204 The Potato Crops 



well. The warts are at first brown, but later become black 

 and show decay. The disease Is very destructive. Control: 

 Report the disease promptly to the State experiment station 

 and receive recommendation for control. 



Mosaic. — Affected leaves are frequently wrinkled and pre- 

 sent a mottled appearance, light green or yellow areas alteri- 

 nating with the normal green of other parts of the leaf. Con- 

 trol: Plant seed from mosaic-free fields. 



Leaf boll.— A rolling of the leaves, beginning with the lower 

 ones, accompanied by a change in color to a pale green is 

 characteristic of this disease. The development of the plant is 

 checked and the yield is greatly reduced. Control: Seed tubers 

 should be obtained from fields free from disease. 



PowDEBT SCAB {Spongospora subterranea) . — In the early 

 stages of this disease small blisters are formed on the skin 

 of the tubers. Later these blisters rupture, exposing a dark 

 powdery mass and appear as raised pustules surrounded by 

 the torn skin of the potato. Control: The use of disease- 

 free tubers, seed treatment and crop rotation are important. 



FusARiuM WILT (Fuswrixi/m oxysporium) . — Rolling and wilt- 

 ing of the leaves, together with yellowing of the foliage 

 and premature death of the vines, are characteristic. Stems 

 of afifected plants show a blackening of the sap tubes. Con- 

 trol: The planting of field-selected or certified seed is advis- 

 able, and crop rotation is important. 



Blackleg {Bacillus phytophthorus) . — This is a bacterial 

 disease carried on the seed tubers. It may cause rot of the 

 seed tuber and thus occasion an uneven stand. The stems of 

 affected , plants become black at the base. Diseased plants 

 show lack of vigor and usually die without setting tubers, 

 ' although they may become .diseased after the tubers are 

 formed. Control: Affected tubers should never' be used for 

 seed. Seed from disease-free fields should be employed and 

 seed disinfection is advisable. 



CoLOEAno POTATO BEETLE (Leptinotarsa decemlineata) . — The 

 adult is a convex black- and yellow-striped beetle about % in. 

 long, which passes the winter in hibernation in the ground. 



