Identification and Control of Bacterial Brown Spot Disease in Orchids

1. Damage: This disease was first described in 1946. It has a wide host range and has been recorded both domestically and abroad. The main orchidorchidspecies affected include: Cattleya, Phalaenopsis, Aerides, Ascocentrum, Cymbidium, Dendrobium, Gastrochilus, Peristeria, Oncidium, Rhynchostylis, Vanda, and Vandopsis, among others. The damage caused by this disease is severe. In some orchid nurseries in Guangdong Province, patches of plants often die en masse due to brown spot damage, causing serious losses. 2. Symptoms: Generally speaking, the initial symptoms of this disease are water-soaked spots, which gradually expand into round, oval, or elongated lesions. The tissue in the center of the lesion turns brown or black and necrotic, surrounded by a distinct yellow halo. When touched by hand, there is a noticeably hard sensation. The disease develops rapidly and often causes large numbers of seedlings to die. On older plants, the pathogen can infect any part of the leaf. In severe cases, the entire leaf yellows or dries up, and if it spreads to the growing point, it causes the death of the entire plant. Considerable bacterial ooze often appears at the site of infection. On Cattleya-typeorchids, the symptoms on the leaves are sunken, black, water-soaked lesions, generally confined to one or a few leaves, and do not kill the plant. 3. Pathogen: The pathogenic bacterium was identified as Pseudomonas cattleyae (Pavarino) Savuleacu, the same pathogenic bacterium as reported in the United States. The bacterial cells are rod-shaped, single or in pairs, measuring 1.4-0.6 um X 2.4 um, with single or bipolar flagella, motile, Gram-negative. On nutrient peptone medium, the colonies are relatively large, smooth, moist, and creamy white. Liquid cultures become turbid within 24 hours. The optimal growth temperature is 25-30°C. According to the results of physiological and biochemical tests on this pathogenic bacterium, it can hydrolyze starch, reduce nitrates, and give a catalase reaction, but cannot liquefy gelatin, produce indole, or produce hydrogen sulfide. It can ferment glucose, galactose, fructose, sucrose, glycerol, xylose, raffinose, and melibiose. 4. Disease Characteristics: This disease is prevalent in spring and autumn during periods of high temperature and abundant rain. The bacteria enter the plant through stomata on the leaf surface or occasionally through broken wounds. Under high humidity conditions,cultivated'sorchidsare susceptible to infection when their leaf surfaces are moist, because the stomata are open and the bacteria can enter through the water film. The bacterial ooze from diseased parts of infected plants can be spread to other plants through watering, becoming an important source of infection. 5. Control Methods: ① Horticultural Control: Carefulcultivation, taking care to avoid causing wounds, paying attention to watering methods, avoiding overhead irrigation and water splashing; promptly removing diseased leaves to eliminate the source of infection, not placing plants too densely together, improving ventilation conditions, and preventing orchid leaf surfaces from becoming too moist. ② Chemical Control: Spraying Physan at a 1:1500 dilution is an effective measure for controlling brown spot. If the rot has already spread into or near the top of the plant, the plant, pot, and other disinfectable items should be soaked in a 1:2000 dilution of 8-hydroxyquinoline sulfate or sodium orthophenylphenate for 60 minutes; using ferbam 75% wettable powder at a 1:1000 dilution to spray diseased plants and susceptible plants and their surroundings often has significant results. Liu Zhongjian