Symptoms of orchid rust disease and its prevention and control methods

OrchidsRust disease has not been systematically reported in China; it was recently discovered in Shenzhen, China, but no systematic and in-depth research has yet been carried out. In other parts of the world, the occurrence of rust disease is still quite common. Its pathogenic species are numerous and its host range is broad, because certain spore generations that cause rust disease are numerous, and their color changes from yellowish-brown to dark brown, and when the spores are released, the infected areas all appear rust-colored, which is why this type of disease is called rust. Generally speaking, this type of disease is not too devastating to herbaceous ornamental plants, but sometimes it can also seriously hinder healthy plant growth and even cause seedling death. Rust fungi are generally obligate biotrophs. In nature, they have infective ability only on living plant tissues, and will not grow on dead organic matter. For most plants, rust fungi infect the leaves, but sometimes they also infect the stems or flowers; usually the lower leaves are susceptible to infection, which indicates that leaf age is related to the infection process. Sometimes rust infection can be very severe, even destroying an entire leaf or several leaves, yet it rarely causes the entire infected plant to die. For certain types of leaves, infection can form an abscission layer, and then severely infected leaves fall from the plant. In the rust disease oforchidsfound at the Wutong Mountain Nursery General Farm in Shenzhen, China, the damage often occurs on the leaves, and its typical symptom is the formation of rust-brown or black longitudinal streaks on the leaves, parallel to the leaf veins; in severe cases they cover the leaf surface, making the appearance extremely unsightly, and in severe cases the diseased spots become brittle and split. Symptoms can occur on the leaf surface, the leaf underside, or both sides. However, the host species damaged by this rust disease are not widespread, and are limited to certain particular varieties, generally the "Fuji Nishiki" of theCymbidium ensifoliumgroup, and the "Huangdao", "Hezhihua", "Xuhuangjin", and "Wandaifu" of theCymbidium sinensegroup, as well as varieties with central translucent variegation. Rust fungi, like most typical fungi, also require condensed moisture for leaf infection, so that the spores can germinate and infection can occur. The life cycle of many rust fungi is divided into two distinctly different stages: in one stage the rust fungus cells have only one nucleus (called the monokaryotic or gametophytic phase), and in the other stage the cells have two nuclei (called the dikaryotic or sporophytic phase). Some rust fungi also produce two distinctly different spore types during the monokaryotic phase, and during the dikaryotic phase sometimes produce as many as three spore types. In some rust fungi, the two stages occur simultaneously on completely unrelated host plants; such rust fungi are called heteroecious rust fungi, for example, they may show one stage on herbaceous plants and another stage on woody plants. In contrast to heteroecious rust fungi are autoecious rust fungi, whose entire spore stages occur on one type of host plant. Regarding the life cycle of rust fungi, the actual infection of plants includes only 3 of the 5 spore generations, each spore stage has a particular function and name, and the 5 spore stages are as follows: Uredospores: Uredospores are the most common spore stage; the spores released at this stage first make the infected site appear rust-colored, hence the name rust disease. Uredospores have the ability to germinate and infect immediately, and the result is usually an increase in the number of uredospores. In heteroecious rust fungi, the uredospore stage can be repeatedly formed on the same host. Uredospores are single-celled, and their color ranges from yellow to dark brown depending on the rust species; they are asexual reproductive spores, have two nuclei, and have rough or spiny cell walls. In some imperfect rust fungi, uredospores may be the only spores; they usually form on the underside of leaves, and the site where they grow is called a uredinium. The uredinium causes the leaf tissue on the upper side (and sometimes the surrounding area) to fade in color. The number of this spore stage is D. Teliospores: Teliospores are usually resting spores and can be produced at the same site as, or around, where uredospores grow. They are often produced in large numbers when the plant begins to mature or senesce. Teliospores are darker in color than uredospores, often appearing black, and depending on the rust fungus they may be single-celled, two-celled, or multicellular. Teliospores are initially dikaryotic cells, then the two nuclei fuse and divide when the spore germinates. The teliospores of many rust fungi are resting spores and can germinate only after a period of dormancy. In some short-cycle rust fungi, the teliospores sometimes germinate at the site where they are produced; nevertheless, each cell of the teliospore produces four very small spores (these spores are called basidiospores). Teliospores do not infect plant tissues; the tissue in which these teliospores and spores are formed is called a telium, and the telium is often a raised rust-spot area. The number of this spore stage is III. Basidiospores: After germination, each cell of the teliospore produces a small structure bearing four very small basidiospores; the spores of short-cycle autoecious rust fungi infect the same host and produce teliospores on this host. The teliospores of heteroecious rust fungi infect another host in the cycle. The number of this spore stage is IV. Pycnospores: These very small spores do not infect plants, but instead enter the sexual phase through the union of mycelia. In this way the sporophytic phase begins. Pycnospores are borne in a small structure called a pycnium or spermogonium, usually formed on the upper surface of the leaf. Pycnia are very small and can produce obvious or inconspicuous black dots within brightly colored round spots. The number of this spore stage is 0. Aeciospores: These very pale spores (from pale yellow to orange) are usually produced in special structures on the underside of the leaves of herbaceous plants. This spore-producing structure is called an aecium; it ruptures in the infected tissue, often raising and discoloring the tissue along with it. In heteroecious rust fungi, the aeciospores infect the alternate host (on the original host only uredospores and teliospores are produced). Aecia are often found together with pycnia, either surrounding the pycnia or borne on the leaf surface opposite the pycnia. Aecia are generally the largest rust fungus structures on heteroecious plants, and are also called aecial cups; together with their easily distinguished appearance, this helps in the identification of rust fungi. The number of this aeciospore stage is 1. The prevention and control oforchidrust disease is very difficult; one can only carry out control by supplementingcultivationmeasures with a control plan. First, reduce the sources of infection: removing diseased leaves in the early stage of rust disease gives notable control efficacy; eliminate alternate hosts of rust fungi in the surroundings, so weeds that rust fungi commonly parasitize should be removed and planting should be isolated from other ornamental plants susceptible to rust disease. Second, in orchidcultivationmanagement, for example, when watering, care should be taken not to wet the leaves, especially at night, because wetting the leaves favors the growth of rust fungi. Third, use chemical agents to control orchid rust disease. In the early stage of an orchid rust disease epidemic, protective fungicides should be sprayed on the orchids; considering that weathering will cause the chemicals to lose effectiveness, regular spraying every 7 to 14 days is necessary to achieve good control results. Such protective fungicides include ferbam, zineb, maneb, and chlorothalonil, among others. When using them, it is advisable to use several fungicides in combination. Fourth, biological control, but at present there is no effective method for the biological control oforchiddiseases. Some believe that snails, slugs, and certain beetles are attracted to plant tissues infected with rust fungi, and they will bore into and eat away the rust-infected parts on the leaves. However, when the disease is severe, this method cannot achieve control. Liu Zhongjian