Orchid soft rot is a bacterial disease

1. Damage:Orchidsoft rot can occur on many vegetables and ornamental plants. Although it is not very common onorchids, it may be one of the most destructive diseases. It has been reported to infect not only Cattleya-typeorchidsbut also some species in the following genera, including Ascocentrum, Cymbidium, Oncidium, Paphiopedilum, Phalaenopsis and Vanda. This disease has been reported in both the United States and Japan. It once occurred severely in Fujian and caused the death of large numbers of orchid plants. Among Cymbidium species,Cymbidium ensifolium、Cymbidium faberiand others can be susceptible. According to reports from Taiwan, China, the pathogen of orchidsoft rotdisease is the same as the pathogen of orchid soft rot. 2. Symptoms: Bacteria enter the plant through wounds and quickly form a foul-smelling wet rot. In fresh, tender, succulent tissues, the rot develops rapidly, but in more mature tissues, especially in parts such as pseudobulbs and rhizomes, the rot develops somewhat more slowly. The disease occurs mainly on orchid shoots, followed by leaves. At the initial stage, water-soaked lesions the size of mung beans appear at the base of the shoot. After 2-3 days, they rapidly expand upward and downward, becoming dark green, scald-like large patches, reaching the outside of the shoot sheath and appearing as dark brown rot; diseased leaves are easy to pull out. The pathogen also infects the leaf surface, causing dark green water-soaked patches on the leaves. 3. Pathogen: The pathogen of orchid soft rot issoft rotErwinia carotovora (Jones) Holland. It is short rod-shaped, with somewhat rounded ends, measuring 1.2-1.9 um X 0.5-0.9 um. It is Gram-negative, with 2-4 peritrichous flagella, and does not produce capsules or spores. Cut a small piece of tissue from a diseased plant, preferably from the junction between diseased and healthy parts, place it in a drop of water on a slide, cover with a cover slip and press lightly, then immediately examine it under a low-power microscope field (120X); a large amount of turbid bacteria often oozes out from the cut. 4. Disease characteristics: The pathogen of orchid soft rot overwinters in diseased tissues. The large accumulation of diseased parts of infected plants on moist ground is conducive to the reproduction and survival of the pathogen. The pathogen spreads via water flow and can invade only through wounds. This disease occurs in July. When continuous rainy weather in summer reaches more than 15 days per month, the temperature is relatively low (25-30°C), and especially after typhoons and torrential rain, it is prone to outbreak and epidemic. For fragile plants, heavy rain can damage the cuticle of new leaves, which favors bacterial infection. Improper water and fertilizer management, overly wet potting soil, and excessive nitrogen application also make the disease more likely to occur. 5. Control methods: (1) Horticultural control: Using acidic planting material as the potting medium can reduce the damage (because this bacterium prefers to grow under slightly alkaline conditions). Control the potting medium so that it is not overly wet. Because the pathogen can spread only via water flow, avoid splashing when watering, so that no free water remains in the medium. For seedlings and plants with fragile leaves, cover them to prevent direct rain. (2) Chemical control: When new shoots emerge 4-5 cm above the ground, spray with 0.01% boric acid solution, or spray 50% dikushuang, shakujing and other fungicides at 1000 times dilution, which has a certain control effect. Once infection is found, diseased leaves should be removed and destroyed. A 800 times solution prepared with Physan can effectively preventsoft rot. If the disease develops rapidly, the entire diseased plant should be destroyed, and the growing bench should be disinfected with 10% Clorox (0.5% sodium hypochlorite). To prevent soft rot of pseudobulbs and rhizomes, attention should be paid to sanitary operation during division. The knife used for division should be sterilized in boiling water or in a solution of 1 part formalin plus 7 parts water, and the cut surface from division should be coated with a layer of captan paste to prevent bacteria from invading through the wound. (Liu Zhongjian)