What pesticide should be used for cabbage downy mildew? What are the main symptoms?
Cabbage downy mildew, commonly known as white mildew disease, frosty leaf disease, etc., is a fungal disease very widely distributed in cabbage-growing regions across the country. So what pesticide should be used for cabbage downy mildew? What are the main symptoms?
Cabbage downy mildew, commonly known as white mildew disease, frosty leaf disease, etc., mainly damages the leaves. At first, grayish-white, pale yellow or yellow-green lesions with indistinct edges appear on the upper surface of the leaves, then expand into yellow-brown lesions. Because the lesions are restricted by the leaf veins, they take on a polygonal or irregular shape, and dense white frosty mold grows on the underside of the leaves. When there are many lesions, they connect with each other, the leaves turn yellow and withered, causing part or all of the diseased leaf to die. The disease develops from the outer leaves toward the inner leaves, and diseased plants often dry up layer by layer from outside to inside. In severe cases, the plant cannot form a head, or only a tiny heart-leaf ball remains.
Disease pattern
1. Mode of transmission:
In the north, the downy mildew pathogen overwinters and oversummers in the soil as oospores with diseased plant residues, or as mycelium inside seed plants. On rainy days or during irrigation, it is splashed onto plants by water; after germination it produces germ tubes that directly invade the host tissue through stomata or the epidermis, causing primary infection. It then develops into mycelium, spreads in the intercellular spaces, causes pathological changes in the host tissue, and produces sporangiophores and sporangia from the diseased areas. The sporangia spread through airflow, causing secondary infection and leading to disease spread. Mycelium overwintering inside seed plants can also produce sporangiophores and sporangia from the inside in the following spring, causing secondary infection.
2. Conditions for disease occurrence:
The occurrence and epidemic of downy mildew are affected by many environmental factors, but it is most closely related to temperature and humidity. Temperature determines how early the disease appears and how fast it develops. The optimum temperature for the formation and germination of sporangia is 7-13°C, the optimum temperature for infecting the host is 16°C, and the optimum temperature for mycelial growth and development is 20-24°C. Therefore, when the temperature is 15-24°C and the difference between day and night temperatures is large, it is most favorable for the occurrence and epidemic of this disease. Above 20°C, lesions develop fastest, and especially at 24-25°C, yellow-brown lesions are very easily produced.
Main symptoms of downy mildew
Chinese cabbage can develop downy mildew at every stage of growth, and all parts of the plant can be damaged, but the leaves are damaged most severely, followed by the stems, flower stalks and seed pods. During the seedling stage, white downy mildew-like material appears on the underside of the leaves, while there are no obvious symptoms on the upper surface; in severe cases, the leaves and young stems turn yellow and die. At the adult plant stage, the disease generally starts first on the outer leaves. At first, pale green or yellow-green spots appear on the upper surface of the leaves. In the middle stage of the disease, the lesions gradually expand and their color also turns pale yellow or light brown. The expansion of the lesions is restricted by the leaf veins, so they become polygonal or irregular. In the late stage of the disease, the lesions turn yellow or yellow-brown. After entering the heading stage, if environmental conditions are suitable, the disease develops very quickly, the lesions increase rapidly, and the leaves gradually turn yellow and dry from the outside inward, until finally only a head remains; when wind blows or people walk in the field, a “white smoke” rises. When the stems and flower stalks of seed plants are infected, the diseased parts become swollen, bent and deformed, commonly known as “dragon-head crook”; when seed pods are damaged, the fruits set few or no seeds. White downy mildew-like material grows on all diseased parts.
Cabbage downy mildew has a long occurrence period, starting from the seedling stage, with the peak period from early September to the end of October. Its occurrence characteristics are:
1. Variety factors
Practice has shown that there are significant differences in disease resistance among Chinese cabbage varieties; generally, the tall-pile type with dark green leaves has high resistance;
2. Planting and management factors
Early sowing, continuous cropping land, high density, low-lying terrain, water accumulation, high humidity, poor drainage, lack of fertilizer during the heading stage, and plots with weak growth vigor are favorable for the occurrence of the disease.
3. Climatic factors
Large temperature differences between morning and evening, heavy fog and dew, alternating sunny and rainy weather, and relatively high humidity make the disease more likely to occur; more rainfall, frequent fog and dew, and relatively high temperatures make its occurrence more severe. The occurrence of downy mildew is related to climatic conditions, variety resistance, cultivation measures, etc., among which climatic conditions have the greatest influence.
Chinese cabbage downy mildew occurs and develops quickly, so prevention should be the main approach in control, and chemical control must be carried out in the early stage of the disease. Specific measures:
1. Strengthen water and fertilizer management
On the basis of applying sufficient organic fertilizer, phosphate fertilizer, potassium fertilizer and other base fertilizers, topdressing should be mainly nitrogen fertilizer. At the seedling stage, rosette stage and heading stage, the amount of urea applied each time should generally not exceed 30 jin per mu. Watering should be done little but frequently, and drainage should be timely after rain. During the rosette stage, priority should be given to promoting growth, with timely watering to meet the water needed for growth.
2. Thin seedlings early, finalize seedlings late, and control seedling growth moderately
3. Foliar fertilization
Combined with the control of pests and diseases, spraying foliar fertilizer can improve plant disease resistance.
4. Chemical control
Choose the following pesticides: 69% dimethomorph, i.e. Anke, 20 grams; 68.75% fluopicolide propamocarb, i.e. Infinito, 30 milliliters; 30% kresoxim-methyl, i.e. Cuibei, 20 milliliters; 66.5% propamocarb, i.e. Previcur, 40 milliliters; 72% Kelu, 20 grams; 72% Kekangling, 25 grams. Choose one of the above pesticides, add 30 jin of water per sprayer and spray on the leaves. They can be used alternately, at an interval of 5 days, for 2-3 consecutive sprays. During the disease period, special attention should be paid to application after rainfall. The pesticides used to control cabbage downy mildew can also be mixed with pyrethroids, emamectin benzoate, thiamethoxam, pymetrozine, imidacloprid and foliar fertilizers, which can also control leaf-eating insects, boring insects and whiteflies, achieving simultaneous control of pests and diseases and reducing the number of applications.