Orchid rust

Orchid rust has not been systematically reported in China, but it has not been systematically studied recently in Shenzhen. Rust is common in other parts of the world. There are many kinds of pathogens and a wide range of hosts, because the number of some spore generations that cause rust is large, the color changes from yellowish brown to dark brown, and when the spores are released, the place of infection is rust, so this kind of disease is called rust. Generally speaking, this kind of disease is not too destructive to herbaceous ornamental plants, but sometimes it can seriously hinder the healthy growth of plants and even cause the death of seedlings.

Rust is generally an obligatory living parasite. In nature, they only infect living plant tissues, but do not grow on dead organic matter. For most plants, rust infects their leaves, but sometimes they also infect stems or flowers, and the lower leaves are usually susceptible to infection, indicating that leaf age is related to the infection process. Sometimes the infection of rust is so serious that it destroys the whole leaf or several leaves, but rarely leads to the death of the whole infected plant. For some kinds of leaves, the infection can form an abscission layer, and then the seriously infected leaves fall off from the plant. In the orchid rust disease found in the main field of Wutong Mountain nursery in Shenzhen, the harm often occurs on the leaves. Its typical symptom is the formation of rust brown or black longitudinal stripes on the leaves, which are parallel to the veins of the leaves, and are covered with leaves in severe cases. The appearance is extremely indecent and the disease spot is brittle in severe cases. Symptoms can occur on the surface and back of the leaf, as well as on both sides. However, the host species damaged by this rust are not common, and they are limited to some special varieties, such as "Fuji brocade" of Jianlan, "zodiac", "crane flower", "Xuzhuo brocade", "Wandaifu" and Zhongtuoyi varieties. Like most typical fungi, rust also needs condensed water to infect leaves in order for spores to germinate and infection to occur.

The life cycle of many rust is divided into two distinct stages: one stage in which the rust cell has only one nucleus (called monokaryon or gametophyte stage), and the other stage in which the cell has two nuclei (called binucleate or sporophytic stage). Some rust fungi also produce two distinct spore types in the uninucleate stage, and sometimes as many as three spore types in the binucleate stage. The two stages of some rust occur on completely unrelated odd plants at the same time. This type of rust is called transgenic parasitic rust, for example, it shows one stage in herbaceous plants and another in woody plants. In contrast to the transgenic parasitic rust, the single parasitic rust occurs in all its pool stages on one type of host plant.

With regard to the life cycle of rust, the actual infection of plants consists of only three stages of five spore generations.

Each spore stage has a special function and name, and its five spore stages are as follows:

Summer spores (UredosPores): summer spores are the most common spore stage. The spores released at this stage first rust the infected area, hence the name rust. Summer spores have the ability to germinate and infect immediately, which usually results in an increase in the number of summer spores. The summer spore stage of transitional parasitic rust can be repeated in the same host. Summer spores are single-celled, dark brown in color, depending on the type of rust. They are asexual spores with two nuclei and rough or prickly cell walls. In some incomplete rust fungi, summer spores may be the only spores. They usually form on the back of leaves and grow in a place called summer spore pile. The heap of summer spores fades the color of its frontal leaf tissue (and sometimes the surrounding area). The number of this spore stage is D.

TeliosPores: teliospores are usually dormant spores and can be produced in or around the same place where summer spores grow. They are often produced in large numbers when plants begin to mature or age. Teliospores are darker than summer spores and often appear black. Depending on rust, they may be unicellular, bicellular or multicellular. Teliospores produce binuclear cells at the initial stage, then the two nuclei fuse and divide during spore germination. The winter spores of many rust fungi are dormant spores and can only germinate after dormancy. The teliospore hands of some short-cycle rust sometimes precede the site of production. Nevertheless, each cell of the tetraspore hand produces four very small spores (these spores are called BasidiosPores). Teliospore does not infect plant tissue, and the tissue that forms this kind of teliospore and spore is called Telia, which is often a raised embroidered spot. This spore stage is numbered as a dish.

Basidiospores (BasidiosPores): after germination, teliospores each cell produces a small structure with four very small basidiospores. The spores of a short-cycle single-host parasitic rust infect the same host and produce teliospores on this host. The winter spores of the parasitic rust infect other hosts in the cycle. The number of this spore stage is IV.

Pycnospores: these very small spores do not infect plants, but enter the sexual stage through the binding of mycelium. Thus the sporophytic stage (Sporophytic) begins. Sexual spores are born in a small structure of foliar sporangium (Pycnia) or seminal organ (Spermagonia), usually formed on the adaxial side of the leaf. The sexual spore organ is very small and can produce obvious or inconspicuous black dots in brightly colored spots. The spore stage of this stage is numbered O.

AeciosPores: these light-colored spores (from yellowish to orange) are usually formed in a special structure on the back of the leaves of herbaceous plants. This spore-producing structure, called Aecia, ruptures in infected tissue, often with tissue uplift and discoloration. The rust spores of the parasitic rust fungus infect the host (only summer spores and winter spores are produced on the original host). The rust sporangium often occurs together with the sexual sporidium, either surrounding the sexual sporidium or growing on the wheat flour opposite to the sexual sporidium. Generally speaking, the rust sporidium is the largest rust structure on the host parasitic plant, also known as the spring spore cavity, which is easy to distinguish in appearance, so it is helpful to the identification of rust. The number of rust spore stage is 1.

It is not easy to control orchid rust, so cultivation measures can only be supplemented by control plan. One is to reduce the source of infection, to remove the diseased leaves in the early stage of rust, and to remove the parasitic parasites of the surrounding rust, so the weeds often parasitized by rust should be removed and planted in isolation from other ornamental plants susceptible to rust; second, in the cultivation and management of second flowers, such as watering, be careful not to wet the leaves, especially at night, because wetting leaves is conducive to the growth of rust. The third is the use of chemicals to control orchid rust. In the early stage of orchid rust epidemic, protective fungicides should be sprayed on orchids. Considering that weathering will invalidate the drugs, orchids must be sprayed regularly every 7-14 days in order to achieve good control effect. These protective fungicides include Fumei Iron, Dyson Zinc, Dyson Manganese and Chlorothalonil. When in use, a variety of fungicides should be used together; the fourth is biological control, but there is no effective method for biological control of orchids at present. Some people think that snails, slugs and some beetles will be attracted by plant tissues infected with rust. They will eat the rust-infected parts of the leaves. However, when the disease is serious, this method can not play a role in prevention and treatment.

Liu Zhongjian