230 Diseases of Economic Plants 



When the potatoes are dug, there is rarely any external 

 evidence of the disease. The major portion of the crop pro- 

 duced from wilted plants is nevertheless infected. If these 

 potatoes are stored in a cold place, but little change occurs; 

 but when infected potatoes are kept in a heated room or 

 left out-of-doors in a warm climate, rot soon develops. 



The presence of this disease can be detected, in otherwise 

 normal tubers, only by making a thin slice across the stem 

 end and searching for the brown discoloration. Such pota- 

 toes are not desirable for seed, but are not objectionable for 

 food. 



Soils which have once borne a sick crop will infect future 

 crops. The general means by which a field may become 

 infested are discussed on page 27. In addition to these 

 means, the potato wilt is often carried to new fields by seed. 

 It is a matter of record that this disease was largely in- 

 troduced into California from Oregon by means of infected 

 tubers. This could have been prevented by proper quaran- 

 tine restrictions. 



If potatoes must be stored for any length of time, all that 

 are badly diseased should be rejected, and the remainder 

 kept as cool as possible without freezing. It is best to sell or 

 consume all affected tubers at once. 



Another very similar wilt is due to Vertidllium alboatrurn 

 Reink. Several types of tuber rot due to various Fusaria 

 are distinguished, e. g., the jelly-end-rot, a soft rot of the 

 stem end; dry-rot, also of the stem end. A tuber dry-rot is 

 also caused by Phoma. 



Wilt ^'*' (Pseudo7nonas solanacearuni (EFS.)). — This wilt, 

 chiefly prevalent from Maryland southward, reported also 

 from Indiana, Nebraska, and Iowa, is identical with that of 

 eggplant, toliacco, and tomato, and is to a large extent 

 transmitted by insects. 



The leaves wilt and dry up; the stems and tubers show 

 yellowing and l)lackening in the veins, and the tubers give 

 way to a soft rot. Squeezing the fliseased tubers causes ex- 

 udation of creamy drops from the diseased veins. 



