166 TORREYA 



barley, sometimes obtaining simple relations, but identifying several distinct 

 factors for resistance to a specific race of the powdery mildew. 



7. Disease control. Viewed by the practical man, the control of disease 

 is the primary consideration, and the emphasis is placed on securing adequate 

 methods for avoiding the losses due to the destructive diseases. The selection 

 of resistant varieties is one method of procedure, but many others have been 

 employed. The prevention of disease, rather than an attempted cure of infected 

 plants, is recognized as of first importance. Ward (1882), in connection with 

 the coffee disease, clearly emphasized the idea of preventive treatment. It is 

 essential that the toxic material be applied so that it is on the leaves when the 

 spores of the pathogen are germinating. Whatever material is used, it must be 

 applied at the right time. 



In a few cases curative measures are successful. In loose smut of wheat 

 and barley, the invasion of the parasite occurs just after the period of polliniza- 

 tion and as the grain ripens the fungus passes into a dormant condition, and 

 may be killed by the hot water treatment. There are a few other illustrations, 

 particularly in the case of virus diseases, as discovered by Kunkel. 



Previous to 1867 there were two diseases of plants which were more or 

 less effectively controlled by chemical substances. One was the powdery mildew 

 of the grape by the use of sulphur, discovered by Tucker (1847), and the other 

 the bunt of wheat by a method of seed treatment with salts of copper, as worked 

 out by Prevost, Kiihn, and others. Since 1867 great strides have been made 

 in the control of diseases by chemical means. Many sprays and dusts have been 

 utilized, one of the most important being Bordeaux mixture, discovered by 

 Millardet in 1882, which was effective against the downy mildew of the grape. 

 This spray, with modifications, is still one of the standard materials in the 

 control of many diseases. Lime-sulphur was accidentally discovered in 1885 

 as an effective control of the peach leaf curl. Pierce (1900) giving the history 

 of its use. Scott ( 1908) reported experiments on the value of self -boiled lime- 

 sulphur, which was effective in the control of peach scab and brown rot, and 

 was successfully used to control apple scab in 1910. Great emphasis has been 

 placed upon the use of dusts instead of sprays in the control of fruit diseases. 

 Whetzel and his associates have been active in the development of suitable 

 dusts. 



Copper, mercury, and sulphur remain, at the present time, the principal 

 materials for the chemical control of disease. However, great advances have 

 been made in the use of these elements in new types of compounds and in the 

 physical make-up of the dust or the spray. Investigations have been carried out 

 on the proper methods of applying the material, the discovery of suitable 

 spreaders and stickers, and methods of control involving the combination of 

 insecticides and fungicides. Important changes have occurred in developing 



