278 DISEASES OF ECONOMIC PLANTS 



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

 yellowing and blackening in the veins, and the tubers give 

 way in a soft rot. Squeezing the diseased tubers causes ex- 

 udation of creamy drops from the diseased veins. 



Tubers from diseased vines rot rapidly and should not 

 be stored if it can be avoided. If stored, they should be 

 kept at as low a temperature as possible. It is unwise to 

 plant potatoes in land which has shown this disease upon 

 any of its hosts without a long intervening rotation. The 

 precautions suggested under soil diseases, use of clean seed 

 and special attention to insect control, are preventive meas- 

 ures. 



Cercosporose, leaf blotch (Cercospora concords (Casp.) 

 Sacc). — Cercosporose of the potato has been known in 

 Europe since 1854, but was not known in the United States 

 until 1902, when it was found in Vermont by L. R. Jones. 

 C'ercosporose appears when the plants begin to blossom. 

 The lower leaves show obscurely defined pale spots, 3-5 mm. 

 in diameter, sometimes blending to form larger areas. 

 A pale gray or violet fungus becomes conspicuous upon the 

 lower surface. As the disease progresses the spots encroach 

 upon the upper leaves. The affected spots sometimes die, 

 much resembling the alternariose spot, but more often the 

 entire leaf slowly turns yellow and dies. 



The damage, especially to late potatoes, is considerable, 

 reducing the yield one fourth or more. In Europe the dis- 

 ease is troublesome, and its spread in this country may be 

 anticipated. 



The following varietal resistance was noted by Jones and 

 Pomeroy : ^ — 



1 Jones, L. R., Vt. Agr. Exp. Sta. Rpt. 19, 255. 



