Orchid round spot disease

1. Harm: the disease was found for the first time in Shenzhen and mainly harmed Cymbidium sinense and C.kanran. The leaf incidence rate was 75% and 64% respectively, and the disease index reached 64.8% and 43 respectively.

2. Symptoms: the disease harms the leaf and shows two typical symptoms: first, the disease spot is round, which is semicircular, small, 1~3mm in diameter, black when it occurs at the leaf edge, does not change color at the junction of the disease spot and healthy tissue, is sunken on both sides of the leaf, and can occur from the leaf tip, leaf edge, middle and leaf stem. The disease spot is mostly uniformly distributed on the whole leaf, and its fruiting body can hardly be seen on both sides of the leaf. When the disease is serious, the whole leaf is covered with black spots and dies. Second, the symptom is locally known as "iron cannon", showing a typical satellite-shaped symptom, the disease spot appears on the leaf, there is an obvious disease center, from the central disease spot to the surrounding radiation, resulting in many small disease spots. The central lesion is oval, semicircular (from the edge of the leaf) or irregular, dark brown, larger in diameter, 10~25mm, the edge is reddish brown, sunken on both sides of the leaf, the surrounding lesion is less, and shrinks with the increase of the distance from the central spot, round, dark brown, wherever it goes, the surrounding leaf tissue is yellowed, and the central spot can occur from the top or base of the leaf, leaf margin or interior, and its radiation expansion ability is strong. Sometimes the central disease spot appears at the base of the leaf, and the small disease spot around it can extend all the way to the top of the leaf along the leaf edge, and the fruiting body can not be seen on both sides of the leaf.

3. Pathogen: the pathogen of orchid round spot disease was identified as Stagonospora curtisii. On PDA medium, the colony was round, the edge was neat, showing two forms, one colony was reddish brown on the back, the ochre red on the front, the colony was flat like velvet, and the other was dark green on the back, the hyphae on the front was loose, raised in the middle, and grayish brown. After 4-5 days of culture, milky red conidia mucus was produced on the colony. The carrier is a conidium, oblate, buried in the substrate, with an opening at the top, 120~180um in diameter, the conidia are short, cylindrical or rod-shaped, grow from the bottom cells of the conidium, the vesicular cells are stalked, the conidia are long oval or cylindrical, colorless, 15~25um*5~7um, the septum is 1 or 3, the initial septum is not obvious, the septum is obvious during germination, and the septum is slightly constricted or not constricted. Contains big oil balls. When the conidia germinated, the bud tube could grow from both ends or the middle, and the constriction and septum were obvious during germination. Conidia can germinate in clear water, but 1% glucose solution can promote its germination, and the temperature can germinate at 15: 30 ℃, but the optimum temperature is 2O~28 ℃, the optimum temperature is 25 ℃. In 1% glucose solution, 25 ℃ for only 12 hours, the germination rate can reach more than 90%. The temperature range of mycelial growth was 10: 40 ℃, the optimum temperature was 25: 35 ℃, the optimum temperature was 28 ℃, the mycelium could hardly grow over 40 ℃ or less than 10 ℃, the pH range of mycelial growth was 4: 11, the suitable pH range was 6: 9, and the optimum was 7.

4. The characteristics of the disease: the investigation results showed that the disease began to occur in Shenzhen more than in the middle of February, and the peak period was from April to May. The weather conditions of high temperature, overcast and rain and high humidity were favorable for the occurrence of the disease, and the most suitable temperature was 2O~26 ℃. After June, the temperature rose above 30 ℃ and the disease stopped developing. The germs overwintered in diseased leaves.

5. Prevention and control methods:

(1) strengthen quarantine, maintain the hygiene of orchid nursery, remove diseased leaves and reduce the source of pathogen infection.

(2) the pathogen of the disease can be co-infected with daffodils, so it should be planted separately from daffodils and other flowers and plants.

(3) spray with 50% carbendazim, 64% alum, 58% Manganese Zinc, 50% benzimidazole, Xiafangfan copper 80% bengshuangling 1000 times solution. (Liu Zhongjian)