All plant viruses so far isolated are similar in chemical composition and contain two essential constituents, nucleic acid and protein.

Nearly all of the plant viruses that have been studied extensively have been found to be complexes of strains. The strains vary in virulence, kind of disease produced, host range, or other characteristics. Curly top virus and tobacco mosaic virus are made up of innumerable strains. Some of these are weak and cause little injury even on susceptible plants. Others are more virulent and cause severe injury. Some strains of curly top virus attack tobacco and tomato; others do not. Two strains of potato yellow dwarf virus appear to be transmitted by different specific vectors.

It may be assumed that these strains have arisen, one by one, from parent strains during the course of years by a process similar to mutation. The relatively large numbers of strains of some of the more common viruses may indicate that some viruses are less stable than others, under the environmental conditions to which they are subjected, hence mutate more readily. This tendency to mutate gives to viruses a degree of adaptability that may be comparable to that possessed by plants and animals.

Recovery from the more severe phases of virus disease is a rather common phenomenon in plants but is limited usually to partial or complete recovery from obvious symptoms and very rarely extends to loss of the virus by infected plants.

One of the first examples of recovery to be discovered was that of the recovery of tobacco from ring spot. A few days after plants become infected with this virus, marked necrosis is produced on a ring of new leaves, but subsequent growth is normal or nearly normal.

Plants from cuttings of recovered plants remain free of severe symptoms, and the diseased plants may be propagated indefinitely by vegetative means without the reappearance of the severe phase of the disease.

Turkish tobacco recovers to a very high degree from curly top. Tomato plants are killed by curly top if the virus introduced by means of the beet leafhopper; if it is introduced by means of scions from recovered tobacco, however, they usually show only mild symptoms.

Water pimpernel, Samolus parviflorus, also shows a high degree of recovery from most strains of curly top virus. Recovery in sugar beet is much less marked. Plants of susceptible varieties usually show little evidence of improvement after they become diseased.

With some viruses and perhaps with most of them, infection by one strain protects against infection or injury from a second strain of the same virus, but no protection is afforded against infection with a totally different virus. Protection of this type is very marked between strains of the tobacco mosaic virus. Juice from a plant with tobacco mosaic, when rubbed gently over the surface of a leaf of a healthy plant of Nicotiana sylvestris, produces many small necrotic lesions, which mark points of separate infections. If the inoculated plant has already been invaded by a strain of tobacco mosaic virus, however, no lesions are produced by re-inoculation with a second strain of the same virus.

After recovery from one strain of curly top virus, Turkish tobacco plants are very resistant to injury by other strains of this virus. Also, when tomato plants are inoculated with one strain of the curly top virus by means of scions from recovered tobacco plants, they arc very resistant to injury by most other strains. Tomato plants "immunized" in this way have been grown successfully under field conditions where all the non-protected plants were killed.

In contrast, the plants of Nicotiana glutinosa and water pimpernel, although they recover from the severe effects of most strains of the curly top virus, are not protected against injury by strains of the virus more virulent than the one already present. No strain of curly top virus offers appreciable protection against either infection or injury in sugar beet.

Although protection between related strains of some viruses is not always evident or complete, cross-protection tests have been of value in identification and classification of viruses from different sources and host plants.

The cross-protection phenomenon may prove to be of value also in certain phases of control of virus disease, particularly diseases such as swollen shoot of cacao and tristeza of citrus. Where either is present, it is known that all susceptible trees must sooner or later become infected from virus sources that are impossible to eliminate. Since this is true, it may prove worth while first to infect all the young planting stock with a weak and relatively harmless strain of virus in order to protect against later infection by more virulent strains of the same virus.

After a plant is infected with a virus disease nothing, as a rule, can be done to restore its health. Therefore, methods of control are directed almost wholly toward prevention of infection or toward development of disease-resistant varieties.

Many practices and precautions are employed to prevent infection with virus diseases. Control of mosaic on tobacco and tomato is obtained largely by avoiding spread of virus by contact during transplanting and cultural operations. Fortunately, few of the virus diseases are so easily transmitted and ordinary cultural operations can be carried on with most plants without danger of spreading virus diseases by contact.

Destruction of plants that serve as sources of infection is of value in the control of a number of virus diseases. Spring infection of fields of sugar beets with mosaic and virus yellows comes largely from infected beets that survive the winter or from beets that are carried through the winter for seed production. Elimination of such sources of virus usually gives a high degree of control. The spread of X-disease on peach can be prevented by removing all infected chokecherries within 500 feet of peach orchards, and the virus diseases of raspberries can be controlled, in most instances, by destroying all wild and escaped brambles in the immediate vicinity of plantings, provided the plantings themselves are not already infected.

Reducing the population of insect vectors by spraying or by other means has value in the control of some virus diseases. Usually it is not possible, however, to reduce the insect populations sufficiently or soon enough to obtain completely satisfactory results. Some virus diseases can be partly controlled by destruction of the hosts of the insect vectors. Extensive reduction of the weed hosts of the beet leafhopper in the Western States would correspondingly reduce the amount of curly top virus carried from desert plants to cultivated fields. In much of this area, reduction in weed hosts comes about naturally under systems of land management in which annual and perennial grasses and other nonhost plants are allowed to replace the weed hosts of the beet leafhopper. Fall spraying to kill leafhoppers on weeds in uncultivated areas has been resorted to also in the program to control curly top.

Virus-free nursery stock is extremely important in the control of virus diseases of strawberry and raspberry. Natural virus spread often is not extensive enough to cause serious damage during the life of plantings started with virus-free nursery stock. That is true also of some of the virus diseases of tree fruits.

G. W. BENNETT is a pathologist of the division of sugar plant investigations, Bureau of Plant Industry, Soils, and Agricultural Engineering, who has had more than 30 years of experience in the study of Plant viruses and virus diseases.