196 The Potato 



disease is the selection of tubers from healthy plants and 

 a wide rotation of crops as outlined under the chapter on 

 Control Measures. 



Vertidllium wilt 



This wilt is caused by the fungus Verticillium albo-atrum 

 and shows symptoms much like the Fusarium wilt, and 

 the two are very easily confused in the field. The fungus 

 Verticillium gains entrance to the plant in much the same 

 way and also inhabits the sap vessels of the plant. Af- 

 fected plants show yellowing, wilting and dying of the 

 leaves, beginning with the lower ones and progressing 

 rapidly upward. The plants have a straggling appear- 

 ance, usually die earlier than is normal and yield poorly. 

 As with Fusarium wilt the tissues underlying the outer 

 layers of cells show a brown discoloration, which in this 

 case extends upwards into the branches as well as down- 

 ward into the roots, stolons and tubers. The tubers 

 show a blackened ring in a section of the stem end. The 

 fungus passes the winter in such infected areas of the 

 tuber and is also capable of living in the soil. The same 

 methods of control should be practiced as for Fusarium 

 wilt. 



Fusarium dry rot (Plate XI) 



This rot, caused by one or more species of Fusaria, is 

 a dry rot extending deep into the tuber. The surface 

 of the rotted area is much wrinkled and often shows 

 numerous clusters of a white, moldy growth. The fun- 

 gus gains entrance to the tuber through the skin wounds 

 or through lesions caused by other fungi, so that the rot 

 may occur at any point on the tuber. The powdery 

 dry rot caused by Fusarium trichothecioides, common in 



