Description
Viruses are complex, submicroscopic particles composed of proteins and nucleic acids. They are much smaller than fungi or bacteria, and do not have cell walls. A few virus diseases of plants are serious or fatal. These are usually transmitted by insects or spread by hands or equipment. Most viruses only slightly impair the growth of the plant, and manifest symptoms only under certain conditions. They are usually transmitted when diseased stock is propagated by cuttings or grafting. Viruses may spread slowly from cell to cell, eventually infecting all of the plant tissue. Or they may spread quickly through the nutrient-conducting vessels to specific parts of the plant. Infection usually decreases the plant's ability to manufacture food. Less chlorophyll (green plant pigment) is produced, and the amounts of nitrogen and stored foods in the plant are reduced. Some of the common types of viruses, such as mosaics, ring spots, and stunts, produce specific symptoms. Mosaic viruses cause the foliage to become mottled or streaked. Ring-spot viruses cause pale rings to form on the leaves. Stunt viruses cause stunting of plant foliage. Often the symptoms overlap. In some hosts, these viruses cause leaf thickening, curling, and distortion; slow growth; and reduced yields of fruit, flowers, and vegetables. The severity of virus infections varies, depending on the host plant and strain of virus. Sometimes viruses infect certain host plants without showing symptoms of disease. In other cases, symptoms appear with changes in the environment, such as a rise or drop in temperature.
Control
There are no chemical cures for virus infections. Aphids, leafhoppers, certain other insects, and nematodes transmit viruses while feeding, so you can control infection by keeping down the populations of these organisms. (For more information about controlling virus-carrying insects, see Vectors of Plant Diseases.) Pulling out infected plants will also help reduce the spread of virus diseases. Because seeds, bulbs, corms, and cuttings taken from infected plants are likely to be infected, it's best to purchase healthy plants and seeds from a reputable nursery, and discard seeds and cuttings from diseased plants. Some virus diseases are readily transmitted by hands and equipment. When handling plants, thoroughly washing hands and equipment between plants can reduce the possibility of diseases spreading. Discard obviously virus-infected plants. Wash the container and your hands in hot soapy water.
