Identification and control methods of orchid fungal diseases

OrchidThe causes of diseases, as with other plants, are infection by pathogens such as moulds, bacteria, viruses, viroids, mycoplasma-like organisms, etc., parasitism by parasitic plants, or physiological factors such as nutrients, water, temperature, pollution, etc. The following takes fungal diseases as an example to describe them separately: OrchidThe most important diseases are caused by fungi, accounting for at least more than 2/3 of all diseases. Fungi are mostly small organisms that can only be seen under a microscope. They have no chlorophyll and cannot manufacture nutrients; they must absorb nutrients from living plants or dead plants, or even animals. Fungi are composed of many branching filaments, namely hyphae. Hyphae can reach 1Oum in diameter and several metres in length, and can produce large numbers of spores. Spores are spread through moving air, water, insects or other animals. InfectionOrchidThe fungi are all parasitic. They absorb nutrients from the host, destroy tissues, and thereby cause the host to produce many symptoms, such as rot (the infected part decays), rust (the affected part turns rust-coloured), damping-off (rapid wilting, leading to death of the whole seedling), blight (turning brown, then dying), canker (localised decay or sunken lesions), anthracnose (ulcer-like necrosis appearing black and sunken), scab (slightly raised or sunken, may crack), spot (leaf surface damage, including tissue death and collapse), mould (the infected part covered with a powdery substance), tissue death, and plant decline (weakened metabolism, blocked vascular bundles, etc.). Some fungi parasitise the surface of the orchid plant body, inserting special hyphae into epidermal cells; some live between cells or around living cells; but most fungi are indiscriminate, passing through tissues (including cells) and causing them to die. Examination of fungal diseases must be carried out under a microscope or transmission electron microscope; infected parts need special treatment, then sectioned for observation. Identification requires the aid of specialised reference books. Because pathogenic fungi are spread through air, water, insects, other animals, humans or direct contact with the plants themselves. Therefore, disease prevention must first pay attention to cleanliness and hygiene, develop good horticultural practice habits, and control transmission vectors in order to eliminate sources of infection. Only when the above measures are inadequate will fungal diseases be caused, thus forcing the use of chemical agents. As far as is currently known, the fungi harming orchidflowersnumber about 30 or more genera and over 100 species. Among them, the more common are some species of the following genera: Colletotrichum, which causes anthracnose, Sclerotium, which causes southern blight, Botrytis, which causes flower and fruit blight, Phytophthora, which causes black rot or heart rot, Fusarium, which causes leaf spot or blight, and Uredo, Coleosporium, etc., which cause rust. (Chen Xinqi, Ji Zhanhe)