeing, pay attention to removing systemically infected diseased seedlings to reduce the infection source in the field. Practice rational close planting and apply more phosphorus and potassium fertilizer.
5. In the early stage of the disease, spraying 40% chlorothalonil suspension concentrate at 600 times dilution, 50% carbendazim wettable powder at 500 times dilution, or 25% metalaxyl wettable powder at 800 times dilution can control the occurrence of soybean downy mildew.