Control of several main diseases of onion

The main diseases of onions are downy mildew, rust and purple spot. In recent years, the occurrence and harm of these diseases have been aggravated all over the country, especially in protected cultivation, which has a great impact on the yield and quality of onions. The main symptoms, characteristics and control methods of these three diseases are introduced below.

1. Downy mildew

(1) main symptoms. Downy mildew is an important disease of green onion and onion, which not only harms green onion and onion, but also harms garlic, leek and so on. Downy mildew mainly affects leaves and pedicels, and there is a layer of white mildew on the yellowish-green spot, which is the mycelium and sporangium of the pathogen. In the later stage, the disease is saprophytic by other bacteria, resulting in black mold.

(2) the condition of onset. The occurrence of downy mildew likes the climatic conditions of low temperature and high humidity, and the optimum temperature for the disease is 15 ℃. At this temperature, the incubation period of the pathogen is 5-10 days. In spring and early summer, if the weather is warm during the day, cool at night, and rainy, it is conducive to the occurrence of the disease, especially in the continuous heavy fog, the epidemic of downy mildew often occurs. In addition, low-lying, heavy soil, flood irrigation, too close planting, poor growth of the land is also serious.

(3) Prevention and control methods. To adhere to rational crop rotation, rotation should be carried out with non-onion and garlic crops for more than 2 to 3 years. Under the premise of not affecting the growth, reduce the humidity in the field as far as possible, enhance the plant growth and improve the disease resistance. To strengthen chemical control, spray protection should be started from the early stage of the disease. the commonly used agents are: 72% DuPont Kelu wettable powder 800 times, or 64% poison alum 800 times, or 58% Ruidu manganese zinc wettable powder 700 times, or 70% ethyl phosphorus aluminum manganese zinc wettable powder 500 times. In order to improve the adhesion of the liquid, 500 times liquid spray can be mixed with washing powder, once every 7-10 days, even 3-4 times; in case of rainy days, the spraying time should be shortened. When downy mildew is popular in the season, if you encounter foggy weather, it will aggravate the disease, so you should spray clear water immediately after the fog has dispersed to prevent and control the outbreak of the disease.

two。 Purple spot disease

(1) main symptoms. Purple spot mainly affects leaves and pedicels on scallions, and bulbs on onions, resulting in soft rot and shrinkage. In addition to onions can also harm garlic, the initial stage of the disease showed water-immersed white spots, the disease spots expanded rapidly, and quickly formed wide 1~3cm, long 2~4cm spindle-shaped sunken spots, first light brown, and then turned brown to bluish purple, surrounded by yellow halos. After that, some gradually fade and form concentric wheel lines, and black-brown pulverized coal mildew is produced on the spot when the humidity is high. If the spot enlarges around the leaf or pedicel, it can be broken from the spot. Bulbs often occur in the neck, and the disease shrinks, turning reddish or yellow, and mildew occurs when it is wet. The disease is characterized by spindle-shaped spots, slender upper and lower parts, dark spots, and rarely whole-leaf death, which can be distinguished from downy mildew.

(2) the condition of onset. In the southern region, conidia are often harmful to onion crops. In the northern cold area, after the mycelium overwintered in the host or on the rest of the disease, the conidia were produced, which were transmitted by airflow and Rain Water and invaded through stomata and wounds. The condition of the disease was warm and humid, and there was no disease when it was lower than 12 ℃. The wound inhaled by onion thistle is an important way for bacteria to invade. Therefore, when thrips occur seriously, the disease can be aggravated.

(3) Prevention and control methods. Crop rotation should be carried out for 3 to 4 years with non-onion and garlic crops. Because the pathogen can be attached to the seeds and spread, the seeds should be disinfected carefully before sowing. 50% thiram, 50% carbendazim or 50% topazin wettable powder can be used to mix seeds according to 0.4% of the seed weight. At the initial stage of the disease, 75% chlorothalonil wettable powder, 64% disinfectant alum wettable powder, 70% mancozeb 700x liquid, or 40% Dafudan wettable powder, 58% metalaxyl manganese zinc to water, or 50% propofoin wettable powder to water can be sprayed in time. 1500 times. If onion thrips are harmful at the same time, you can choose one of the above pesticides mixed with 1000 times of 80% dichlorvos EC or 90% trichlorfon crystals, or 2.5% deltamethrin or 20% cypermethrin EC 3000 times spray to treat onion thrips.

3. Onion rust

(1) main symptoms. Onion rust harms not only green onions and onions, but also garlic and leeks. Some of the yellow spots on the diseased leaves are the summer spores of the rust pathogen, and the orange powder inside is its summer spores. In the later stage, some dark-brown dots grow in the disease department is the germ's winter spore pile, which contains thousands of winter spores.

(2) the condition of onset. The occurrence of rust likes the climatic conditions of low temperature and high humidity, the most suitable temperature for spore germination is 918%, and the incubation period is 10 days. When the temperature is above 24 ℃, the disease is affected. Therefore, when it is overcast and rainy and the temperature is low in spring and autumn, the disease is serious. If the winter is warm and humid, which is conducive to the overwintering of spores, the rust is serious in spring next year; in summer, if it is low temperature and rainy, the disease is serious in autumn. The lack of field fertility and poor plant growth are also a serious factor in the disease.

(3) Prevention and control methods. The main methods to control rust are as follows: improve soil fertility, apply more phosphorus and potassium fertilizer to enhance the disease resistance of plants; fields with serious disease should be harvested in advance, and avoid planting onion and garlic vegetables nearby; greenhouse cultivation should pay attention to heat preservation and dehumidification to control the disease conditions. Early stage of the disease should be timely spray prevention and treatment, the use of agents are: 25% Chitong San (triadimefon) EC 800x liquid, or 12.5% he Guoli (diniconazole) wettable powder 1500 times liquid spray, once every 10 days, continuously sprayed 2 times 3 times.