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 alter- 

 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. 



Powdery scab {Spongospora suljterranea) . — 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 (Fusariu/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 affected 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 pliytophthorus) . — 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. 



Colorado potato beetle (Leptinotarsa decemUneata) . — The 

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

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



