388 AGRICULTURAL BACTERIOLOGY. 



possible that they are comparatively few, but those who have 

 studied the subject most insist that bacterial diseases of plants 

 are very numerous, and will probably, in the end, be found to 

 be as numerous as bacterial diseases among animals. At the 

 present time, however, only a few can be regarded as definitely 

 proved, although there is a long list, attributed with more or 

 less reason to bacterial action. The best known and the best 

 attested bacterial diseases of plants may be briefly given and 

 are as follows : 



Pear and Apple Blight. Fire Blight of the Pear Twig. An- 

 thrax of the Fruit Tree. The disease known under these 

 various names is a common one in apple and pear trees. It 

 has been known for over a century. It is characterized by small 

 dead spots on the bark of the tree, which spread somewhat, 

 eventually killing the young twig, and even extending into 

 the stem. The leaves turn a brownish color and may exude 

 a dark fluid. The trees are sometimes ruined, though some- 

 times the infection is not so serious. The cause of this disease 

 is now known positively to be a bacillus, to which the name 

 B. amylovorus has been given. This organism has been care- 

 fully isolated from trees showing the disease, has been culti- 

 vated, and has been successfully used for inoculation experi- 

 ments. In the normal growing plant it appears that the 

 bacillus finds its entrance into the plant through the flowers, 

 and feeds upon fluids in the nectary. 



The Twig Gall of the Olive. This is a well-known disease, 

 though one, of course, of little interest in the United States 

 from the fact that we do not here produce olives. If 'The disease 

 is characterized by knots on the branches, from the size of peas 

 to the size of hazelnuts. It is produced, as has been shown 

 through culture and inoculation experiments, by a bacillus 

 named B. olece. A somewhat similar disease, probably pro- 

 duced by a similar cause, is found on the willow, birch, 

 and pine. 



