Virus Diseases of Tomato
Ned Tisserat Extension Specialist, Plant Pathology http://www.plantpath.ksu.edu/pages/extension Figure1. Leaf symptoms of tomato mosaic virus.
Figure 2. Fruit symptoms of tomato mosaic virus.
Disease
Several virus diseases of tomato occur in Kansas, although they generally are not as prevalent as the wilt and foliar diseases. Three of the more common virus diseases are tomato mosaic (also referred to as tobacco mosaic), cucumber mosaic, and tomato spotted wilt. The tomato mosaic virus can attack a wide range of plants, including tomato, pepper, eggplant, tobacco, spinach, petunia, and marigold. On tomato, virus infection causes light and dark green mottled areas on the leaves. The dark green areas tend to be somewhat thicker than the lighter portions of the leaf. The leaf mottling is
seen more easily if the affected plant surface is partially shaded. Stunting of young plants is common, and often is accompanied by a distortion and fern-like appearance of the leaves. Older leaves curl downward and may be slightly distorted. Certain strains of the virus can cause a mottling, streaking, and necrosis of the fruits. Infected plants are not killed, but they produce poor quality fruit and low yields.
The cucumber mosaic virus has one of the broadest host ranges of any of the viruses. The disease affects a number of important vegetables and ornamentals including tomato, pepper, cucumber, melons, squash, spinach, celery, beets, and petunia. Tomatoes infected with the cucumber mosaic virus develop a slight yellowing and
mottling of the older leaves. Expanding leaves typically become twisted, curl downward, and develop a 'shoestring' appearance as a result of a restriction of the leaf surface to a narrow band around the midrib of the leaf. Diseased plants are stunted and produce small quantities of fruit.
The tomato spotted wilt virus also has a wide host range and can affect a number of ornamental plants as well as tomato. Early symptoms of spotted wilt on tomato are
difficult to diagnose. Young, infected plants may show an inward cupping of leaves, and the foliage may appear off-color or have a slight bronze cast. In some cases, leaves with exhibit dark purple flecks or small necrotic concentric rings.
Figure 3. Bronze cast to leaves caused by tomato spotted wilt.
Figure 4. Purple flecking of young leaves caused by tomato spotted wilt.
As the disease progresses, plants may develop dark brown to black streaks on the main stem. Occasionally the top portion of the plant wilts.
The most characteristic symptom of spotted wilt appears on the fruit. On young fruit, white to yellow concentric rings, one-half inch in diameter, develop on the fruit skin. a bumpy or warty appearance. The bright yellow rings on red, mature fruit are quite striking and are easily diagnosed as spotted wilt. The area within the ring typically is raised, which gives the fruit.
Figure 5. Fruit symptoms of tomato spotted wilt.
The leaf distortion associated with some of these viral diseases often is confused with injury resulting from phenoxy herbicide exposure. Generally, a phenoxy herbicide (2,4-
D) will uniformly affect all tomato plants in a garden or greenhouse. The herbicide also will cause leaf distortions on a number of other broadleaf plants in the area. Plants
exposed to low levels of the herbicide may recover. In contrast, virus diseases normally will not affect all plants at the same time, or cause injury to other types of plants in the area. Once the plant is infected, it will not recover.
Causes
Tomato mosaic, cucumber mosaic, and spotted wilt are incited by viruses. The tomato mosaic virus is very stable and can persist in dry contaminated soil, in infected tomato debris, on or in the seed coat. The virus is transmitted readily from plant to plant by mechanical means. This may simply involve picking up the virus while working with infected plant material, then inoculating healthy plants by rubbing or brushing against them with contaminated tools, clothing, or hands. Aphids are not vectors of the tomato mosaic virus, although certain chewing insects may transmit the pathogen.
The cucumber mosaic virus overwinters in perennial weeds and may be transmitted to healthy plants by aphid vectors (although tomatoes are not the preferred host of aphids) or by mechanical means.
The cucumber mosaic virus cannot withstand drying, or persist in the soil. It also is more difficult than tobacco mosaic to transmit mechanically. Thus, cucumber mosaic tends to progress more slowly than tobacco mosaic in a field or garden.
The spotted wilt virus is transmitted from plant to plant by several species of small insects called thrips. Thrips are less than one-quarter inch in length, light green to brown, and are extremely difficult to find on the plants. Several weedy hosts and ornamental plants may serve as alternate hosts for the virus.
Control
Virus diseases cannot be controlled once the plant is infected. Therefore, every effort should be made to prevent introduction of virus diseases into the garden. Many varieties of tomato are available with resistance to tomato mosaic virus. Sanitation is the primary means of controlling the other two virus diseases. Infected plants should be removed immediately to prevent spread of the pathogens. Perennial weeds, which may serve as alternate hosts, should be controlled in and adjacent to the garden. Avoid planting tomatoes next to cucurbits, spinach, or other vegetables and flowers susceptible to these diseases. Control of insects, especially aphids and thrips, will help reduce the likelihood of cucumber mosaic and spotted wilt.
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