PLANT DISEASES 435 



In a third group the parasites neither destroy living cells 

 nor live peaceably with them, but invade the water-conduct- 

 ing vessels (woody fibres) and live in the sap. This inter- 

 feres with the movement of water, and if the parasites 

 develop so as to block the vessels, the water supply is cut 

 off and the plant wilts. These " wilt diseases " are very 

 common and destructive, especially in the case of seedlings. 



Fiu. 84. A spot disease of maple leaf. After SORAUER. 



141. Diseases of the first group. In this group of dis- 

 eases the parasite kills living cells. No list of these diseases 

 can be given, but a few representative cases will illustrate 

 them. 



Pear blight. This is one of the common diseases, not 

 only of pear trees, but of apple trees and other fruit trees. 

 It is sometimes called " fire blight " or " twig blight," and 

 these names suggest the appearance of trees with this dis- 

 ease. The flowers and branch tips begin to wilt and finally 

 blacken, and this may extend to every branch tip, until the 



