Causes and control of corn downy mildew

Corn downy mildew is a fungal disease that originally occurred in tropical and subtropical regions. In recent years, it has been in Jiangsu, Sichuan, Yunnan, Shandong, Ningxia, Hebei, Liaoning, Gansu, Shanxi, Shaanxi and other provinces, cities and districts. It has also occurred and has caused serious losses in some areas.

There are four main pathogens causing corn downy mildew, which are all special oomycetes in fungi and are obligate parasites. Among them, Phytophthora sojae can produce oospores, and several other pathogens mainly rely on mycelium to overwinter in soil and manure with diseased plant residues, because oospores and diseased bodies can spread long distance with seeds and form new diseased areas. Therefore, corn downy mildew is also an important entry and domestic quarantine object in China.

It has been observed that the symptoms of downy mildew caused by infection of different pathogens at different times will be quite different. In the early stage, especially in the seedling stage, the pathogen can enter the stalk through the leaves and sheaths and reach the growth point. This will lead to malformation of the whole plant, including seedling dwarfing, internode shortening, stalk bending or twisting, abnormal male and female ears, etc., and some diseased plants are long, the leaves are wide, no tassels or no pollen. Usually the diseased seed will die or the adult plant will die early. This symptom is similar to the onset of millet white, so it is also called "mad top disease." Another characteristic of the symptoms is that the leaves show white streaks or yellow-green stripes, and white frosty mildew is produced on the back of the diseased leaves, which releases zoospores. It can spread by wind and rain, causing the onset of leaves and sheaths.

Because the pathogens rely on zoospores to spread, and the production, survival, germination, and infestation of zoospores require water, so in the rainy years and low-lying areas, the flooding of the corn seedlings and the accumulation of paddy fields are heavier. Some places in the north reflect the seriousness of heavy and wheat-covered corn fields, and increasing planting density is conducive to disease occurrence.

In view of the above-mentioned pathogenesis, the following prevention measures can be taken. Use resistant varieties. Water conservancy should be built to prevent stagnant water, especially in the seedling stage of corn; in the autumn, the straw in the field should be removed, and the decomposed organic fertilizer should be applied in the spring; the diseased plants should be removed in the growing season and treated centrally. Implement quarantine measures and do not transfer from infected areas. In the early stage of the disease, spray 90% aluminum triethyl acrylate wettable powder 400 times liquid or 64% ecstasy (antivirus cockroach) wettable powder 500 times liquid, 72% gram dew (cream urea manganese zinc) wettable powder 700 times liquid, 12 % green milk copper emulsifiable concentrate 600 times. In areas where the above-mentioned fungicides are resistant to chemicals, a 69% acryl manganese (dimethomorph + mancozeb) wettable powder or a water-dispersible granule 1000 times solution may be used instead.

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