Mycosis-- Orchid White Silk Disease

Orchid white silk disease (Orchid southern blight) is also known as orchid base rot (Orchid basal rot).

Distribution: worldwide; common in southern China.

Pathogen: Sclerotinia neatly (Sclerotium rolfsii).

Hosts: Cartland (Cattleya), Cymbidium (Orchid), Oncidium (Oncidium), Phalaenopsis (Phalaenopsis), Spathoglottis (Orchid), Vanda (Orchid), etc.

Symptoms: the earliest sign of plant infection is the appearance of yellow to light brown running water spots at the base of the stem. The pathogen mainly destroys the base of the plant and infects young leaves and roots. The injured leaves turn yellow, wither and die. Then the roots and pseudobulbs wither and rot rapidly. If it spreads upward, the stem will appear an annular groove, and then rot, resulting in the death of the whole plant. The net of the mycelium is a small circle called sclerotia, which is about 2~4mm in diameter, first white and then brown or black. It can survive for many years in soil, on benches or on flower racks.

Prevention and treatment: once there is a signal of the disease, cut off the diseased stem immediately, and soak the whole orchid in 1% copper sulfate bath solution (that is, 10g copper sulfate with 1L water) to disinfect the contaminated flowerpot. Culture soil, flower rack, etc., can be sprayed with 50% Dysen zinc 500~10O0 solution for disinfection. The spread of the disease can also be controlled by using 50% carbendazim wettable powder (5~10g/ square meters) and fine soil mixed with soil or spraying rhizosphere soil with 1000 times liquid.

Note: the disease is more common in the south and is often found in infected orchids. It is reported that a kind of base rot disease similar to white silk disease has been found in Shanghai, which is harmful to Jianlan. The leaf base of the diseased plant first appeared light brown watery spots, then extended to the whole leaf, and finally rotted, resulting in death. However, the pathogen is not clear, and the prevention and control methods are the same as above. (Chen Xinqi, Ji Zhanhe)