The Latest Control Methods for Southern Blight in Orchids
Southern blight in orchids is one of the major diseases of orchids, also known as white silk disease, sclerotium disease, etc. Its harm is second only to anthracnose, but once it occurs, the damage and losses it causes will far exceed those of anthracnose. It usually begins infecting in April to May, with the peak incidence period from June to August. The disease develops rapidly, causing the affected orchid plants to rot, wither and die. Now let us take a look at the control methods for southern blight in orchids!
Symptoms
Southern blight starts from the stem near the ground of the orchid plant, initially appearing as yellow to light brown water-soaked lesions, then turning brown to dark brown rot, and spreading on the rhizosphere soil surface and stem base, destroying the stem and infecting young leaves and roots, with white mycelium produced on leaf sheaths and root clusters. The affected parts are water-soaked, rotting and softening, turning black, until the leaves wither. The pathogen spreads to the pseudobulbs, and the sclerotia change from white to reddish brown to tea brown necrosis. In severe cases, the entire leaf cluster of the whole plant dies.
Disease Characteristics
The pathogen of southern blight in orchids is a fungus of the subphylum Deuteromycotina. The pathogen overwinters as sclerotia in soil, fertilizer, and undecomposed residues. In areas with higher winter temperatures, the mycelium can also overwinter on undecomposed residues. The sclerotia germinate the following year, spread in the soil, and invade and cause damage from the base of orchid leaves. The sclerotia have strong resistance to adverse environments and can survive in soil for several years. They are spread by flowing water, irrigation water, rain splashing, and fertilization. The disease is most severe in acidic soil, and hot and humid weather easily induces the disease. It begins infecting in April to May, with the peak incidence period from June to August. The disease develops rapidly, causing the affected orchid plants to rot, wither and die.
Control Methods
1. Southern blight in orchids is prone to occur when the substrate is acidic (pH value less than 5.3), so mixing plant ash at a volume ratio of 1/10 into the substrate, or applying 0.3% lime water to raise the pH value to around 6.5, can reduce the occurrence of white silk disease, with obvious effects.
2. Disinfect the orchid substrate soil with 0.2% quintozene 500-fold solution or 50% carbendazim wettable powder 600-fold solution. Alternatively, spray the rhizosphere and soil with quintozene 500-fold solution or 70% thiophanate-methyl wettable powder 800-fold solution, or mix 50 grams of quintozene powder with 4 to 5 kg of semi-dry wet fine sandy soil and spread it at the root and stem of diseased plants, which can inhibit the spread and transmission of the disease.
3. In the rainy season from May to June in the south, soil humidity is relatively high, and southern blight is prone to occur on the pot surface. The plants should be quickly moved out of the orchid growing area for “isolation”, and the substrate should be pried open, the substrate contaminated with mycelium removed, and the diseased roots cut off. Spray with potassium permanganate solution 1000-fold for disinfection. Alternatively, spray the root system and wounds with 70% thiophanate-methyl 1500-fold solution, and spread fresh plant ash around the orchid roots, then cover with substrate to protect the roots. Water only after 2 to 3 days; if the substrate is wet, delay watering for 5 to 7 days. The control effect is good.
4. When southern blight occurs in orchids, use medical chloramphenicol injection 2000-fold aqueous solution to drench the diseased plants, once a day, for 3 consecutive applications. Once the disease occurs, the diseased leaves should be cut off, ventilation conditions improved, and the whole orchid plant soaked in 10% copper sulfate solution for disinfection. For nearby orchid plants, a comprehensive spray should also be applied, wetting the base and pot surface, spraying once a day for 2 consecutive applications, which can control the disease with good control effects.
5. After new orchid shoots grow out of the soil surface, spray with 0.05% chloramphenicol once a week for 2 to 3 consecutive sprays. If the potting soil is dry, this medicine can be used for drenching to prevent bacterial infection. Aspirin 1500-fold solution can also be applied by watering or spraying, which can enhance the immunity of orchid plants and prevent pathogen invasion and spread.
6. During the new shoot elongation period and leaf expansion period of orchids, medical penicillin, streptomycin, chloramphenicol, or a penicillin mixture can be diluted to 2000-fold solution to drench the diseased plants, and a comprehensive spray should be applied, once every 3 to 4 days, for 2 consecutive sprays, with good control effects.
7. After the disease occurs in orchids, spray with 50% benomyl wettable powder 1000-fold, or 50% carbendazim wettable powder 500-fold, or 50% thiophanate wettable powder 500-fold solution, or 70% chlorothalonil wettable powder 800-fold solution, once every 7 to 10 days, for 2 to 3 consecutive sprays, with good control effects.
8. After the disease occurs in orchids, pesticide can be applied before and after cloudy or rainy days. Use 50% procymidone powder 500-fold, or 50% vinclozolin (vinclozolin) powder 500-fold solution, oxadixyl 500 to 600-fold solution, 5% jinggangmycin aqueous solution 300-fold, and spray on stems, leaves, and pot surface, with remarkable control effects.