236 Agricultural Bacteriology. 



incomplete and methods of prevention can, therefore, not 

 be so successfully used as in the case of diseases caused 

 by other fungi. 



Pear blight. A disease that is most common in the 

 pear is known as blight, or fire blight, since the diseased 

 parts appear as though they had been injured by fire. 

 The disease affects apple, quince, apricot, and plum 

 trees. It is said to affect the mountain ash and several 

 species of hawthorne. The bacteria enter the tissue most 



FIG. 23. PEAR BLIGHT. 



The bacteria causing the shriveling- of the fruit en- 

 ter through, the blossom. 



often through the blossom, being carried from flower to 

 flower by bees and other insects. They may also enter 

 through wounds on any part of the tree. The bacteria 

 grow in the inner bark, gradually working their way 

 down the twig, causing the leaves to turn brown and to 

 become dry, and the bark to blacken and shrivel. The 

 growth may extend to the trunk and the tree be de- 

 stroyed. 



The growth of the bacteria in the tree is most rapid in 

 the spring and summer while the new tissues are tender 



