Identification and Control Methods for Common Orchid Diseases

Orchid diseases, like those of other plants, are caused by infection by pathogens such as fungi, bacteria, viruses, viroids, and mycoplasma-like organisms, parasitism by parasitic plants, or physiological factors such as nutrients, water, temperature, and pollution. The following uses fungal diseases as an example to describe them one by one: The most important diseases of orchids are caused by fungi, accounting for at least 2/3 or more of all diseases. Fungi are mostly small organisms that can only be seen under a microscope. They have no chlorophyll and cannot manufacture nutrients, so they must absorb nutrients from living plants or dead plants, or even animals. Fungi are composed of many branched filaments called hyphae. Hyphae can reach 1Oum in diameter and several meters in length, and can produce large numbers of spores. Spores spread through moving air, water, insects, or other animals. The fungi that infect orchids are all parasitic. They absorb nutrients from the host, destroy tissues, and thereby cause the host to develop many symptoms, such as rot (infected parts decay), rust (affected parts turn rust-colored), damping-off (rapid wilting, leading to death of the entire seedling), blight (turning brown, then dying), canker (localized decay or sunken lesions), anthracnose (ulcer-like necrosis appearing as black sunken areas), scab (slightly raised or sunken, may crack), spots (leaf surface damage, including tissue death and collapse), mold (infected parts covered with a powdery substance), tissue death, and plant decline (weakened metabolism, blocked vascular bundles, etc.). Some fungi parasitize the surface of the orchid plant body, inserting special hyphae into epidermal cells; some live between cells or around living cells; but most fungi indiscriminately penetrate tissues (including cells), causing them to die. Examining fungal diseases must be done under a microscope or transmission electron microscope. Infected parts need special treatment, then sectioning, in preparation for observation. Identification requires the aid of specialized reference books. Because pathogenic fungi spread through air, water, insects, other animals, humans, or direct contact with the plants themselves, disease prevention must first emphasize cleanliness and hygiene, develop good horticultural practice habits, and control transmission vectors to eliminate sources of disease. Only when the above measures are ineffective will fungal diseases occur, forcing the use of chemical agents. As currently known, there are about 30 or more genera and over 100 species of fungi harmful to orchids. Among the more common are some species in the following genera: Collectotrichum, which causes anthracnose; Sclerotium, which causes southern blight; Botrytis, which causes flower and fruit blight; Phytophthora, which causes black rot or heart rot; Fusarum, which causes leaf spot or blight; and Uerdo, Coleosporium, etc., which cause rust. (Chen Xinqi, Ji Zhanhe)