Greenhouse Disease Management
Greenhouse Viral Diseases
Viruses are ultra-microscopic, infectious particles composed of nucleic acid surrounded by a protein coat. Virus particles multiply only within living host plant cells where they disrupt normal cell functions. They are too small to be seen under a light microscope, they can be observed using an electron microscope. Viruses can spread systemically throughout the host plant, and plants may be infected even when symptoms of disease are not apparent. Many different viruses can infect greenhouse crops. Insects such as aphids, thrips, mites, and leafhoppers are the most important vectors of viruses. At the present time, no effective chemicals will control virus diseases. Therefore, sanitation and use of resistant varieties of plants has been the most effective means of controlling plant viruses.
Symptoms
Virus symptoms are often quite striking and distinctive. Symptoms vary depending with the type of virus, the host plant, how long the host plant has been infected, the strain of the virus, and the environmental conditions. Symptoms depend on the duration of the infection, the age of the plant at the time of infection, and the crop growing conditions. Symptom expression can be temperature sensitive— some viruses are expressed at high temperatures whereas others are expressed at lower temperatures. Viral symptoms can also be masked when the plants are growing vigorously. Sometimes, symptoms may only be apparent when multiple infections are present or when plants become stressed. Some viruses cause stunting. In severely infected plants, the growing point of the plant may die. Some virus-infected plants are grown and propagated because of their attractive appearance. Viruses disrupt the normal growth and developmental processes in the plant. Hence, a variety of changes in the leaf and petal coloration, texture, and shape often occur.
Cultural Management Strategies
No chemical controls will eradicate viral diseases. Prevention is the key. The best way to control viruses is to keep them out of production areas. Prevention is the grower's first line of defense against virus infection. Purchase clean, virus-free seed, cuttings, seed and stock plants from a reputable supplier. Virus-indexed plant material may be available for certain crops. If unsure, isolate incoming plants in quarantine type area until you have determined that they are virus-free. Don't rely on visual diagnosis to determine whether or what type of virus is present. You may not become aware of a problem until it is widespread.
Common Greenhouse Plant Viruses
Most virus diseases in the greenhouse are caused by several common viruses: Impatiens necrotic spot virus (INSV), Cucumber mosaic virus (CMV), Tobacco mosaic virus (TMV), and to a lesser extent Tomato spotted wilt virus (TSWV).
Impatiens Necrotic Spot Virus (INSV)
Impatiens necrotic spot virus (INSV) is transmitted by thrips and can cause disease on greenhouse crops. Symptoms of INSV on impatiens include dark black or purple lesions on the stems and leaf veins and dark ringspots or blotches on leaves. Infected plants are stunted, and young leaves may be small and misshapen (See Figure 28.13).
Cucumber Mosaic Virus (CMV)
Cucumber mosaic virus (CMV) has a wide host range of over 400 species of plants. CMV has been reported on ajuga, aquilegia, campanula, delphinium, dahlia, lilium, petunia, and phlox. Symptoms vary widely depending on the host plant; possibilities include distortion of foliage, mosaic, mottling, stunting, and necrosis (See Figure 28.14). Symptoms can resemble herbicide injury. This virus is spread by several aphid vectors.
Tobacco Mosaic Virus (TMV)
Tobacco mosaic virus (TMV) has a wide host range but is especially a concern on solanaceous crops. TMV has been reported on ajuga, calibrachoa, cyclamen, epimedium, gerbera, helianthus, impatiens, lisianthus, lysimachia, New Guinea impatiens, nicotiana, pepper, petunia, penstemon, tomato, and torenia.
Tomato Spotted Wilt Virus (TSWV)
Tomato spotted wilt virus (TSWV) is a tospovirus that is transmitted by thrips, which are small insects that feed on a variety of plants by puncturing the leaves and sucking the plant’s contents. Thrips contract the virus while they are in the larval stage and feed on infected plants, such as weeds.
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