L. D. Christenson, Floyd F. Smith.
The Russian scientist D. Iwanowski demonstrated in 1892 that sap from tobacco plants with a mosaic disease is infectious after passing through a bacteria-proof filter. It was the first discovery of an amazing group of agents that cannot be seen with ordinary microscopes and that now are called viruses. Many of our most serious and difficult plant-disease problems have been shown to be the results of infections of plants by these minute entities, which are smaller than bacteria. A few of the many different kinds of viruses are even smaller than the largest molecules known to chemists.
Tulip mosaic, peach yellows, aster yellows, sugar-beet curly top, phloem necrosis of elm, tobacco mosaic, raspberry mosaic, blueberry stunt disease, potato leaf roll, pea mosaic, tomato spotted wilt, and sugarcane mosaic are examples of plant diseases caused by viruses. Virus agents also cause serious diseases of man and animals smallpox, measles, mumps, the common cold, rabies, distemper, and foot-and- mouth disease. Others, like the sac-brood virus of honey bees, infect invertebrate animals.
For a long time we knew little about the nature of viruses. Now, as a result of the studies of W. M. Stanley, F. C. Bawden, N. W. Pirie, and others, they are believed to consist of complex nucleoproteins that have some of the attributes of living organisms. Like living organisms, the individual virus particles can reproduce or multiply. They also can change or mutate during the multiplication process. They do not seem able to grow or multiply, however, except within the living cells of their hosts, and, unlike living organisms, they cannot carry on the complicated processes of respiration, digestion, and other metabolic functions.
Most of the plant viruses have been discovered since 1900, but they are not of recent origin. Old Dutch masters recorded in their paintings the variegations in the petals of tulips caused by a virus now known as tulip mosaic. Dutch bulb growers knew as early as 1637 how to graft healthy bulbs with variegated bulbs to get the coveted many-colored flowers even though they did not know what caused them. Potato viruses had become so abundant in Europe by 1775 that the production of potatoes had to be abandoned in many areas because of what was then termed the "running-out" of potatoes. In the United States, the virus disease now known as peach yellows was described as early as 1791. We have evidence that it was doing damage in peach orchards as early as 1750.
Only a few viruses kill the plants they infect. Plants affected by most of them never recover, but they do not die as a result of the infection. Their growth and productivity may be seriously affected. Some species and varieties of plants apparently are not attacked by viruses. Others may be tolerant of them or only mildly affected when their tissues are invaded by the virus particles. Trees, shrubs, other plants in uncultivated areas, and weeds on farms may be infected by viruses that also attack cultivated plants. When that is so, the wild plants serve as important sources of danger to the cultivated plants. Otherwise the viruses in the uncultivated plants are not economically important. No viruses are yet known that attack coniferous trees, such as pine and spruce.
Our cultivated crops annually suffer heavy losses because of virus diseases. Phony peach has plagued peach growers in the Southeastern States for at least half a century, making it necessary for them to take out more than 2,600,000 peach trees. Years ago in the Northeastern States, peach yellows destroyed the productiveness of hundreds of thousands of trees. Sometimes it was necessary to destroy entire orchards. Tobacco mosaic has been estimated to cause an annual loss of millions of pounds of tobacco. Viruses have seriously affected the production of potatoes each year. To reduce their losses, the growers here and in England and other countries have to expend large sums to get healthy seed potatoes grown in areas where potato viruses are not serious. Production of head lettuce in the East has not been profitable because of infection by the virus known as aster yellows. Losses caused by the curly-top virus in sugar beets have been so severe in the Western States that some sugar factories have had to be abandoned. The same virus has caused crop failures in tomato fields. Similar heavy tolls may be levied by the viruses that attack many of our ornamental plants and flowers.
Plant diseases caused by viruses spread in several ways. Some are so infectious that contact between the leaves of normal and diseased plants is all that is necessary. Highly contagious diseases such as these may be spread by mechanical means. A few instances of spread through seeds are known. A serious method of spread is through the use of parts of infected plants to start new plantings. For example, viruses that persist from year to year in potato tubers, in bulbs, and in rhizomes infect plants growing from them. Viruses may also be spread through cuttings or suckers from infected plants and through budding and grafting procedures employed in nurseries.
Insects are the worst spreaders. A Japanese scientist in 1901 found that a leafhopper could transmit stunt disease of rice from diseased rice plants to healthy plants.
The first insect to gain prominence in North America as a carrier of a plant virus was the beet leafhopper. It was found to be spreading curly-top disease in sugar-beet fields in Utah and other Western States only a few years after the discovery of the insect carrier of stunt disease of rice. We now know that many of our plant virus disease outbreaks are the result of insect-carrier activity, and it is suspected that insects are involved in the spread of many other plant virus diseases.
Insect carriers of plant viruses are known to occur in only six of the major orders of insects the Homoptera (aphids, leafhoppers, whiteflies, mealy-bugs, scales) , Thysanoptera (thrips), Heteroptera ( plant bugs, lace bugs), Coleoptera (the beetles), Orthoptera (grasshoppers), and Dermaptera (earwigs). Most of the carriers have sucking mouth parts, and among them the aphids and leafhoppers seem to be the most proficient. A few insects with chewing mouth parts, such as grasshoppers and leaf-feeding beetles, also spread certain virus diseases.
To accomplish transmission, the vector has to get the virus from a diseased plant, which it does while feeding, and then move to a healthy plant, which it infects during the feeding process. With the sucking insects, the virus particles apparently are injected into Plants with the saliva.
The relationships between plant viruses and their vectors have commanded the attention of many entomologists, plant pathologists, and other biologists. Striking advances have been made, and we now know a great deal about many insects that transmit viruses, something about what happens to the virus during its period in the insect body, and something about the factors involved in the transmission process. There is still much to be explained, however: We do not know why certain species can transmit viruses while other similar insects cannot, or why certain insects can transmit so many different kinds of plant viruses but not others. The many other vectors awaiting discovery also remain a challenge.
PLANT VIRUSES are considered as belonging to two general groups.
In the group called the nonpersistent viruses, the insect carrier can transmit the virus soon after feeding on a diseased plant. This ability to cause new infections is quickly lost, however, after the insects feed on healthy or immune plants. A starvation period before feeding on infected plants usually increases the transmission efficiency of the vectors of the viruses, which usually can be transmitted by mechanical means, as by wiping the sap of an infected plant over the leaves of a healthy plant. The insect carriers sometimes include many different kinds of insects. Many viruses transmitted by aphids and chewing insects belong to this group. Perhaps some of the non-persistent viruses are transmitted through contamination of the mouth parts of the insect carriers with virus particles, but for many others the transmission process does not seem to be that simple.
The other group includes the persistent viruses. When they are taken in with the food of their vectors, an interval (the incubation, or latent, period) is necessary before the insects can infect healthy plants with them. Once having the ability, insect carriers of persistent viruses usually can transmit them to healthy plants for an extended period, often for life. In two instances involving leafhoppers, persistent viruses are transmitted to the succeeding generation through the eggs. Some of these viruses are transmitted by only one or a few closely related insects. Most of the viruses that leafhoppers transmit are persistent viruses. A few aphids or other insects also transmit persistent viruses.