DISEASES OF THE PEAR TREE. 201 



DISEASES AND ENEMIES. 



Blight. A most formidable difficulty in the cultivation of 

 the pear, is the blight, known in its modifications, supposed 

 or real, by the names fire-blight, insect-blight, frost-blight, 

 and frozen sap-blight. The causes may be various, but the 

 apearances are the same, a sudden withering and turning 

 black of the leaves on certain limbs during rapid growth, 

 and while the rest of the tree remains apparently in full 

 vigor, the evil extending downwards, unless naturally or 

 artificially checked, till the whole tree is destroyed. 



After a close investigation for years, by the most skilful 

 cultivators of the country, a satisfactory explanation, appli- 

 cable to all cases, has not been made. 



The earlier theory was, that the hot rays of the sun pro- 

 duced the disaster, and hence the original name fire-blight. 

 This was confirmed 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 fre- 

 quently attacked as those fully exposed to the action of the 

 hot sun. 



It was subsequently discovered that a small insect, (Sco- 

 lytis pyri,) by the supposed infusion of poison, caused the 

 death of the branches, but no general or wide destruction of 

 the pear could be traced to this source. 



More recently, the frozen-sap theory has been extensively 

 adopted. The explanation by this theory is as follows : 

 A damp and warm autumn causes a late and unripened 

 growth of wood, imperfectly 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 so as 

 to result, sooner or later, in disease and partial decomposi- 

 tion 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 de- 



* A modification of the frozen-sap blight, known sometimes by the name of frost- 

 blight, occurs early in summer, immediately after unusuul and sharp nocturnal frosts. 

 The young and tender shoots and the branches below them, are observed to die sud- 

 denly wiihiu a few days, but protracted in some instances to several weeks. The 

 sap from the dead limbs does not in this case appear 'to be so deleterious as in other 

 forms of blight, and although sometimes destroying large portions of trees, they more 

 frequently escape after a partial loss of their tops. 



