According to the forecast of the agricultural department, due to the climatic characteristics of this spring, bulb vegetables are common diseases, and purple spot disease may occur more seriously. Purple spot disease mainly harms onions and green onions, and also infects garlic and leeks. Generally, it attacks the leaves, peduncles and bulbs of onions in April ~ May. The harm to garlic is mainly manifested on the leaves in mid-April, which makes the leaves dry up, loses the ability of photosynthesis, makes the garlic head smaller, and reduces the yield and quality. The damage to green onions is mainly in May, manifested in the leaves and green onion white (bulb), the leaves are dry, and the green onion white turns brown. The occurrence of purple spot disease poses a great threat to the yield of bulb vegetables, and it should be prevented and controlled in time once it occurs.
Present with underlying symptoms
Purple spot disease first begins at the tip of the leaf or forms at the wound of thrips, and the initial lesion is small, gray to light brown, with a purplish central purple, and a blackish-gray mold. The lesion quickly expands to an oval or spindle-shaped, sunken, dark purple color, forming concentric ring patterns with a yellow halo around it. When the humidity is high, the lesions expand to the whole leaf, which often makes the leaf yellow, die or break off from the bottom up, which seriously affects the yield and quality of the product.
Onset conditions
The pathogen of purple spot disease is a fungus of the subphylum Semi-Epimycetes, which overwinters in the soil with hyphae attached to the diseased residue. Or it can be caused by tossing and turning on bulbs, spreading through air currents and rainwater, and invading from stomata, wounds or directly through the epidermis. It is easy to develop disease under warm and humid conditions, and the bulb is susceptible to disease at the bulb expansion stage, and the fields with sandy soil, dry land, early seedlings, old seedlings, lack of fertilizer, weak onion growth, and thrips are seriously affected. The local red-skinned onion has a mild disease, the yellow-skinned onion is seriously susceptible, the disease is serious early transplanting, the deep ditch has high moisture and fast drainage, the soil has good air permeability, the susceptibility is mild, and the occurrence of irrigation in the recent past is serious.
Prevention and control methods
1. Agricultural prevention and control: clean the farm, before planting bulb vegetables, remove the surrounding weeds, fallen leaves, dead branches and other debris from the planting plot. During the growth period, the diseased plant residues should be removed and treated in time to reduce the source of infection, reduce the number of pathogenic bacteria, and reduce the incidence rate.
Strengthen management: apply more organic fertilizer as base fertilizer, with an appropriate amount of compound fertilizer. Biological or functional fertilizers are applied to make the plants grow robust and enhance disease resistance. Reasonable watering, timely squatting seedlings at the seedling stage, timely drainage in rainy days, and reducing field humidity. Irrigation should be controlled after the onset of the disease to reduce soil moisture.
Cut off the route of disease transmission: find thrips, and prevent thrips as soon as possible to avoid wound infection.
2. Pharmaceutical prevention and control. The seeds were soaked in 40% formaldehyde 300 times for 3 hours, and washed in time after soaking.
At the beginning of the disease, spray 600 times of 75% chlorothalonil wettable powder, 500 times of 64% alum wettable powder, 500 times of 58% methamalin-manganese zinc wettable powder, 500 times of 70% manganese-zinc wettable powder, 50% promethane 1500 times, 50% antimicrobial wettable powder 500 times, 50% Suclin 30-50 grams, onion and garlic fungus net and other agents. Any one of the above agents can be selected, and various agents should be used alternately, sprayed once every 7~10 days, and continuously prevented and treated 3~4 times. Adding 5~10 grams of neutral laundry detergent to every 10 kg of liquid medicine can increase the adhesion of the liquid medicine and improve the prevention and control effect.