VIRUSES 56 



distemper of dogs, both of them virus diseases. The former represents a constant 

 threat to the health of human beings. A most serious virus disease of animals 

 is the foot and mouth disease, one which causes economic losses to the extent of 

 a million dollars or more per year in England alone. 



Even though diseases of plants do not threaten mankind directly, yet they 

 have a tremendous influence upon his economical well-being. Perhaps the great- 

 est tragedy resulting from plant disease was the famine in Ireland in 1845-46. 

 This famine was caused by the destruction of the potato crop by the late blight. 

 Countless thousands of the population of Ireland died of starvation as a result 

 of this one plant disease. The great wave of migration of Irish to this country 

 was a direct result of this event. The present name of the common potato, Irish 

 potato, stems from the attention called to the importance of the potato in the 

 Irish economy by this great famine. Virus diseases of plants are also extremely 

 important. The virus disease called curly top, which affects sugar beet plants, 

 caused the abandonment of 10,000 acres of beet cultivation about the year 1925* 

 Chester estimated that the virus disease, sugar cane mosaic, reduced the yield 

 of sugar from 400,000 tons to 50,000 tons in the state of Loulsiauia over an 18- 

 year period. Tobacco mosaic virus is estimated by Chester to cause a reduction 

 of tobacco yields of from 35 to 40 million pounds annually. While this latter 

 loss cannot be considered as affecting the nutritional well-being of man, it 

 certainly does affect his economic status. Some of the indirect effects of to- 

 bacco mosaic virus are potentially even more serious, for this disease can be 

 transferred to tomatoes where it causes up to a ^0% loss in the crop. Certain 

 virus diseases of the potato cause vast damage. It is estimated that the farm- 

 ers of England spend between three and four million dollars annually for the 

 importation of disease-free stock from Scotland and Ireland. Probably enough 

 has been said to indicate the intimate relationship between the well-being of 

 mankind and virus disease not only of man but also of plants and animals. 



The remainder of this chapter consists of a discussion of several selected 

 diseases of direct importance to mankind and of measures which have been found 

 useful for combatting them. First let us consider diseases of plants. In gen- 

 eral there are five Important methods which might be effective in the control of 

 plant virus diseases. The simplest and most obvious is the careful inspection 

 of a crop and the immediate elimination of any diseased plants, thus cutting 

 down the foci of infection. Some virus diseases of crop plants are spread through 

 tubers or other organs of propagation. An effective control measure is to obtain 

 carefully-inspected, disease-free seed in such cases. Many plant virus diseases 

 are spread by insect vectors. Thus another important control measure is the de- 

 struction or at least the control of the insect population by standard agricult- 

 ural processes, A fourth approach to the control of plant virus diseases is 

 through the development of resistant, immune or carrier strains of plants. Sev- 

 eral virus diseases of economically important plants have been brought under 

 control by this method. A final possible means of control is one analagous to 

 vaccination as practiced with animals. Symptomless strains of plant viruses can 

 be selected through laboratory study. If a plant is deliberately diseases with 

 this strain, it is immune to the natural virus. Some plants carry symptomless 

 strains of virus naturally, ^or example, in this country virtually all potato 

 plants carry a virus called the potato X virus. As far as can be told, this vi- 

 rus does not produce adverse effects in the potato plant. However, whenever the 

 virus is transferred to the tobacco plant severe symptoms are detected. The pro- 

 cess of immunizing plants by infecting them with a mild virus is relatively 

 dangerous, because the plant carries the mild virus infection throughout its life 

 and is, therefore, a potential source of infection for other plants wiiich might 

 show signs of the disease. Further, there is always danger of mutation occurring 

 and of a virulent virus being produced. Thus, the process of vaccination of 

 plants has never seemed very practical. 



Several examples of the control of plant virus diseases by the second and 

 fourth methods can be cited. The British have been experiencing considerable 



