There are numerous viruses found on the different plants grown in commercial fields as well as in the home vegetable garden. Most viruses are spread by insects (mostly but not always by aphids, thrips and white flies) and can also be seedborne. The following are just a few of the viruses found in vegetable hosts (as well as flower and weed plants). Lettuce mosaic - symptoms can vary greatly. Young infected plants seldom grow to harvest size, are stunted, deformed, & may have a mosaic/mottled appearance (intermingled patches of normal colored tissue & light green or yellowish tissue) depending on variety. Also, head lettuce plants rarely form a head. If plants are more mature when the plants often reach full size. However, the outer leaves are yellow and twisted. Developing head lettuce may have deformed heads with dead brown spots. This virus is seedborne. The virus can infect other plants including weeds such as, shepherd's purse, common lambsquarters, chicory, and henbit. Other food crops that may be infected include plants such as endive, pea, spinach. This virus may also be found in flowers such as asters, Shasta daisies and zinnias. Aphids are a primary carry of this virus.
Cucumber mosaic virus symptoms also include a mosaic/mottling pattern (as described above) as well as the stunting. This virus can be found in other vegetables such as tomato, carrot, celery, cucurbits, legumes, lettuce, spinach, and pepper. It may be found in the following flowers: dahlias, delphiniums, columbines, geraniums, petunias, phlox, zinnias and violas. Weeds, such as chickweed, pokeweed and milkweed are also hosts.
Squash mosaic virus is spread mainly by infected seeds and both the spotted and stipped cucumber beetles. A mosaic mottling, a green veinbanding, and distortion of leaves may be the symptoms on young seedlings. A very dark green mosaic appearance with blistering (looks like a herbicide was applied) are symptoms on mature plants.
Tobacco mosaic virus and the closely related tomato mosaic virus cause mosaic/mottling (sometimes with a blister-like appearance) of leaves, flowers, and fruit and cause the stunting of plants. Plants seldom die but quality and quantity of the yield is affected. With virus infected tomatoes, the leaves are mosaic/mottled but sometimes change shape. Fruit set is often reduced and sometimes blemishes and distortions of the fruit occur. Symptoms can vary on other host plants which can include pepper, muskmelon, cucumber, squash, &spinach. Flower hosts include delphinium, marigold, petunia, snapdragon, celosia, impatiens, ground cherry, phlox, and zinnia. Weeds include some types of ivy, plantain, night shade, and jimson weed.
Whether or not your vegetable plants get a virus depends on buying disease free seed, the virus being present on other host plants in your area and whether the insects that usually carry the viruses are also present and carrying the virus. As mentioned above sometimes virus symptoms may look-like herbicide injury as well as air pollution damage, mineral deficiencies, and other plant diseases. In addition, there are at least two other diseases that may be confused with some virus symptoms, they are big vein agent (caused by a virus like agent) and aster yellows (caused by a phytoplasm). Therefore positive identification of virus infection is frequently required.
Managing the insects, does not necessarily prevent virus infection. Depending on when some viruses get into the plant determines whether or not the crop is saleable or needs to be destroyed. The closer to harvest, the better the chances are that all or some of the crop may be eatable and therefore saleable. With other viruses, the infected plants need to be destroyed by burning or burying the crop at least 6 inches deep. For most viruses, to minimize their spread, the sooner the infected plants are destroyed, the better. The longer most virus infected plants are left, the more time insects that are virus free have to feed on the infected plants and thus help spread the virus infection to other plants. Many of the virus as well as the phytoplasm carrying insects overwinter in tall grassy ditches, fence lines or other insect sheltering areas. Mowing grasses growing in the ditches and in and along fence lines helps reduce the number of disease carrying insects that survive winter weather. People using tobacco products should wash their hands thoroughly before handling plants.