43 



leaf, and this appeared to be a mechanical result of growth. No 

 parasite appeared at any subsequent stage, notwithstanding the 

 increase in the size of the holes. 



As the disease was traced farther and farther into the opening bud 

 of the growing shoots, it was felt that an adequate cause for the 

 disease must present itself, and suspicion fell upon a microscopic 

 insect larva belonging to the Thripsidse. In the course of the investi- 

 gations several species of Thrips have been seen in apricot buds (and 

 also in those of the peach), and these may also be injurious, but not 

 to any such extent as the larval form first mentioned, which may 

 belong to another genus of the Thripsidse. 



This latter was uniformly present in apricot buds on trees showing 

 shot-hole, one or more inhabiting each bud. This was found to be the 

 case in numerous orchards in Cumberland county. 



I had not been long studying the various holes to be found in the 

 leaves of the apricot before I had come to the conclusion that the holes 



Fig. 61. Young apricot leaf at the time 

 it was pricked with an exceedingly 

 fine-pointed needle which had been 

 wiped on the surface of an ant. The 

 resulting shot-holes are shown in 

 Fig. 62. 



Fig. 62. Mature apricot leaf showing row of 

 five shot-holes, produced by very slightly 

 puncturing the young leaf with an exceed- 

 ingly fine-pointed needle. This is the leaf 

 shown in Fig. 61. 



were due to a variety of causes, and that they might arise as the result 

 of mechanical injury and be in no way connected with parasites of any 

 kind. Soon afterwards I was pleased to learn that other observers 

 had already come to the same conclusion. The apricot leaf illustrated 

 on this page, and showing five holes nearly in a line, was the result of 

 one of my trials to see what effect mechanical irritation, in this case a 

 series of minute punctures, would have in producing holes. Other 

 experiments and observations proved that merely whipping the leaves 

 about when they were young would cause holes to appear at a later 

 stage of their growth, and that wind storms are in this way a cause of 

 shot-holes. In a similar manner certain gnawing insects were observed 

 to cause shot-holes. These were accustomed to gnaw away the tissue 

 on the under surface of the leaves in small spots. These spots 

 ultimately became " shot-holes." 



