Tomato - tobacco mosaic virus disease
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The plant disease caused by tobacco mosaic virus is found worldwide. The virus is known to infect more than 150 types of herbaceous, dicotyledonous plants including many vegetables, flowers, and weeds. Infection by tobacco mosaic virus causes serious losses on several crops including tomatoes, peppers, and many ornamentals. Tobacco mosaic virus is one of the most common causes of virus diseases of plants in Minnesota.
Figure 1. Tomato leaves infected with tobacco mosaic virus. Note the mosaic patterns, leaf distortions and blistering of the leaves.
Many viruses produce mosaic-like symptoms on plants. Mosaic-like symptoms are characterized by intermingled patches of normal and light green or yellowish colors on the leaves of infected plants (Figure 1). Tobacco mosaic damages the leaves, flowers, and fruit and causes stunting of the plant. The virus almost never kills plants but lowers the quality and quantity of the crop, particularly when the plants are infected while young.
Virus-infected plants often are confused with plants affected by herbicide or air pollution damage, mineral deficiencies, and other plant diseases. Positive identification of tobacco mosaic virus in infected plants often requires the services of a plant pathologist and the use of an electron microscope. Although it may take a plant pathologist to diagnose tobacco mosaic virus in many ornamental plants, the majority of tomato plants showing mosaic symptoms usually are infected by tobacco mosaic virus.
Common plant hosts
In Minnesota, common plant hosts for the mosaic virus are tomato, pepper, petunia, snapdragon, delphinium, and marigold. Tobacco mosaic virus also has been reported to a lesser extent in muskmelon, cucumber, squash, spinach, celosia, impatiens, ground cherry, phlox, zinnia, certain types of ivy, plantain, night shade, and jimson weed. Although tobacco mosaic virus may infect many other types of plants, it generally is restricted to plants that are grown in seedbeds and transplanted or plants that are handled frequently.
Figure 2. Tomato fruit infected with tobacco mosaic virus. Note the distortion of the fruit and the intermingled patches of light colored areas on the fruit.
In tomatoes, the foliage shows mosaic (mottled) areas with alternating yellowish and dark green areas (Figure 1). Leaves are sometimes fern-like in appearance and sharply pointed. Infections of young plants reduce fruit set and occasionally cause blemishes and distortions of the fruit (Figure 2). The dark green areas of the mottle often appear thicker and somewhat elevated giving the leaves a blister-like appearance (Figure 1). Symptoms on other plant hosts include various degrees of chlorosis, curling, mottling mosaic, dwarfing, distortion, and blistering of the leaves. Many times the entire plant is dwarfed and flowers are discolored. Symptoms can be influenced by temperature, light conditions, nutritional factors, and water stress.
Viruses differ from fungi and bacteria in that they do not produce spores or other structures capable of penetrating plant parts. Since viruses have no active methods of entering plant cells, they must rely upon mechanically caused wounds, vegetative propagation of plants, grafting, seed, pollen, and being carried on the mouth parts of chewing insects. Tobacco mosaic virus is most commonly introduced into plants through small wounds caused by handling and by insects chewing on plant parts.
Figure 3. Photograph taken with an electron microscope of the rather stiff, rod-shaped virus particles of tobacco mosaic virus from infected tomato. The bar represents 200 nanometers or 0.000008 inches.
The most common sources of virus inoculum for tobacco mosaic virus are the debris of infected plants that remains in the soil and certain infected tobacco products that contaminate workers hands. Cigars, cigarettes, and pipe tobaccos can be infected with tobacco mosaic virus. Handling these smoking materials contaminates the hands, and subsequent handling of plants results in a transmission of the virus. Therefore, do not smoke while handling or transplanting plants.
Once the virus enters the host, it begins to multiply by inducing host cells to form more virus (Figure 3). Viruses do not cause disease by consuming or killing cells but rather by taking over the metabolic cell processes, resulting in abnormal cell functioning. Abnormal metabolic functions of infected cells are expressed as mosaic and other symptoms as previously described. Infected plants serve as reservoirs for the virus and the virus can be transmitted easily (either mechanically or by insects) to healthy plants.
Unlike fungicidal chemicals used to control fungal diseases, to date there are no efficient chemical treatments that protect plant parts from virus infection. Additionally, there are no known chemical treatments used under field conditions that eliminate viral infections from plant tissues once they do occur. Practically speaking, plants infected by viruses remain so. Thus, control of tobacco mosaic virus is primarily focused on reducing and eliminating sources of the virus and limiting the spread by insects. Tobacco mosaic virus is the most persistent plant virus known. It has been known to survive up to 50 years in dried plant parts. Therefore, sanitation is the single most important practice in controlling tobacco mosaic virus.
Control for seedling growers and gardeners
The most common method of transferring the virus from plant to plant is on contaminated hands and tools. Workers who transplant seedlings should refrain from smoking during transplanting and wash their hands frequently and thoroughly with soap and water. Tools used in transplanting can be placed in boiling water for 5 minutes and then washed with a strong soap or detergent solution. Dipping tools in household bleach is not effective for virus decontamination. Any seedlings that appear to have mosaic symptoms or are stunted and distorted should be removed and destroyed. After removing diseased plants, never handle healthy plants without washing hands and decontaminating tools used to remove diseased plants.
Persons purchasing small tomato plants for transplanting should beware of any plants showing mottling, dwarfing, or stunting. Avoid the purchase of any affected plant. Gardeners are advised to follow the same procedures recommended for greenhouse workers when handling tomato transplants. Other control methods for home gardeners include roguing (removal of diseased plants), destruction of diseased and infected plants, and control of weeds and chewing insects. When roguing and destroying mature diseased plants from the home garden, be sure to wash hands and decontaminate any tools used in the process before contacting healthy plants.
Control for commercial producers
Commercial greenhouse producers of tomatoes should follow control practices for seedling production as stated above. It is essential for commercial growers to constantly inspect and rogue diseased production plants while the plants are in the seedling stage. An experienced individual, who is familiar with the tobacco mosaic virus symptoms, should do the initial inspection.
Roguing of young production plants is recommended and should take place before workers are allowed to prune or tie up production plants. When removing diseased plants, also remove one plant on either side of the diseased one. The reason for this is that it is almost impossible to remove a diseased plant and not contaminate the healthy adjacent plants. Never attempt to transplant a healthy tomato into the soil from which a diseased plant was removed. Roots from diseased plants will remain in the soil and provide the virus inoculum for the new transplant.
As a matter of routine, soils from which production plants have been removed, following harvest, should be steam sterilized before the introduction of new seedlings. Steam sterilization can be accomplished by steam or air-steam mixtures. In the preparation of soil for steam sterilization, sift it to remove clumps and large pieces of organic matter. The total soil mixture will have to be heated to a temperature of 200° F for 40 minutes. Since high temperatures are required, steam sterilization must be done in an enclosed system. Temperatures within the steam sterilization system should be monitored by high temperature thermometers to make sure the desired temperature has been reached. Steam sterilization of soil also will eliminate fungi, insects, nematodes, and weeds from the soil. Steam sterilization also is recommended for gravel mixtures used in hydroponic operations following the same procedure described above.
Grow individual production plants in separate containers so that the soil or growing media can be removed when roguing infected production plants. Remember that the soil harbors old root tissues that may serve as inoculum when new roots are introduced. Growing production plants in separate containers is also useful for the control of root diseases caused by fungi and bacteria.