Entomological Department. 323 



The disease is not known to attack any tiling but the pear in this 

 country.* 



This Pear Leaf Blister is sometimes mistaken for a common fungus 

 disease, the Pear Scab, whifih attacks the pear, also forming blackish 

 spots O'l the surface of the leaves which have a slight resemblance on 

 the blisters. The fungus, however, does not produce the blister-like 

 corky appearance ; nor is there scarcely any thickening of the leaf 

 where it is attacked by the fungus. 



These diseased portions of the leaf are termed galls, as are the vari- 

 ous abnormal vegetable growths jiroduced by true insects. Figure 2 

 represents several of these galls, or blisters as they are sometimes 

 called by fruit growers ; the galls are magnified about six diameters 

 and well illustrate their blister-like corky appearance. In figure 3 is 

 shown a section of a pear leaf through one of the galls, made doubtless 

 while the gall was in its red stage. Here the leaf is seen to be greatly 

 thickened at the diseased part. And in addition to the swelling of 



31 



Fig. 3.— Sectiou of a leaf ; g, gall in its red stage; n, n, normal structure of the leaf; o, opening 



of the gall; e, eggs. (After Sorauer.) 



both surfaces of the leaf, its internal structure is seen to be modified. 

 In some parts there is a multiplication of the tissue cells, and in others 

 a large part of the cells have been destroyed. 



As the season advances and the galls become dry and brown, the 

 thickening of the leaf becomes less marked, especially on the upper 

 surfaco. Figure 4 represents a section of a leaf collected and studied 

 in October. Here the tissues in the diseased spot are dead, and there 

 has been a shrinkage of the affected parts until the gall is but slightly 

 thicker than the uninjured portion of the leaf. 



If these galls be examined from the lower side of the leaf with a 

 hand lens (an instrument which every fruit grower should own) there 

 can be seen near the center of each a minute round hole. It can be 

 discerned in some of the galls represented in figure 2. This hole leads 



* Mr. Crawford records the finding of similar diseased spots on a fern (a 

 Oleichenia) growing among infested pear trees in Australia. Although the 

 inhabitants of the spots differed in color from those on the pear, he believed 

 this to be due to the difference of the food and. concluded that the disease was 

 the same on the fern and pear. 



