Comprehensive Prevention and Control Methods and Technical Key Points for Orchid Diseases

Southern blight is a disease caused by fungi and is one oforchidthe major diseases, second only to anthracnose. Once this disease occurs, the damage and losses caused will far exceed those of anthracnose! 1. Pathogen:orchidThe southern blight pathogen is caused by Sclerotium rolfsii. Its sclerotia are small, formed by the aggregation of white silky mycelium, initially milky white, gradually turning cream yellow, and finally becoming dark brown, resembling the size of rapeseed. The optimal temperature for growth is 25-35°C; growth stops below 10°C or above 40°C. It dies when exceeding 100°C. 2. Symptoms of southern blight: On infected orchid plants, the stem base near the pot surface and the fleshy roots initially show browning with rot. Soon after, white silky threads appear on the surface, spreading radially and continuing to extend across the potting soil surface around the roots. Later, many granular bodies (sclerotia) resembling rapeseed in size form on the white silky threads. The sclerotia are initially white and gradually turn dark brown. 3. Disease characteristics:orchidThe southern blight pathogen mainly overwinters as sclerotia in diseased plant residues or soil. The disease begins to occur in May-June. This pathogen has a very wide host range. The pathogen can spread with contaminated potting medium, and mycelium spreads through the potting medium, invading from the base of orchid leaves. The incubation period of southern blight is very short; symptoms can appear within 1 week after invasion. Acidic soil favors the occurrence of this disease, and high-temperature, high-humidity environments make it even more susceptible. Therefore, during the plum rain season with high temperatures and frequent rain, this disease is very easily triggered! Southern blight mainly attacks orchidflowerbuds and seedlings. The mycelium starts from the leaf sheath base and gradually extends to the central leaf base of the seedling, reaching the young pseudobulbs and tender roots. After infection, the young tender tissues disintegrate, exude yellow liquid, and rot, ultimately causing orchid leaves to collapse and the entire seedling to die. If one seedling is infected and not treated promptly, all plants in the same pot will not be spared. 4. Prevention methods: (1). Create a suitable environment for orchid growth: The occurrence of orchid southern blight is related to high environmental humidity. Potted orchids should avoid prolonged direct rain exposure, especially during the plum rain season in May-June and the hot months of August-September. Therefore, orchids are best grown under a shedcultivation, with rain shelter and shade facilities built, creating a ventilated and light-transmitting environment. (2). Use clean potting medium for planting: Orchid southern blight is a soil-borne disease. The mycelium can also die under intense sunlight exposure. Therefore,cultivationsoil should be subjected to high-temperature sterilization treatment, and un-decomposed plant residues should be sifted out. When planting, avoid using fine powdery substrate. (3). Manual control: Keep the pot surface slightly dry, which helps inhibit sclerotia germination. Orchid pots should not be placed too close together. Organic fertilizers must be fully fermented and decomposed before application. Promptly remove weeds from the pot surface (weeds are also host plants of southern blight) to reduce sources of pathogen infection. 5. Chemical control: During peak disease seasons, use 70% thiophanate-methyl wettable powder at 1000x dilution or 75% chlorothalonil wettable powder at 800x dilution, spraying onto the pot surface or leaf bases, once every 1 week, for 2-3 consecutive times for better results. In addition, 25% carbendazim at 1000x dilution and 50% thiram wettable powder at 800x dilution also have good effects on controlling orchid southern blight. Once plants are found to be infected with southern blight, severely affected plants should be cut off and burned, and sprayed with thiram or chlorothalonil at 800x dilution to prevent further infection. When spraying, focus on the orchidflowerstem base; combining with root drenching will produce even better results. During this period, watering should be reduced. At the same time, conduct daily inspections. If the disease continues to occur, the pot should be repotted, all rotten roots cut off, and the entire orchid soaked in chlorothalonil at 800x dilution for half an hour. Then replace with new potting medium and plant in a small pot for isolated cultivation, and stop fertilizing, focusing on light maintenance. In summary, the prevention and control of orchid southern blight should achieve: prevention first, rain-shelteredcultivation, use of clean potting medium, enhanced ventilation and light transmission, preventive medication during disease periods, and early removal of diseased plants with isolated planting after infection. (Xinyuanxuan)