the; prevention op potato scab. 83 



CAUSE OF SCAB. 



Contrary to the opinion frequently expressed by practical 

 growers, the cause of the potato scab is a specific organism, and 

 is not due as a first cause to any character or condition of the 

 soil. The amount of moisture, nature of the fertilizer used, the 

 alkalinity or acidity of the soil may and do influence to a large 

 degree the amount of scab present on a given crop, but if the 

 germs of the disease are not in the soil or introduced into it by 

 means of infected seed tubers or from some other outside source 

 these factors alone are unable to produce the disease. Ashes, 

 lime, stable manure and chip dirt are of themselves incapable of 

 producing scab, but if the land or the seed is already infected 

 their action upon the soil is such that favorable conditions are 

 produced for the development of the scab fungus which mani- 

 fests itself upon the crop. For this reason a belief that these 

 materials themselves produce the disease is more or less held by 

 practical growers. 



The real cause of potato scab was unknown up to 1890 when 

 Dr. Roland Thaxter of the Connecticut experiment station dis- 

 covered the fungus, to which he afterwards gave the name 

 Oospora scabies, and demonstrated the true relationship 

 between the fungus and the disease. Since that date his con- 

 clusions have been accepted quite generally by American plant 

 pathologists. 



Experiments have shown repeatedly that scab does not develop 

 on new land unless it is infected from some outside agency. If 

 clean seed is used and other precautions are taken a clean crop 

 will result. If scabby seed is used a more or less scabby crop 

 is almost sure to be produced. Because of the readiness with 

 which the disease may thus be spread it follows that most of the 

 infection of new areas comes from scab infested seed. It is 

 probable that scab germs are sometimes introduced into the soil 

 by means of tools or manure and one case is recorded where soil 

 plainly became infected by the water draining off from a potato 

 field on higher ground.* 



* Minn. Exp. Sta. Bui. 32. p. 223, (1S93), 



