INTRODUCTION 7 



fungus may show itself in slight physiological disturbances of the 

 host are too numerous for special consideration. 



Control measures. Control measures for the prevention of fun- 

 gous diseases should be a part of every study which is undertaken. 

 Reference has already been made to the very rapid development 

 of systematic methods of control. Control may be developed along 

 one or more very different lines. In the first place, it may concern 

 itself more particularly with a maintenance of the host plant in a 

 thoroughly sanitary environment, or in one which renders it more 

 resistant to the attacks of fungi. It may again concern itself with 

 the application of deleterious substances (fungicides) to the host, 

 in order that the germination and growth of the fungous spores 

 may be prevented. This use of fungicides may take the form of 

 disinfection of the seed or of propagative parts, the application of 

 reagents to the soil in order to prevent the growth of the fungus 

 in the vicinity of the host plant, or the application of fungicides to 

 the aerial vegetative portions of the host, which is commonly accom- 

 plished by the operation of spraying. This latter operation has been 

 practiced to a considerable extent for a long period of time, but the 

 really substantial development of the work began with the discovery 

 of Bordeaux mixture, to which reference has already been made. At 

 the present time a great variety of spraying mixtures are employed, 

 a large number of which contain copper compounds, or copper 

 combined with lime, subsequently discussed in detail. There are, 

 however, a great many directions in which the development of de- 

 sirable fungicides may yet go forward. At the present time the 

 use of lime-sulfur washes and sprays is rapidly taking an impor- 

 tant place. It is particularly in connection with control measures, 

 or facts concerning the life history of the organism suggesting such 

 measures, therefore, that the study of fungous diseases of plants 

 makes for itself a place among practical sciences. The amount of 

 injury annually suffered by the various crops, due to fungous dis- 

 eases, may be more or less definitely ascertained, and this repre- 

 sents the possibilities to which control work may be pushed. On 

 the other hand, the relation of the crop in unsprayed regions to that 

 in regions where spraying is used may permit one more or less 

 roughly to determine the actual saving through the present imper- 

 fect and rather haphazard practice of control measures. Estimates 



