How to prevent and control southern blight in orchids? Finding the right nemesis is key

Promote Chinese orchid culture, spread orchid knowledge

After reading the article "The Whole Story of Treating Southern Blight" written by Zhou Weiqi of Zhejiang, published in the first issue of "Chinese Orchids" in 1999, I deeply resonate: the southern blight fungus in orchids is stubborn and grows rapidly, often causing the death of orchid plants within 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 southern blight under control. This method is good, but it is only a passive method of treating southern blight, 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 enthusiasts an active prevention and control method for southern blight that can promote orchid growth: spreading plant ash. That was in 1993, when I worked at the state-owned 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 Tiegusu had died. On closer inspection, 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 southern blight had occurred.

As per routine, a thiophanate-methyl spray sterilization was carried out for the whole garden, all diseased orchids were repotted, rotten roots and rotten shoots were cut off, the diseased plants were first washed with potassium permanganate solution, then soaked in 1000 times diluted thiophanate-methyl solution for 10 minutes, and after drying off the moisture, they were replanted with new understory humus soil plus 30% stone bone (shale) and placed in isolation.

A week later, white mycelium grew out on the pot surface again. When the potting soil was tipped out 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 prevention 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 soil. The suitable 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 methods can be used to disinfect the soil for prevention and control. Based on the characteristic that southern blight grows suitably in an acidic environment, plant ash was spread once on 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 diseased orchids, the aforementioned conventional method was adopted, with an added procedure of soaking in plant ash water, and after potting, a thin layer of slaked lime was spread 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 arrival of the plum rains, drawing on the prevention and control experience of 1993 and striving to take the initiative, a comprehensive spreading of plant ash was carried out once on several hundred pots of orchids in the nursery orchid garden. As a result, that year southern blight left no trace. 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 an even thickness of 1 mm on the pot surface. After spreading, spray with clear water to wash the leaves.