254 FUNDAMENTALS OF AGRICULTURE. 



is to plant clean seed in clean ground; if either ground 

 or seed is infected, the crop will be scabby; and the 

 amount of scab will depend on the amount of original 



POTATO SCAB. 



fungus infection and the favoring or retarding influ- 

 ence of the surroundings. 



Bacterial Wilt of 'Cucumber, Melon and Squash.-^- 

 The first indication of this disease is the drooping of 

 a single leaf during the day, at night it recovers, but is 

 more wilted the next day, and is soon beyond recov- 

 ery, while other leaves near by are similarly affected. 

 Finally the whole branch dies. This disease is caused 

 by the growth and spread of bacteria in the sap tubes 

 of the plant; these reduce the supply of water for parts 

 beyond, and cause the wilting. If stem or leaf-stalk 

 of an infected portion is cut across and pressed be- 

 tween the fingers, a milky, somewhat sticky fluid, con- 

 taining bacteria, will exude from the sap tubes, instead 

 of the clear watery sap. These bacteria are trans- 

 ferred readily from diseased to healthy leaves on the 

 mouth parts of insects. Since they are well protected 

 in the tissues of the plant, sprays have little or no di- 

 rect effect in checking the disease, but a coating of Bor- 



