SHADE TREES. 



189 



Diagnosis of Disease. 



A successful diagnosis of disease necessitates a thorough knowledge of 

 the normal and abnormal functions of the organism, together with an 

 understanding of the specific reactions of the plant to various external 

 and internal agencies or stimuli that may affect it. The specific reactions 

 of plants are so little understood as compared with those of animals which 

 have been studied for centuries that it often requires considerable study 

 to make a complete and accurate diagnosis of some of the troubles affect- 

 ing plants, especially without knowledge of 

 the conditions to which they have been 

 subject. Plants have their peculiarities, 

 like animals, and the large number of 

 different species which are normally adapted 

 to a great variety of conditions and which 

 are likely to be subject to disease renders 

 the problem of diagnosis often quite diffi- 

 cult. The reactions of plants to stimuli are 

 manifold, and much more depends upon 

 the nature of the organisms stimulated, as 

 regards the nature of the response, than 

 upon the particular stimuli giving rise to 

 the reaction. The same agency may produce 

 a variety of reactions even in the same 

 organism, and different agencies will often 

 produce like effects. 



It might be difficult to tell whether a 

 particular plant was affected by coal gas, 

 hydrocyanic acid gas, burned sulfur, for- 

 malin vapor, or other gases without other 

 evidence than that afforded by the plant, 

 unless the observer possessed a special 

 knowledge of the effect of these gases. 

 But there are distinct symptoms displayed 

 by plants which enable one, after much 

 experience and careful investigation, to de- 

 termine with some degree of accuracy the exact cause of injury resulting 

 from injurious agencies. 



In diagnosing diseases it is first necessary to distinguish between primary 

 and secondary causes. A tree may be subject to borers and fungi, but these 

 may not be the primary cause of the trouble; indeed, they are more 

 often merely secondary effects. A tree may sometimes winterkill and 

 become subject to fungi, but one would not be justified in diagnosing the 

 case as injury from fungi, although in the diagnosis of disease secondary 

 causes are often mistaken for primary ones. It should be borne in mind, 

 however, that no plant ever dies without some definite cause. In deter- 



FIG. 62. Open mesh tree guard 

 with protective springs at top. 

 (From the Wright Wire Com- 

 pany.) 



