Some Important Pear Diseases. 



295 



2. Historical. 



Pear blight has probably been known in this country for over a 

 century. It was crudely attributed to one or another of countless 

 causes until 1880. At that time, through the studies cf Professor T. 

 J. Burrill, of the University of Illinois, we became acquainted with 

 ., the indisputable means by which this disease is induced and dissemi- 

 ' nated. The germs of pear blight belong to that class of minute 

 organisms known as bacteria, so many species of which are concerned 

 in the many contagious diseases of man and animals, and so many 

 others useful in various processes of fermentation. From 1883 to 

 1887, while connected with the Geneva station, Arthur greatly increased 

 our knowledge of this disease. Lastly, Waite of the U. S. Depart- 

 ment of Agriculture, has cleared up many points regarding the disease 

 in the flower, its spread by bees and other insects, with added sugges- 

 tions concerning its eradication. 



3. Special Features of the Disease. 



{a.) The Bactefia. — We may well dwell upon some of the interesting 

 points concerning the life and action of these pear bhght bacteria. 

 Often, when young shoots are affected, 

 slight ruptures in the bark may be 

 noted, and from these ruptures may 

 be exuded minute gummy droplets. 

 Examining under high powers of the 

 microscope such exudation, or even 

 the sap from diseased parts, we find 

 innumerable little oval bodies floating 

 about singly or in chains of two or 

 more, each individual being a plant 

 with great power of rapidly growing 

 longer and dividing again and again 

 into similar simple bodies (see figure 



171). These individual bacteria are so small that it requires a mass 

 of them to be evident to the unaided eye ; indeed, to attempt to see 

 the individuals in the little gummy droplet, or in the sap, would be 

 like an attempt to see the separate rain drops in the approaching 

 storm more than a mile away. 



171. — The bacteria 0/ pear blight. 



