Identification and Control Methods for Crop Viral Diseases

Viral disease is a special type of disease, and can be called a stubborn problem among plant diseases. In recent years there has been a trend of gradual worsening. Because it differs considerably from ordinary diseases in symptom characteristics, patterns of occurrence and control measures, in order to facilitate correct identification and control, some key points for diagnosis, identification and control based on practical experience and reference materials are described in detail below: I. Diagnosis and Identification 1. Symptom diagnosis can be roughly divided into the following types: (1) Discoloration: includes 3 types: "mosaic", "mottling" and "breaking". "Mosaic" refers to patches of alternating dark and light green appearing on diseased leaves. "Mottling" refers to chlorotic spots on diseased leaves, with the spots being relatively large, with indistinct edges and uneven distribution. (2) Chlorosis, yellowing: the whole plant or some organs appear light green or yellow. (3) Spots, streaks: often occurring on leaves, stems and other parts, appearing as necrotic spots, necrotic streaks, chlorotic spots or chlorotic streaks. (4) Ringspot, oak-leaf: both mostly appear on leaves; concentric ring-shaped spots are called "ringspot". Discoloration patterns along the leaf veins shaped like oak leaves are called "oak-leaf". (5) Vein clearing, vein yellowing, vein banding: vein clearing and vein yellowing are the early stages of mosaic symptoms; first the leaf veins become transparent, called "vein clearing", then the leaf veins turn yellow, called "vein yellowing"; "vein banding" is when the tissue along the leaf veins turns dark green. (6) Leaf wrinkling, leaf curling: deformation of local tissues or organs. Leaf vein growth is inhibited while the mesophyll still grows, the leaf blade becomes wrinkled, and the leaf margin curls upward or downward. (7) Enations: protrusions form on the leaf veins on the underside of diseased leaves, shaped like ears, called "enations". 2. Observation and analysis in ordinary practice is of great significance for disease diagnosis. In terms of symptom characteristics, viral diseases are easily confused with non-infectious diseases, especially diseases caused by nutrient deficiency, air pollution and so on. If healthy plants are near virus-infected diseased plants, they should be isolated immediately. Once a plant develops the disease it often cannot recover. Non-infectious diseases mostly occur in patches; such diseases can be restored to health by increasing nutrition and improving environmental conditions.OrchidsAmong viral disease symptoms, mosaic and yellowing are the most common, and they often appear as systemic infection, with rotting and wilting symptoms being relatively rare. Of course, viral symptoms sometimes easily change; different viruses on different host species and varieties will differ, and some viral diseases may show latent symptoms due to changes in temperature and so on. 3. Diagnosis using inclusion bodies After plant cells are infected by a virus, they produce tiny individuals different from other cell structures; the shapes and compositions of these individuals all differ, and they are called "inclusion bodies". By observing inclusion bodies with special shapes under a microscope, viral diseases can be diagnosed, but diagnosis using inclusion bodies alone is not reliable. II. Modes of transmission: 1. Sap transmission Sap transmission usually occurs only in mosaic-type viral diseases. During cultivation, some viral diseases can be transmitted through mutual rubbing between the branches and leaves of diseased and healthy plants or through artificial contact rubbing. Other management operations such as pricking out seedlings, trimming, and cutting asexual propagation material cause the fingers or tools to be contaminated with diseased sap and thus spread the virus. 2. Vector transmission Mainly by insects, especially aphids and leafhoppers, which are the most common, followed by soil nematodes and fungi. 3. Transmission by asexual propagation material Because viral diseases are whole-plant infections, once a virus is contracted, the host plant and its various parts generally all carry the virus. For example, tubers, corms, bulbs, tuberous roots, runners and so on can all spread viral diseases. 4. Transmission through growing media is actually contact transmission between organisms in the growing medium and the host plant; it is divided into two kinds: one is transmission by nematodes and fungi in the soil, and the other is transmission by virus-carrying organic matter in the soil. Therefore, spreading viruses is very easy. III. Control methods: Although no effective cure has yet been found, the patterns of disease occurrence can be explored and grasped through production practice, and a whole set of comprehensive preventive measures can be adopted to keep it under control. 1. First, when introducing plants, inspection of whether orchid plants carry viral diseases should be strengthened, and virus-free planting material must be strictly used. 2. Control virus-transmitting vectors by using various methods to eliminate aphids, leafhoppers and nematodes, cutting off transmission routes and preventing the occurrence of viral diseases. 3.CultivationDuring the process, strive for intensive and careful cultivation, properly adjust every link such as water and fertilizer, light, temperature and humidity, ventilation and light penetration, so that no loopholes appear, creating an excellent small growing environment, making the plants grow robustly so that viruses cannot take advantage of any weakness. 4. Environmental hygiene and tools should be disinfected regularly, and transmission of viruses through fingers and tools should be avoided as much as possible. 5. If orchid plants suspected of viral disease are found, first do a good job of isolation and then control; if effective control cannot be achieved, they should be pulled out and burned to avoid spreading and infecting other plants. 6. Chemical control In recent years, with scientific and technological progress, many chemicals effective against viral diseases have been developed, such as Zhibingling, Junduqing, Bingdubike, Bingduiling, Bingdute, Bingkeke and other chemicals. Author: Xu Maojin