234 Diseases of Economic Plants 



Both diseases are usually of such mild form during the 

 first year as to be unnoticed; but appear in much more 

 severe form during the second year. Though the causal 

 organism is unknown, both are also contagious, and infection 

 passes readily from plant to plant in the field. Such infection 

 takes place chiefly through the soil and has been observed to 

 occur at a distance of 2 meters. The disease is seed-borne and 

 probably infestation does not persist in the soil. Therefore 

 seed should be taken from a field free of these diseases, or 

 seed-plants which grew fully 2 meters distant from diseased 

 plants should be selected. 



Minor diseases 



Leaf-blotch -^^ (Cercospora concors (Casp.) Sacc.) appears 

 when the plants begin to blossom. The lower leaves show 

 obscurely defined, pale spots, 3-5 mm. in diameter, these 

 sometimes blending to form larger areas. The spots some- 

 times much resemble the early-blight spot, but more often the 

 entire leaf slowly turns yellow and dies. The damage, espe- 

 cially to late potatoes, is considerable. 



Leak -^- {Rhizopus nigricans Ehr. and Pythium, deharyanum 

 Hesse.). — Orton has described a wet, soft rot of potato in the 

 peat lands of California. It is characterized by dull brown 

 discoloration of the skin and light brown or buff coloring of 

 the flesh, accompanied by softening. Upon pressure a clear 

 brown liquid without disagreeable odor, exudes. 



The disease spreads rapidly in warm weather, and entire 

 shipments may decay en route to market. To prevent infec- 

 tion the skin should not be broken in handling, and all refuse 

 should be destroyed by fire. Diseased tubers should be scru- 

 pulously sorted out of lots intended for storage or shipment. 



Silver-scurf {Spondylocladiimi atrovirens Harz.). — Dark 

 blotches spotted with minute black specks appear on the 

 surface of the tubers. Later these assume a silvery-gray tint. 

 The disease, recently introduced from Europe, is widely 

 distributed, but n^t of great importance. Infected seed 

 should not be used. 



