VEGETABLE PATHOLOGY AN ECONOMIC SCIENCE 167 



ever, plants may be sickened by poisonous substances in the soil, 

 may be injured by extremes of heat and cold, or injuriously affected 

 by other environmental conditions so that disease results. After 

 these unfavorable conditions have passed away, the plants are not 

 prepared to respond to normal conditions. The ecologist under- 

 stands the effect of environment on plants, their adaptations and 

 struggles in competition with other plants and with unfavorable 

 environment. When the plant succumbs wholly or partially in this 

 struggle it becomes a pathological subject. Here ecology blends 

 into pathology. 



Bacteriology, although somewhat allied to mycology, deserves 

 special mention as an aid in the study of plant diseases. A con- 

 siderable number of plant diseases no one knows how many as 

 yet are caused directly by parasitic bacteria. The study of these 

 bacterial diseases forms in itself a very important department of 

 vegetable pathology, and has enabled us to understand with consider- 

 able clearness a very large group of plant diseases. The usefulness 

 of bacteriology is, however, still wider than this. The special methods 

 developed in bacteriological research have quite revolutionized the 

 life-history studies of fungi. I refer especially to the use of artifi- 

 cial culture media and the methods of isolating, cultivating, and 

 inoculating parasitic germs. By these processes results which were 

 only possible by the most laborious and uncertain methods thirty 

 years ago can now be accomplished with the greatest facility by 

 a student of only a few months' training. 



Zoology, the sister science of botany, has contributed along many 

 lines to botany, and therefore to pathology. Animal physiology, 

 anatomy, and cytology have been extremely helpful. Many of the 

 methods of sectioning and staining, especially the finer cytological 

 and embryological methods, have been first worked out on animal 

 tissues. 



Entomology is connected with plant pathology in many ways. 

 While the study of insects is itself quite distinct from the study 

 of fungi, yet not infrequently the plant diseases produced by these 

 diverse agents are difficult to distinguish till properly studied. The 

 pathologist must know a good deal about insect injuries and diseases 

 to distinguish them readily from fungus troubles. Furthermore, 

 insects are largely concerned in the distribution of bacteria and the 

 spores of fungi, and are thus important agents of infection. In 

 the treatment by spraying in many cases an insect pest and fungus 

 disease are killed at the same application. For example, the lime 

 sulphur salt spray on dormant peach-trees kills the curl-leaf fungus 

 and the San Jos scale. Bordeaux Mixture with Paris green or 

 arsenate of lead kills the apple-scab and the coddling moth. 



The utilization of physiological botany, the methods of staining, 



