168 



FUXGI AXD FUXGICIDES 



usually. Add half a pound of London purple or Paris 

 green to each barrel of the mixture whenever the Col- 

 orado beetles threaten injury. 



The Potato Scab 



Oospora scabies 



The ^'scab'^ of potatoes is familiar to every experi- 

 enced farmer. Like the pear blight, the theories con- 

 cerning its cause have been many and varied ; but the 

 recent investigations of Messrs. Thaxter and Bolley have 

 established, with considerable certainty, that the injury 

 is really due to the attacks of a low form of fungus life,, 

 which develops in the tissues of the growing tubers. 

 Two forms of scab, the ''deep scab^^ and the ^'surface 

 scab,^^ have been recognized by some writers, but there 

 is a strong probability that these are simply different 

 phases of the same injury. At any rate, there is little 

 difference, from a practical standpoint, and they will 

 not be separately discussed here. 



It has often been noticed that scabby j^otatoes are 

 more abnndant in fields heavily fertilized by barnyard 

 manure. This is now believed to be largely due to the 

 fact that if scabby potatoes are fed to stock, the spores 

 of the fungus can pass through the alimentary canal 

 uninjured, and can multiply abundantly in the manure 

 heap. Consequently they will be transferred to the field, 

 and when the potato tubers are developing, will attack 

 them and produce the injury. 



The fungus-producing scab is an obscure one, which 

 botanists have studied only recently. It can generally 

 be seen on j^artially developed scabby potatoes, in the 

 shape of fine white threads running over the surface. 

 It reproduces by means of spores. Eepeated experi- 

 ments have shown that if scabby potatoes are planted, a 

 large percentage of the resulting crop will be scabby. 



