Viruses, a Scourge of Mankind

C. W. Bennett.

Few kinds of living organisms are immune to viruses. Man, domesticated and wild animals, insects, plants, and even bacteria succumb to their attack. They have been a scourge of mankind since before the dawn of recorded history. Smallpox, for example, existed in China in 1700 B. C. Measles, mumps, influenza, and scarlet fever are other virus diseases that plague humans.

The virus diseases of plants also probably have existed for many centuries. Their importance on crop plants has increased tremendously in the past 50 years since 1900 more than 200 new plant viruses have been discovered. Many of them have done widespread damage to crop plants.

Curly top caused almost complete abandonment of the sugar beet industry in parts of western United States from 1916 to 1932 and still causes severe injury to tomatoes, beans, and a number of other crops. Sugarcane mosaic caused extensive losses to the sugarcane industry in the United States, Argentina, Brazil, and other countries beginning about 1917. Spotted wilt has become widespread and now causes losses to tomato and other crops in many parts of the world. Since 1940, swollen shoot has caused extensive damage to the cacao industry of west Africa. Virus diseases of citrus have become more destructive and from 1936 to 1946 tristeza caused the loss of 7 million orange trees in the state of Sao Paulo, Brazil, alone. It has attacked or now threatens other millions of trees in various tropical and subtropical areas.

This increase in destructiveness of virus diseases and in the number of known viruses has come about largely as a result of the expansion of agricultural enterprises and the increased movement of plants and plant products in recent years.

Disease-producing agents particularly viruses apparently have originated in local areas all over the world. Probably through long association have native plants developed a tolerance to the local viruses that enables infected individuals to survive with little injury and often with but little evidence of being carriers of viruses.

When crop plants are introduced into areas where they have not been grown before, they frequently become subject to infection with the native viruses, against which they have had no opportunity to develop resistance. Such a virus may cause extensive losses to a crop plant, not only in the areas of original distribution of the virus, but also in the other areas to which it may spread on the recently attacked crop plant.

The appearance of the yellow wilt virus on sugar beet in the Rio Negro Valley of Argentina when attempts were made to develop a sugar beet industry there in 1929 probably is a typical example of the transfer of a virus from wild plants to a new crop plant. From the beginning of the attempt to establish the sugar beet industry there, the sugar beets were attacked by a virus disease, previously unknown on sugar beet or on any other plant. It caused a yellowing, wilting, and death of all infected plants. Injury was so severe that attempts to grow sugar beets were abandoned about 10 years later after considerable loss to growers and processors. This disease appears still to be limited to South America, but it continues to constitute a potential threat to sugar beets elsewhere.

Many plant viruses have spread over extensive areas from their points of origin. The movement of the tristeza virus of citrus is an example of the extensive spread of a destructive virus disease. Tristeza virus probably originated in South Africa or in southwestern Asia. For many years, possibly for centuries, it appears to have had a limited distribution but since 1930 it (or a similar malady or maladies) has been reported in Australia, Java, California, Louisiana, Argentina, Uruguay, and Brazil. Apparently the chief method of long-distance spread has been by means of infected budwood or nursery stock moved from infested to noninfested areas. After it was introduced into Brazil and Argentina it was spread rapidly by the oriental citrus aphid, Aphis citricidus.

Virus diseases produce a wide range of symptoms and types of injury on plants. Sometimes they kill the plant in a short time, as with spotted wilt and curly top on tomato. More often they cause lesser injuries that result in reduced yields and lower quality of product. With respect to general type of the symptoms produced, most viruses are of two rather clearly defined groups: Those that cause mottling or spotting of leaves and those that cause a yellowing, leaf curling, dwarfing, or excessive branching, but little or no mottling or spotting.

The first group, by far the larger, includes such important viruses as those causing cucumber mosaic, peach mosaic, and the tomato spotted wilt. Mosaic diseases are characterized by the production of chlorotic, or yellowish, areas in the leaves and sometimes in the blossoms and other parts. The chlorotic areas may be more or less circular or they may be irregular. The spots vary in size from very small areas to large blotches. Sometimes the chlorotic areas cover a large share, or even all, of the surface of individual leaves.

Some of the mosaic viruses cause conspicuous mottling, spotting, or striping of petals of flowers of some ornamental plants. Many of the variegated tulips are only virus-infected plants of non-variegated varieties. The Rembrandt variety is a virus-infected strain of the variety Princess Elizabeth. Variegation, or "breaking," of blossoms due to viruses commonly occurs also in wallflower, stock, gladiolus, and flowering peach.

The attractively variegated leaves of the ornamental shrub Abutdon striatum are also the result of a virus infection. In the United States the causal virus apparently has no insect vector and variegated and non-variegated plants of A. striatum grow side by side indefinitely with no spread of the variegated condition to the non-variegated plants. In Brazil the virus is transmitted by the whitefly, Bemisia tabaci, and there healthy plants may soon become variegated.

Other mosaic-type viruses cause circular or irregular necrotic dying or chlorotic spots on leaves, stems, and fruits, as in spotted wilt on tomato. Ring spots and oak-leaf patterns are symptoms of other virus diseases.

Some viruses produce all these types of symptoms and others as well under specific conditions or on different host plants.

Diseases of the leaf curl and yellows group are caused by viruses that appear to be associated with the vascular system of the plant and produce symptoms that are characteristic of disturbances in this type of tissue. With the leaf curl diseases, such as raspberry leaf curl, sugar beet curly top, and tobacco leaf curl, growth of veins is retarded and the leaves roll or become crinkled. Leaves are sometimes deeper green than normal. The yellows types, such as aster yellows and sugar beet yellow wilt, cause yellowing, stunting, and various types of leaf deformations. Other diseases, such as strawberry stunt, cause dwarfing of the plant and some leaf rolling and thickening. Still others stimulate the production of clusters of thin, wiry shoots as in Witches'-broom of oceanspray, or they cause the production of wiry shoots on steles or on main limbs and trunks if trees are attacked. Viruses of the leaf curl and yellows group seem to persist in their insect vectors for longer periods than do viruses of the mosaic group.

Some viruses produce symptoms only under certain environmental conditions or on certain host plants or host combinations. Red raspberry mosaic causes mottling only on leaves produced at low temperatures. Symptoms usually appear therefore only on leaves formed in early spring or late fall. Curly top virus is present in many plants of tree tobacco, Nicotiana glauca, in California, but no symptoms are evident. Tristeza virus produces marked symptoms only on citrus trees of certain top and root combinations. For example, both sweet and sour orange trees grow well even though infected with the tristeza virus. Also, trees of sour orange that are grafted onto sweet orange rootstocks show little evidence of injury. However, if the positions of the two types of orange are reversed in the grafted tree so that the top of the tree is of sweet orange and the rootstock is of sour orange, the tristeza virus causes yellowing and dropping of the leaves, gradual dying back of twigs, production of weak shoots on the main limbs, and eventual death of the tree.

Plants may be infected with two or more viruses at the same time. When a plant with one virus disease is attacked by another, usually symptoms of the second disease are merely superimposed on those produced by the first. Occasionally, however, infection by two viruses results in added symptoms. An example is double-virus streak in tomato caused by infection with both tobacco mosaic virus and potato virus X. Tobacco mosaic virus causes mottling and a certain amount of dwarfing. Potato virus X induces mild mottling. When both viruses are present in the same tomato plant a marked increase in injury occurs with the production of extensive necrosis on leaves and stems not characteristic of either virus alone.

Transmission of virus diseases may be brought about in a number of different ways. Under experimental conditions, many of the mosaic-type viruses can be transmitted mechanically by rubbing juice from diseased plants over the surface of leaves of healthy plants. The addition of a small amount of an abrasive such as carborundum to the juice frequently increases the amount of infection. This method of transmission is of great value in studies of properties, characteristics, and host range of viruses. The majority of viruses, however, have not been transmitted by juice inoculation.

Transmission of sugar beet curly top virus and cucumber mosaic virus by means of dodder, Cuscula subinclusa, was reported in 1940, and a number of other viruses have been transmitted since by it and other species.

The species of dodder used for virus transmission are members of a group of interesting parasitic seed plants belonging to the same family as morning-glory and sweetpotato. They sometimes cause extensive injury by parasitizing clover, alfalfa, and other plants. They attach themselves to their host plants by a type of natural graft union. Because species of dodder have extensive host ranges, plants widely different botanically may be united by a type of natural graft in which dodder forms a connecting link through which several viruses have been found to pass. Thus dodder provides a medium through which viruses may be transferred from diseased to healthy species that cannot be infected by juice inoculation or by insect vectors.

Under field conditions only a few viruses are disseminated by mechanical transfer. Most natural spread takes place through the use of infected vegetative parts for propagation, through infected seeds, and by insects.