Bacterial disease of orchid: Brown spot

1. Harm: the disease was first reported in 1946, and its host range is wide, and it has been recorded at home and abroad. The main orchid species affected by it are: Cartland, Phalaenopsis, fingernail orchid, Dendrobium, Cymbidium, Cymbidium, Oncidium, Orchid, Magnolia, Magnolia and so on. The harm of the disease is serious. In some orchid nurseries in Guangdong Province, death often occurs due to the harm of brown spot, resulting in serious losses.

2. Symptoms: generally speaking, the initial symptoms of the disease are water-like spots, and then gradually expand into round, oval or long spots, the middle tissue of the disease spot is brown or black necrosis, surrounded by obvious yellow halo, there is an obvious sense of hardness with hand touch, the disease develops rapidly, often causing a large number of seedling death. In the older plant, the pathogen can invade any part of the leaf, when the disease is serious, the whole leaf yellowed or dried up, and extended to the growing point will cause the whole plant to die, and quite a lot of bacteria often appear in the diseased site. On Cartland orchids, the symptoms of the disease on the leaves are sunken, black watery spots, which are generally limited to one or a few leaves and do not kill the plant.

3. Pathogen: the pathogen was identified as Pseudomonas cattleyae (Pavarino) Savuleacu, which was the same as that reported in the United States. The bacteria are rod-shaped, simple or twin, J.4~O.6um X 2.4um in size, unipolar or bipolar flagella, swimming, Gram staining negative, and the colony is large, smooth, moist and creamy from white on the gravy peptone medium. Haze appeared within 24 hours of liquid culture, and the optimum growth temperature was 25: 30 ℃. The results of physiological and biochemical determination of the pathogenic bacteria showed that the bacteria could hydrolyze starch, reduce nitrate and come into contact with enzyme reaction, but could not liquefy gelatin and produce indole and hydrogen sulfide. It could react with glucose, galactose, fructose, sucrose, glycerol, xylose, lactose and raffinose.

4. Disease characteristics: the disease is prevalent in high temperature and rain in spring and autumn. Bacteria enter the plant through stomata or occasional wounds on the leaf surface. Orchids cultivated under high humidity conditions are susceptible to disease when the leaf surface is wet, because the stomata are open and bacteria can enter through the water film. The overflow bacteria in the diseased parts of diseased plants can be transmitted to other plants by sprinkling water and become an important source of infection.

5. Prevention and control methods:

① horticultural control: careful cultivation, beware of causing wounds, pay attention to watering methods, avoid sprinkler irrigation, make water splashing; timely removal of diseased leaves, eliminate the source of infection, placed between plants should not be too dense, improve ventilation conditions, so that the surface of orchid leaves will not be too wet.

② chemical control: spraying Physan1500 double solution is an effective measure to control brown spot. If the rot has spread to or near the top of the plant, the plants, flowerpots and other sterilizable things should be soaked in a 2000-fold solution of 8-hydroxyquinoline sulfate or o-phenylphenol sodium for 60 minutes. Spraying diseased plants and susceptible plants and their surroundings with 1000-fold wettable powder often has a significant effect.

Liu Zhongjian