VEGETABLE PATHOLOGY AN ECONOMIC SCIENCE 169 



tory is frequently necessary for full knowledge of the disease. The 

 different stages of the organism must be worked out, where it spends 

 the winter, how it reinfects the host plant, the climatic and other 

 conditions which favor or retard its development or its parasitism. 

 The stage at which it is most vulnerable, as well as the means of 

 reaching and killing it, must be found out. If it is a non-parasitic 

 disease, the causes or conditions which produce it are often still 

 more difficult to ascertain, and the method of removing them or 

 circumventing them is to be discovered. 



The treatment for plant diseases is yet in its infancy, but suc- 

 cessful results have been reached in many specific instances and 

 along several lines more or less distinct. These may be classified as 

 follows: (1) spraying with fungicides; (2) disinfection by means 

 of germicides and fungicides; (3) eradication methods; (4) breed- 

 ing resistant or immune varieties of the host plant; (5) cultural 

 methods. 



Spraying. The discovery of Bordeaux Mixture by Millardet about 

 1885 proved the starting of an important and successful era in the 

 treatment of plant diseases. Spraying has probably accomplished 

 more good up to the present time than any other method, or per- 

 haps than all other methods together. The success of Bordeaux 

 Mixture led to extensive studies of the other compounds of copper, 

 some of which promised for a time to supersede the original prepara- 

 tion. While for certain uses copper acetate, and especially the am- 

 moniacal solution of copper carbonate, have proved to be the best 

 form of copper, yet the Bordeaux Mixture remains by far the most 

 important fungicide. The sulphur compounds, whose use antedates 

 that of copper, have also retained their place in the list of fungicides. 

 The lime sulphur salt or the lime and sulphur mixture, on account 

 of its usefulness in killing at one application certain fungi as well as 

 scale and other insects, has become the greatest spray for dormant 

 trees. During the last year or two dust-spraying, or, more properly 

 speaking, dusting of plants, has come into prominence. The object 

 here is to avoid the carrying and applying of large quantities ' of 

 water, and instead to prepare the fungicide in a dust form, so that 

 it can be thrown on the plant more economically. Rain or dew is 

 expected to supply the necessary water to dissolve and more thor- 

 oughly distribute the fungicide over the plant. It has proved more 

 satisfactory in the prevention of the coddling moth and leaf-eating 

 insects, in the application of insecticides, than it has in the pre- 

 vention of the fungus diseases. These are more difficult to prevent, 

 and in many cases even too difficult to prevent by the finest sprays, 

 so that the results in dusting are not thoroughly satisfactory. 



Bordeaux Mixture has achieved notable triumphs in the prevention 

 of the black rot and the Peronospora of the grape, apple-scab and 



