430 General Botany 



increase or decrease the amount of the infection. The scab of 

 potatoes is increased by liming. 



Plant pathology. The foregoing paragraphs are sufficient to 

 indicate the complexity of the problems of plant pathology, which 

 is the science that treats of plant diseases and their control. They 

 may also serve to show the importance of studying and under- 

 standing the life histories of fungi and bacteria, of their effects 

 on their host plants, and of discovering new and better methods 

 of eradicating them. One of the most important functions of the 

 federal and state agricultural experiment stations is the pro- 

 motion of research for the control of plant diseases. In view of 

 the enormous annual losses to growers of plants, and the im- 

 proved yields already obtained through the discovery of control 

 measures, the expenditure of large sums of money for the agri- 

 cultural experiment stations is more than justified. Every one 

 profits by these investigations through more abundant and 

 cheaper food supplies. 



In the following paragraphs a few common diseases are de- 

 scribed. More specific information concerning them may always 

 be obtained by writing to your State Experiment Station. 



Fire blight is one of the commonest diseases of pear, apple, 

 and quince trees. It becomes noticeable in early summer 

 through the turning brown of the leaves of twigs on these trees 

 as though they had been scorched. The infective agent is a 

 bacterium which lives in the interior of the affected branches, 

 and unless it is checked as soon as it appears, it may extend into 

 other tissues of the plant. The bacteria are apparently spread 

 by insects. After a rain the infected branches bear numerous 

 drops of a gummy nature containing countless numbers of bacteria, 

 and insect visitors carry the bacteria from infected branches to 

 blossoms and other twigs. The bacteria pass the winter in the 

 living tissues at the edge of the cankers on the larger branches, 

 and in the following spring these may become a source of further 

 infection to near-by trees. The only known raethod of control 



