Orchard Pests and their Control 375 



louse. The black cherry-louse has not become thoroughly dis- 

 tributed as yet, and every effort should be made to exterminate 

 it as soon as it is discovered. 



San Jose Scale. See under Plum. 



DISEASES AND INSECTS OP THE PEAR 



Fungous Diseases 



Blight (Bacillus amylovorus). This disease is known under a 

 variety of local names such as pear-blight, fire-blight, apple- 

 blight, and sometimes, according to the parts attacked, as twig- 

 blight, blossom-blight, body-blight, and root-blight. It is most 

 destructive in its attacks on the pear, but is also very injurious 

 to the apple and the quince. It also attacks the apricot and plum 

 on occasion, but its injuries to these hosts have never been severe. 



This, the most destructive of all orchard diseases, has been 

 known in America for more than a century, and it is scarcely 

 necessary to observe that during these many years of disastrous 

 outbreaks, nearly every conceivable theory as to the cause of the 

 disease has been advanced. These notions have been threshed 

 over and over as new localities have been invaded year after 

 year. A number of localities in this region are now having their 

 first experience with blight, and a few are still exempt. Con- 

 sequently, these many theories are being rediscovered and ad- 

 vocated by their adherents. One of the more recent causes to 

 be advanced is a variation of the frozen-sap theory, which was 

 first advocated by A. J. Downing in his " Fruits and Fruit Trees 

 of America," which was published in 1845. The germ theory of 

 the cause of the disease was first advanced by Burrill of the Uni- 

 versity of Illinois in 1878. In 1880 he made a more complete 

 report of his work. He found that he could produce blight in 

 healthy apple and pear trees by inoculating with diseased tissue 

 or by simply pricking a healthy limb with a needle which had 

 been dipped in juice from diseased bark. Although the micro- 

 organisms were found in all cases, such experiments could not be 

 accepted as positive proof. In other words, was it the germs or 

 was it the " poisonous sap " which caused the disease? 



