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Tomato Mosaic Virus
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A mosaic pattern begins to form 10 to 14 days after the plant becomes infected, with light green coloring between yellowing leaf veins of newer leaves. The affected leaves then begin to curl and become “fern-like” (filiform) or pointed in appearance. Infected plant tissues then develop yellow streaking or spotting until they become completely chlorotic. Newer growth is deformed and distorted. Other symptoms may include mottling, necrosis, and stunting.
Some strains of ToMV also infect flowers and fruits. Fruit may exhibit external symptoms such as brown spots or blotchy discolored areas; the insides may display browning or mottling. When infection is severe, the fruit appear distorted or deformed, ripening is delayed and color non-uniform. The plant will also be stunted with poor yields.
Tomato mosaic virus is very stable in the environment and can survive for at least 50 years in dead, dry plant debris. When a susceptible host is available, the ToMV particles, consisting of a single strand of positive sense RNA, enter the plant cells through wounds or natural openings. The virus spreads very easily through mechanical transmission by contaminated tools, hands, clothing, plant debris, grafting, seeds, and tobacco products such as cigarettes. This virus can also be carried on the mouthparts of chewing insects or animals; however, there is no true insect vector for this pathogen. Once the virus enters the host, it begins to multiply by using the host cell’s replication machinery. Although symptoms are dependent on the age of the infected plant, environmental conditions, the viral strain, and the host plant, symptoms usually appear 10 to 14 days post-infection.
Favorable Environmental Conditions
Symptoms caused by ToMV are more severe at 24°C than at higher or lower temperatures. Low light conditions also promote symptom expression.
Often Confused With
- Herbicide Damage – Herbicide (2,4-D and related) damage on tomato plants consists of the curling, folding, and twisting of foliage. Leaves also become chlorotic, especially in the veins, and in some cases, develop necrosis as well. Damaged fruits are irregularly shaped.
- Tobacco Etch Virus and Potato Virus Y – Light and dark green mosaic develops on the leaves that eventually curl and distort. This occurs mostly on younger and new leaves. Fruits also become infected and display internal browning and uneven ripening. These viruses are vectored by aphids.
Throughout the season, plants should be checked for mosaic, mottling, and chlorotic symptoms. Fruits may also develop a mosaic pattern and be deformed. Infected plants should be removed carefully, without touching healthy plants. Diseased plants should be destroyed to prevent spread of the disease. Dipping hands in milk (e.g. reconstituted nonfat dry milk) inactivates the virus.
Currently, there is no threshold information for ToMV.
- Use Host Resistance – There are several tomato cultivars specifically bred for resistance to ToMV. Cross protection is another form of resistance in tomato to ToMV. This process entails infecting tomato plants with a mild strain of ToMV so that it is resistant to more severe strains of ToMV. Mild ToMV strains do not protect plants from TMV. In addition, severe symptoms may be observed if tomato plants are co-infected with a mild ToMV strain and another virus such as Cucumber mosaic virus.
- Start with Virus-Free Plants and Seeds – ToMV spreads from plant to plant through vegetative propagation, grafting, and seeds. Therefore, it is very important to make sure any plants grown in the high tunnel are virus-free. This will prevent mechanical transmission to healthy plants.
- Use Sounds Sanitation Practices – Sanitize tools and dispose of infected materials and plants carefully. Wash hands and dip them in milk after handling TMV-infected plants and before handling any other plants. Keep the high tunnel weed free because weeds may harbor ToMV. Also, do not use any tobacco products when handling the plants in the high tunnels.