ii8 



THE CANADIAN HORTICULTURIST 



Fig. 2754. Diseased Tissue — Magnified 



of its attacking the fruit, producing watery 

 unlers, accompanied by brown discoloration 

 and decay. The disease may be known by 

 its peculiar odor, said by some writers to re- 

 semble putrefaction. 



When the disease is in progress the dis- 

 colored blighted portion blends gradually 

 into the color of the normal bark ; but when 

 the disease has stopped there is a sharp line 

 of demarcation between the diseased and 

 healthy portions. (Waite.) 



Microscopic Appearance of the Dis- 

 eased Tissues. — (See Fig. 2754). The 

 most conspicuous change in the tissues, af- 

 fected with the blight, is the disappearance 

 of the stored starch, and on account of this 

 peculiarity the organism has been named the 

 " starch destroying bacterium " (Bacterium 

 amylovorum) . The germ penetrates from 

 one cell to another and produces a gummy 

 or mucilaginous matter which is found on 

 the exterior of the affected parts. The 

 microbe is found, as a rule, only in the in- 

 ner bark and in the actively growing tissues 

 (called the cambium, which produces wood 

 on the inner side and bark on the outer 

 side). The organism is unable to grow in 

 tissues that are lignified or woody. 



Life History of the Pear Bught 



Germ. — The organism which produces the 

 disease is a small motile bacillus which in- 

 creases with great rapidity in the succulent 

 parts of affected trees. The microbe is of 

 microscopic size, so small that 25,000 placed 

 end to end would only measure an inch. 

 They are able to live and multiply in the 

 nectar of the blossoms, from whence they 

 are carried to other flowers by bees and in- 

 sects which visit the blossoms for honey and 

 pollen. From this locality the germs ex- 

 tend into the tissues and then downward into 

 the branches by way of the inner bark, gird- 

 ling the limbs and causing a large amount 

 of damage. The blight germ also gains 

 entrance to the plant through the tips of 

 growing shoots, thus producing twig blight. 

 The organism is not killed by the winter 

 frosts, but lives in the bark in a dormant 

 condition until spring. As soon as the plant 

 tissues became gorged with sap in the 

 spring the microbes, which have remained 

 alive all through the winter, start to grow 

 and extend into the new bark. This new 

 blight which develops in the spring can be 

 recognized by its moist and fresh appear- 

 ance from the blighted dead and dried bark 

 of the previous summer. A large amount 

 of gum is exuded from the affected bark 

 and runs down the tree and attracts to it 

 bees and other insects which carry the 

 microbes to the early blossoms, and from 

 these first flowers it is carried to others, and 

 thus the disease extends. 



The germ has never been discovered in 

 the soil, although careful search has been 

 made, hence the importance of recognizing 

 the winter form of the disease, for if these 

 affected portions of the tree are cut out and 

 destroyed the pear blight question is solved, 

 for without the microbes there can be no 

 disease. 



Conditions Affecting the Spread of 

 THE Disease. — Fire blight differs in severity 

 in different localities, and there are a num- 



