Detailed Explanation of the Harm and Control Methods of Angelica Powdery Mildew

The pathogen of Angelica powdery mildew (Erysiphe sp.) is a fungus of the genus Erysiphe in the subphylum Ascomycotina. The cleistothecia are clustered, embedded in the mycelium, spherical or flattened spherical, black or dark brown. The cleistothecia contain 3-6 asci. The asci are nearly spherical, oval, colorless, with short stalks, thick-walled, and contain 3-6 ascospores. The ascospores are elliptical, colorless and transparent, single-celled.

1. Harmful symptoms: It damages the leaves. In the early stage of the disease, grayish-white powdery lesions appear on the leaf surface, which then expand and merge into large spots, and small black granules appear, and finally the leaves turn yellow and wither.

2. Disease occurrence pattern: It occurs during high temperature and dry conditions in summer. The pathogen overwinters as cleistothecia or mycelium on diseased residues or seed roots. The overwintering cleistothecia release mature ascospores the following year to carry out primary infection. The overwintering mycelium directly produces conidia in the second year, which spread and cause damage. During the growth period of Angelica, mycelium, conidiophores and conidia are continuously produced on the outer diseased leaves, and the conidia are spread by airflow to cause secondary infection. The suitable temperature for spore germination of Angelica powdery mildew is 18°C~30°C, the humidity is above 75%, and the incubation period is 2~5 days. The cleistothecia for overwintering are only formed in the late growth stage of Angelica. When the relative humidity of the air is low and there is no water film on the plant surface, the conidia can still germinate and invade, causing damage. Soil water shortage, poor ventilation, extensive management, weak plant growth or excessively dense growth of branches and leaves all favor the occurrence of this disease.

3. Control measures: ① Seed treatment: soak the seeds in 500 times formalin solution for 5 minutes or seal the seeds for 2 hours. ② Promptly pull out diseased plants and burn them in a centralized manner. ③ Implement crop rotation and avoid continuous cropping. ④ In the early stage of the disease, spray 1000 times 50% thiophanate-methyl or 500 times 65% zineb every 10 days or so for control, for 3-4 consecutive times. ⑤ Strengthen field management to enhance plant disease resistance.