Purple spot disease, also known as black spot disease, is a common disease of onion vegetables. The main hazards are green onions and onions, but also garlic and leeks. It occurs everywhere, and the general harm is not serious, but it is more severe in rainy years and can cause greater losses.
First, the symptoms
Onion purpurea mainly damages leaves and pedicels, and can infect bulbs during storage and transportation. Damage to leaves and pedicels often occurs from the middle of leaves or pedicels, and spread to the lower part within a few days. At the beginning, it was white, slightly sag, and the center was slightly purple. After the enlargement, the lesions were elliptical or spindle-shaped. The color and size vary depending on the type of host. The lesions on green onions and onions are purple-brown in color. The lesions of 2-41-3 cm in size are yellow-brown and the size is 1-20.5-1.5 cm. When the humidity is high, the diseased part produces brown to black mouldy material. In other words, the conidial stems and conidia of pathogenic bacteria are often arranged in a concentric lenticular pattern, and the lesions continue to expand. Most of the lesions heal and grow into strips and large spots, resulting in dead leaves or peduncles above the diseased part, such as leaves surrounded by lesions or Pedicel, leaves or pedicels are more likely to collapse from the diseased part, leading to poor seed development or wrinkles.
After the onion bulbs are harvested, the germs invade from the bulbs, and the diseased parts are softly rotted, appearing yellow or red, gradually changing to brown or black, which can continue to occur during the storage period and cause the bulbs to rot.
Second, the pathogen
Onion purple spot is caused by the infestation of fungal hyphae. The optimum temperature for development is 22-30°C, and the conidial germination temperature is 24-26°C. The germination and invasion of conidia must be performed under rainy conditions.
Third, the incidence of the law
P. purpurea with hyphae lurks in the host or wintering on the seedlings. Conidia can survive over winter in diseased bodies. The wintering hyphae can produce new conidia in the coming year and become the initial infestation source. Conidia are transmitted by rainwater and air flow. Germ spores germinate out of germ tubes. They can invade from the stomata or wounds and penetrate directly into the epidermis. At a suitable temperature, the incubation period is 4-5 days.
Fourth, prevention and control measures
1. Use sterile seeds or seeds for disinfection. In the absence of disease or mild disease in the fields to stay. The seeds were sterilized with 30% formalin 300 times, soaked for 3 hours, and then washed with water for later use. The onion bulbs are sterilized with warm water of 40-50°C and soaked for 90 minutes.
2, strengthen the cultivation and management. In the case of non-onion vegetables and other crop rotations for more than two years, choose a flat land with good drainage, apply enough base fertilizer, increase phosphorus and potassium fertilizers, pay attention to the removal of diseased plant residues, and implement deep-rolling measures to reduce the disease.
3, chemical control. There are 1:1:200 times effective Bordeaux mixture, 80% of Dasheng M-45 WP, 600-800 times, 75% chlorothalonil 500-600 times, and 50% carbendazim 500 times. In the early stage of onset of spraying control, spray once every 7-10 days, continuous spraying 3-4 times, can have a good control effect. In addition, the spraying of more than 30ppm of antimycin is also effective.
4, timely harvest, low temperature storage. The onion should be harvested after ripening at the top of the onion and properly dried after harvesting. The bulbs should be dried and stored after selection. During storage, ensure low temperature (0 °C) and low humidity (relative humidity below 65%) while paying attention to ventilation and dehumidification.
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