PEAR GROWING IN CALIFORNIA. 



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CHAPTER XIII. 



BACTERIAL AND FUNGOUS DISEASES OF THE PEAR. 



PEAR BLIGHT. 



In 1878 Professor T. J. Burrill of the University of Illinois isolated 

 the organism which causes the disease of pears known as blight, and 

 since that time the various theories that have been advanced by those 

 who were not willing to accept a scientific fact have been disregarded 

 by all horticulturists and plant pathologists. . The causal agent was 



found to be a bacterium and 'was 

 named Bacillus amylovorus. The 

 plant pathologist can easily isolate 

 this species of bacteria from infected 

 orchard material, and inoculations 

 with pure cultures of the organism, 

 resulting in the development of the 

 disease, have been made over and 

 over again, so that the proof of this 

 particular organism being respon- 

 sible for the blight in pears, apples 

 and quinces, but rarely in other trees, 

 is just as positive as that which con- 

 vinces us that tuberculosis, typhoid 

 fever and diphtheria are due to spe- 

 cific bacterial organisms in the human 

 system. 



FIG. 144. The organism which 

 causes pear blight, Bacillus amylo- 

 vorus, greatly enlarged. (After 

 U. S. Department of Agriculture.) 



Distribution. 



The distribution of pear blight is very wide in this country. Its 

 presence in many localities, otherwise well adapted to the cultivation 

 of the pear, has made it necessary for fruit growers to give up grow- 

 ing this fruit. Perhaps in all cases where failure was met with 

 because of this diesase, the adoption of the present careful scientific 

 methods of control which have been evolved because of an exact 

 knowledge of the cause of the disease, would have resulted in suc- 

 cessful pear culture. The degree of virulence differs, however, in 

 different places and the problem is much more difficult under con- 

 ditions that favor its development and the intensity of its attack. In 

 California, this point is illustrated nicely by certain sections where 

 the blight seldom occurs, and where if it does make its appearance 

 during certain periods of time, it is easily controlled. There are other 

 well known sections where the disease is continually present and where 

 it seems to possess a virulence that makes control work not only 

 difficult but a continuous job. There are many factors that enter 

 into the problem, which are responsible for the seriousness or the lack of 

 importance of the disease. Some of these factors are soil, climate, 

 disease carriers and rapidity of growth. Succulent, rapid growing 

 shoots are more susceptible than the hardier slow growing branches 



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