Symptoms and Control Methods of Sclerotium Disease in Orchid Leaves

1. Damage: Orchid leaf sclerotium disease occurs every year in the orchid room of the Shanghai Botanical Garden, causing the entire shoot of orchid leaves to wither and die, with an annual incidence rate of about 1%. Although it is not very common, once infected,orchidsalways die entirely shoot by shoot, and even the whole pot may wither. Amongorchids,Cymbidium ensifoliumthe autumn orchid is more severely affected, while otherCymbidium faberi、Cymbidium goeringiialso occur. 2. Symptoms: Between March and April each year, pottedorchidskept indoors develop this disease. First, the leaf base becomes light brown and water-soaked, rotting, and this can extend upward to most of the leaf or the entire leaf, turning reddish-brown, but the part that has not changed color may remain green for a short time, after which it becomes grayish-white, with small black round or oval sclerotia on it. The sclerotia are 0.5-1.5 mm in diameter, embedded beneath the leaf epidermis, arranged in longitudinal rows parallel to the leaf veins. In plants with this disease, an abscission layer forms at the base of the diseased leaf, making it extremely easy to break off. 3. Pathogen: The pathogen of orchid leaf sclerotium disease is a stem-leaf sclerotium fungus (Ectostroma sp.). This fungus does not produce spores. The sclerotia are black and glossy, small, 0.5-1.5 mm in diameter. When the sclerotium is cut open, the color is uniform inside and out. On potato, sucrose, agar medium, this fungus also readily produces sclerotia, embedded beneath the surface of the medium, while the mycelium is not well developed. The appearance, color and size of the cultured sclerotia are completely identical to those naturally grown in diseased leaves. 4. Disease characteristics: The pathogen of orchid leaf sclerotium disease has no spores, and therefore overwinters as sclerotia in diseased leaves or fallen diseased leaves. In the following spring, they germinate and produce mycelium to infect and cause damage. When the potting soil is excessively moist and the temperature is cold, the disease occurs more frequently. Among Cymbidium,Cymbidium ensifoliumtheSijilanorchidis severely affected. This indicates that resistance differences exist among varieties. 5. Control methods: (1) Manual control: Promptly pull out diseased orchid leaves when found. According to observations, in a pot of orchids, not all orchid plants become diseased at the same time, but rather one after another, so they should be pulled out immediately upon discovery, and a small amount of lime or sulfur powder can be sprinkled into the hole for disinfection. (2) Horticultural control: For orchids overwintering in a greenhouse, do not move them out too early in spring. Generally, orchids should only be moved outdoors after Qingming when frost has ended. When indoors in winter and spring, do not water too much, pay attention to the drainage of the potting soil, and when watering, water should also be poured in from the edge of the pot. (3) Chemical control:cultivationsubstrate should be disinfected with quintozene (as for controlling southern blight). In the early stage of disease, applying 50% dicloran 1000-fold solution also has a certain effect. (Liu Zhongjian)