Chapter 23

Greenhouse Disease Management

Viral Diseases of Greenhouse Plants

Viruses can cause substantial economic losses for greenhouse growers, and proper management is needed to produce healthy plants. Viruses are disease-causing agents that multiply only within living plant cells. They tend to spread throughout the host plant through the vascular system. Many of the important horticultural crops belong to major plant families that are susceptible to viral attack. With the introduction of many new vegetative annuals to the bedding plant industry, viruses have become of greater importance, particularly for those growers who also grow vegetable transplants. Viruses can be spread by sucking insects, such as aphids and leafhoppers, by tools, or by crop handling such as plant cleaning, disbudding or taking cuttings. Many viruses spread via infected cuttings. 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).

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.

Cultural Management Strategies

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. Viruses tend to move to the growing points in the plant, and symptoms are often most severe in the shoot tips. However, some viruses cause distinguishing symptoms on lower leaves. Symptoms can also mimic those caused by other pathogens and abiotic disorders.

Chemical 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. Routine testing of plants showing symptoms and those not showing symptoms is needed, especially before taking vegetative cuttings.

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 23.12).

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 23.13). 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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