i26 



Minnesota Plant Diseases. 



Potato scab {Oospora scabies Thaxt.). Potato scab is an ex- 

 ceedingly common disease of potatoes. The cause has been 

 the subject of some dispute among botanists but it is now gen- 

 erally accepted that the common form of potato scab in Amer- 

 ica is due to a parasitic fungus, while the European scab has a 

 very different cause. The American potato-scab fungus be- 

 longs to the group of "imperfect" fungi and is found amongst 

 the white, loose-spored forms. Scabby potatoes when freshly 

 removed from the soil show a very delicate moldy coating, in 

 which the loose spore-bearing threads are found. The surface 

 of an attacked potato becomes roughened and scabby, hence 

 the common name of the 

 disease. The fungus can 

 remain in the soil for sev- 

 eral years and badly in- 

 fested fields should not 

 be sown to potatoes. 

 Since the fungus is a 

 lurking parasite and 

 gains entrance through 

 the potato skin, it has 

 been found that a treat- 

 ment of the "seed" pota- 

 toes will kill ofif the scab 

 fungus. Immersion in a 

 solution of one pound of 

 corrosive sublimate to 

 fifty gallons of water for 

 one and one-half hours 

 will free the potatoes from scab providing other precautionary 

 measures are taken. After treatment, the potatoes must be 

 kept free from the disease. They must not, for example, be 

 brought into contact with other diseased potatoes or must not 

 be planted in soil which is badly infested with the scab. A 

 formalin solution may also be used as a steep. This solution is 

 made up of one pound of formalin to thirty gallons of water, 

 and the "seed" potatoes are immersed for about two hours. 

 The corrosive sublimate solution is very poisonous ; potatoes 

 treated by the first method must therefore never be fed to stock. 



Fig. 164. Potato scab. After Clinton. 



