Symptoms: Onion downy mildew is a common disease of Welsh onion, which occurs more commonly or significantly reduces yield and quality. The main damage leaves and pedicels. In the early days after infection, ovoid round yellow-white spots were produced on the leaves or pedicels, the edges were not obvious, the lesions expanded and jointed, and they developed tubularly along the leaves and stems, and turned into dead spots when dry. When the middle and lower parts of the leaves are affected, the leaves above the diseased part will droop. The pseudostem is susceptible, the disease is broken, and the plant is bent. Bulb disease, can cause systemic infection, dwarf diseased plants, distorted leaves, pale green, high humidity, the surface of yellow-white velvet-like material, after the dark gray purple mold layer.
Prevention
(1) Agricultural control. Choose land plots with high topography, convenient drainage and irrigation, and implement a two- to three-year rotation with food crops. When harvesting, clear the field sickness, apply organic base fertilizer, increase the application of phosphorus and potassium fertilizer, close-up planting, remove diseased seedlings when planting, and prevent flooding. Strengthen the management of the field, make a row after the rain, grab the earth when it is rushed, ensure that the soil has good permeability, and regulate the microclimate in the field. (2) Seed disinfection. Use seed dressing of 0.2% seed dressing with 25% viremia or soaking in warm water at 50°C for 25 minutes, immersing in cold water, and allow to dry after sowing. (3) Chemical control. In the early stage of onset, spray 90% of BDP 800 times solution plus 65% of dexamethasone WP 400 times; or 58% of metalaxyl Mn 600 times; or 75% of chlorothalonil WP 600 times, Or 6 4% antiviral 500 times fluid; or 72.2% Prokary fluid 800 times fluid; spray once every 5 to 7 days, alternating medication, even spray 2 or 3 times.