Symptoms and Control Methods of Southern Blight in Orchids

Symptoms Southern blight, also known as white silk disease or sclerotium disease, starts at the stem base of the orchid plant near the ground. Initially, it appears as yellow to light brown water-soaked lesions, which then turn brown to dark brown and rot, spreading across the rhizosphere soil surface and the stem base, destroying the stem and infecting young leaves and roots. White mycelium develops on the leaf sheaths and root mass. The affected areas become water-soaked, rot and soften, and turn black until the leaves wither. The pathogen spreads to the pseudobulbs, and the sclerotia change from white to reddish brown to tea brown and necrotic. In severe cases, the entire leaf cluster of the plant dies.

Pathogen and Disease Characteristics The pathogen is a fungus of the Deuteromycotina. The pathogen overwinters as sclerotia in soil, in fertilizer, and on undecomposed plant debris. In regions with higher winter temperatures, the mycelium can also overwinter on undecomposed debris. The sclerotia germinate the following year, spread through the soil, and invade through the base of the orchid leaves. The sclerotia are highly resistant to adverse environmental conditions and can survive in soil for several years. They are spread by flowing water, irrigation water, rain splash, and fertilization. The disease is most severe in acidic soil, and hot, humid weather easily triggers the disease. Infection begins in April to May, with the peak incidence from June to August. The disease develops rapidly, causing the affected orchid plants to rot and die.

Control Measures

1. Changethe pH of the orchid growingmedium. This disease occurs easily when the medium is acidic (pH below 5.3). Therefore, mix plant ash into the medium at a ratio of 1/10 by volume, or apply 0.3% lime water to raise the pH to around 6.5. This can reduce the occurrence of southern blight with noticeable results.

2. Soil disinfection. Disinfect the medium and soil with 0.2% quintozene 500x solution or 50% carbendazim wettable powder 600x solution. You can also spray the rhizosphere and soil with quintozene 500x solution or 70% thiophanate-methyl wettable powder 800x solution, or mix 50 g of quintozene powder with 4 to 5 kg of semi-dry fine sandy soil and spread it around the root crown of diseased plants to inhibit the spread of the disease.

3. Spread fresh plant ash for prevention. In the rainy season from May to June in the south, soil moisture is high and southern blight easily occurs on the pot surface. The plants should be quickly moved out of the orchid growing area for "isolation," and the medium should be pulled apart. Remove the medium containing mycelium, cut off the diseased roots, and spray with potassium permanganate 1000x solution for disinfection. You can also spray the roots and wounds with 70% thiophanate-methyl 1500x solution, spread fresh plant ash around the orchid roots, then cover with medium to protect the roots. Water only after 2 to 3 days; if the medium is wet, postpone watering for 5 to 7 days. The control effect is good.

4. When the disease occurs, use a 2000x aqueous solution of medical chloramphenicol injection to drench the diseased plants once daily for 3 consecutive times. Once the disease occurs, cut off the diseased leaves, improve ventilation, and soak the entire orchid plant in a 10% copper sulfate solution for disinfection. Nearby orchid plants should also be thoroughly sprayed, wetting the base and pot surface, once daily for 2 consecutive times. This can control the disease with good results.

5. After new shoots emerge from the surface, spray with 0.05% chloramphenicol once a week for 2 to 3 consecutive times. If the potting medium is dry, this solution can be applied as a drench to prevent bacterial infection. You can also spray or drench with aspirin 1500x solution, which can enhance the orchid plant's immunity and prevent pathogen invasion and spread.

6. During the new shoot elongation and leaf expansion stages, you can use medical penicillin, streptomycin, chloramphenicol, or a penicillin mixture diluted to 2000x solution to drench the diseased plants and spray thoroughly, once every 3 to 4 days for 2 consecutive times. The control effect is good.

7. After the disease occurs, spray with 50% benomyl wettable powder 1000x, or 50% carbendazim wettable powder 500x, or 50% thiophanate wettable powder 500x solution, or 70% chlorothalonil wettable powder 800x solution, once every 7 to 10 days for 2 to 3 consecutive times. The control effect is good. You can also spray before and after rainy or overcast weather. You can use 50% procymidone powder 500x, or 50% vinclozolin powder 500x solution, or aluminum fosetyl 500 to 600x solution, or 5% jinggangmycin aqueous solution 300x, spraying the stems, leaves, and pot surface. The control effect is significant.