SOME BACTERIAL DISEASES OF PLANTS. 121 



death or deterioration of the bacteria. These bacteria live in the 

 soil and enter the root of the host plant by way of the root hairs. 

 Once in the root they irritate it causing the plant to form these 

 abnormal outgrowths or swellings commonly spoken of as nodules 

 or tubercles. In the cells of the nodules the bacteria multiply 

 rapidly living upon the elaborated food of the host and the free 

 nitrogen of the soil air. 



I shall now take up somewhat in detail a discussion of two of our 

 commonest bacterial diseases of plants, — Fire Blight and Cucum- 

 ber Wilt. I have chosen these because both are very generally 

 distributed over at least the eastern part of the United States and 

 are, therefore, perhaps familiar to most of you and also because I 

 am better acquainted with these than with any of the others. 



Fire Blight of Pears, Apples, Quinces, and Plums. 



This is the commonest and best known bacterial disease in this 

 country. It is a distinctly American disease, having been known 

 as a disease of fruit trees in eastern United States for more than a 

 century. That it is a native of this continent seems quite certain 

 as it is unknown in the old world, or at least has been until very 

 recently. The bacterial nature of the disease as I have already 

 pointed out was not discovered until about twenty-six years ago. 

 The early horticulturists held peculiar views in regard to the 

 nature of the malady as an examination of the early literature on 

 the subject will show. Even at the present time we find some of 

 these superstitions persisting among quite intelligent growers. 

 Among the theories put forward to explain the disease may be 

 mentioned the following : insects ; rays of the sun passing through 

 vapors ; sudden changes from sod to cultivation inducing an excess 

 of sap; freezing of unripe wood in the autumn whicli produced a 

 poison in the sap or "sour sap" as it was called which killed the 

 twigs and branches the following season ; electricity, many believing 

 the blight followed directly after a heavy thunder shower. Many 

 others might be mentioned, in fact, almost every observer had a 

 theory of his own. Only a few weeks ago a gentleman belonging 

 to a prominent horticultural society gravely informed me that 

 Pear Blight was caused by the thawing and freezing of the sap 



