Causes and Control of Corn Downy Mildew

Maize downy mildew is a kind of fungal disease. It originally occurred in the tropical and subtropical regions. In recent years, it has been established in 18 provinces, cities and districts in Jiangsu, Sichuan, Yunnan, Shandong, Ningxia, Hebei, Liaoning, Gansu, Shanxi and Shaanxi. There have also been serious losses in local areas.

There are four major pathogens that cause corn downy mildew, all of which belong to the more specific oomycetes in fungi, and are specialized parasites. Among them, ophiobacter officinalis can produce oospores, and other pathogens mainly rely on mycelium to overwinter in the soil and manure along with the diseased plant residues. As oospores and diseased bodies can spread long distances with the seeds and form new disease areas, Therefore, corn downy mildew is also an important entry and domestic quarantine target in China.

It has been observed that the symptoms of downy mildew caused by different pathogens infecting at different times may be quite different. Early, especially in the seedling stage, pathogens can enter the stem through the leaves and sheaths and reach the growing point. In this way, there will be malformation of the whole plant, including dwarfing of seedlings, shortening of internodes, bending or twisting of stalks, abnormality of male and female ear, etc. Some diseased plants are long and broad, and they do not draw or have pollen. Usually the diseased seedlings will die or the adult plants will die prematurely. This symptom is similar to the white incidence of millet, so it is also called "crazy disease." Another characteristic of the symptoms is that the leaves exhibit white stripes or yellow-green stripes, and the back of the diseased leaves produces white downy mildew, which releases the zoosporangium. It can spread by wind and rain, causing the incidence of leaf and sheath.

Since pathogens depend on zoosporangia, water is required for the generation, survival, germination, and infestation of zoosporangia. Therefore, in rainy years and low-lying areas, flooding of corn seedlings and flooded fields are more severe. In some places in the northern part of the country, the incidence of heavy crops and wheat in corn fields is heavy. Increasing planting density is conducive to disease occurrence.

For the above morbidity, the following preventive measures can be taken. Use resistant varieties. To build water conservancy projects to prevent stagnant water, especially maize seedlings; to clear field crops in the autumn, to apply organic fertilizers in the spring; to remove the diseased plants in a timely manner in the growing season and concentrate on the treatment. The quarantine measures shall not be adjusted from the epidemic areas. In the early stage of onset, spray 90% triethylaluminum phosphate wettable powder 400 times liquid or 64% caofforin (wet poisoning) wettable powder 500 times liquid, 72% gelu (frozen urea manganese zinc) wettable powder 700 times liquid, 12 % Green Milk Emulsion 600 times. In regions where resistance to the above fungicides has occurred, the 69% ank manganese (morphoyl morpholine + mancozeb) wettable powder or water dispersible granules 1000 times can be used.