The Problems of Disease in Plants 



The diseases of plants offer many analogies to the 

 diseases of human beings. Some, the so-called physio- 

 logical diseases, are of internal origin; others are caused 

 by other living organisms, such as bacteria, fungi, and 

 insects, entering into the plant from outside. While 

 it is becoming customary to control the infectious 

 diseases of human beings by the use of serums and 

 vaccines, this method has so far proved impracticable 

 with plants; and the common procedure with them is 

 to apply poisons by spraying, dusting, or treating the 

 seed. This entails a permanent cost on the grower, 

 which in the case of food plants is added to the cost of 

 the human food supply. If, however, races could be 

 produced which were immune to disease, the cost of 

 food production would be by so much reduced. It is 

 now, indeed, generally agreed by plant pathologists 

 that it is in the study of racial and inherited immunity 

 and in breeding for disease resistance, rather than in 

 prophylactic measures with existing races, that the 

 solution of the great problem of reducing the enormous 

 losses to agriculture and horticulture due to diseases 

 of both physiological and infectious origin is to be 

 sought. 



The vast importance of such considerations as these 

 has led the state and national governments to expend 

 large sums in research along the lines of disease control. 

 But the immediately pressing problems have as yet 

 hardly been touched, either for our major agricultural 

 crop plants or for ornamental and decorative horti- 

 cultural types. Such conditions as the leaf scald of 

 the maples, and gummosis in many of the fruit trees 

 and flowering shrubs of the apple family, are examples 

 of widespread diseases whose causes are as yet unknown. 

 The breeding of types resistant to the rusts, smuts and 



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