158 InseEls and Diseases. 



remains apparently in full vigor, the evil extending downwards^ 

 unless naturally or artificially checked, till the whole tree is de- 

 stroyed. 



After a close investigation for years, by the most skilful cultivators 

 in the country, a satisfactory explanation, applicable to all cases, 

 has not been made. 



The earlier theory was, that the hot rays of the sun produced the 

 disaster, and hence the original name fire-blight This was con- 

 firmed by the fact that the blight was often most fatal in the hottest 

 summers ; and weakened by the opposing fact that shaded portions 

 of the tree were as frequently attacked as those fully exposed to 

 the action of the hot sun. 



It was subsequently discovered that a small insect (Scolytus pyri), 

 by the supposed infusion of poison, caused the death of the branches, 

 but no general or wide destrudtion of the pear could be traced to 

 this source. 



More recently, the frozen-sap theory has been more extensively 

 adopted. The explanation by this theory is as follows: A damp 

 and warm autumn causes a late and unripened growth of wood, im- 

 perfectly able to withstand the effects of winter. It is acted upon by 

 severe frosts, not, however, so as to produce immediate death or 

 winter-killing, but resulting, sooner or later, in disease and partial 

 decomposition of the sap, by which it becomes poisonous in its 

 nature, and by passing downwards through the bark, spreads death 

 in its progress. 



This theory is corroborated by many local observations, and by 

 the general fact that the blight is much more destructive in the 

 warm and fertile valleys of southern Ohio, where vegetation con- 

 tinues late, is more succulent in its texture, and where the frosts 

 are sudden and sharp, than in the dryer and cooler climate of New 

 England. But this same reason is also adduced in support of the 

 original fire-blight theory, and indeed it applies with strength to both. 



But after admitting that the different theories may be in part 

 correct, and that the blight may be caused by a combination in a 

 greater or less degree of each assigned cause, we are driven to the 

 conclusion, from a large number of observations, of which these 

 limits preclude even a brief recital, that th,e cause of the blight, like 

 that of the potato disease, remains hidden, in a large number of 

 instances, from our knowledge. And that, whether the latent ten- 

 dency to disease is only increased and developed by changes of the 

 weather, or whether those changes actually produce them, is yet 

 enveloped in doubt. 



