926 Rural School T.eaflet 



supposed to cause this disease, but in this country a fungus named Oospora 

 scabies has proved to be the cause. It is almost impossible to find this 

 fungus on scabby potatoes that have been stored for some time, but if one 

 examines closely such potatoes as they are dug he will notice a film quickly 

 disappearing, composed of a grayish hair-like growth. While it is next to 

 impossible to find evidence of this fungus on scabby potatoes that have 

 been stored, nevertheless it persists until they are planted and may then 

 spread to the young tubers as they are formed. 



The fungus also seems able to persist in the soil for a year or more. 

 Even healthy tubers planted on such land will produce scabby ones if con- 

 ditions favor the fungus. 



It has been noticed that an alkaline condition of the soil is favorable 

 for the development of this fungus. Lime, wood ashes, and often manure 

 tend to make the soil alkaline. The statement often made by farmers 

 that wood ashes make potatoes scabby is explained in this way. The 

 scab, however, is caused by a fungus, and no matter how alkaline the soil 

 may be the tubers will not be scabby unless this fungus is present. 



There are, then, two conditions to be observed in guarding against this 

 disease: plant clean tubers in healthy land. If tubers absolutely free from 

 scab cannot be obtained, the cleanest should be selected and these treated 

 by soaking them for two hours in a solution of formalin made by diluting 

 I pound of commercial formalin (40 per cent formaldehyde) with 30 gallons 

 of water. The treatment should be made before the tubers are cut and 

 preferably just before they are planted, as there is then less danger of 

 subsequent infection. A convenient way is to immerse a sack of tubers 

 to be treated in a barrel of the solution. 



The treated tubers must be planted on land that has not been in pota- 

 toes for the past two years, preferably after a clover sod, and in soil that 

 is not alkaline. It is wise to practice a rotation of crops in which potatoes 

 come once in three or four years. Potatoes, oats, meadow, potatoes, 

 might be such a rotation. When it is necessary to apply lime in order 

 to grow clover, the application should be made in the fall after the potatoes 

 are harvested. By the time the land is planted again to potatoes, the 

 alkalinity will be greatly reduced and the fungus, if present, will have 

 died out. 



In order to insure the potato crop against these two and against other 

 diseases we must, then: (i) reject all diseased tubers; (2) treat selected 

 seed with formalin; (3) plant treated seed on land free from organisms 

 that infest potatoes; (4) spray the potatoes thoroughly during the 

 growing season. Even then they may become affected with some dis- 

 ease, but the chances are good that they will not. One cannot afford 

 to grow potatoes at all if he cannot afford to take these precautions. 



