How to treat white silk disease in orchids? Plant ash is the nemesis

Promote Chinese orchid culture, spread orchid knowledge

After reading the article "The Whole Story of Treating White Silk Disease" written by Zhou Weiqi of Zhejiang, published in the first issue of Chinese Orchids in 1999, I deeply concur: the white silk disease pathogen in orchids is stubborn and grows rapidly, often causing orchid plant death within just a few days after onset. General fungicides such as potassium permanganate and thiophanate-methyl are basically ineffective against it, and the possibility of recurrence is extremely high. Zhou Weiqi consulted orchid experts and used "Orchid Fungus and Insect Clean" and "Daktarin" ointment to bring white silk disease under control. This method is good, but it is only a passive method of treating white silk disease, limited to use in the early stage of onset (within one or two days), and even if the orchid is saved, it greatly affects the growth vigor of the orchid seedlings.

Here I introduce to orchid friends an active prevention and control method for white silk disease that can promote orchid growth: spreading plant ash. That was in 1993, when I worked at the state-run nursery in Longchang County and also managed the county nursery's orchid garden. In early June, I found that the young shoots of several pots of Longyan Su and Tiegu Su had died. Looking carefully, there were white threads on the pot surface and in the potting soil, and some orchid leaf bases had rapeseed-like granular objects - sclerotia. The dreaded white silk disease had occurred.

As per routine, I sprayed the whole garden once with thiophanate-methyl for sterilization, repotted all the diseased orchids, cut off the rotten roots and rotten shoots, first washed the diseased plants with potassium permanganate solution, then soaked them for 10 minutes in a 1000-fold dilution of thiophanate-methyl, and after drying off the moisture, replanted them with new humus soil from under the forest plus 30% shiguzi (shale), and placed them in isolation.

One week later, white mycelium grew on the pot surface again. When I tipped out the potting soil to look, the white mycelium had almost filled the pot. At the same time, several more pots infected with this disease were newly discovered in the garden. Not daring to delay, I quickly consulted professional books to find the cause of the disease and control measures. The pathogen of this disease is Sclerotium rolfsii. It belongs to the fungi imperfecti. The pathogen survives as mycelium or sclerotia on diseased plant residues, on weeds, or in the soil. The optimum temperature for pathogen growth is 29-32 degrees. In seasons of high temperature and high humidity, and in environments where the soil is relatively acidic, the disease is severe. Carbon and other materials can be used to disinfect the soil for control. Based on the characteristic that white silk disease grows suitably in an acidic environment, I spread plant ash once over the potting soil of all orchids in the garden. Plant ash is alkaline, can moderate the acidity of the potting soil, has a sterilizing function, and contains potassium and calcium fertilizers, which can promote orchid growth. For the diseased orchids, I adopted the aforementioned conventional method and added a procedure of soaking in plant ash water. After potting, I also spread a thin layer of slaked lime on the pot surface. In this way, the pathogen was brought under control, but only 40% of the diseased orchids were saved. After spreading plant ash in the garden, no newly diseased orchids were found, and the orchids grew well.

In 1994, in May before the plum rains arrived, drawing on the prevention and control experience of 1993 and striving to take the initiative, I spread plant ash once comprehensively over several hundred pots of orchids in the nursery orchid garden, and as a result, white silk disease was nowhere to be seen that year. The spreading method is: use plant ash, that is, the ash after grass, wood, and tree leaves are burned, and spread it on the surface of the potting soil beneath the orchid leaves. The amount should preferably be a uniform 1 mm thick layer on the pot surface. After spreading, use clean water in a sprayer to wash the leaves.