PLANT DISEASES 283 



For instance, flax wilt is a disease of this sort, living 

 almost entirely in the soil. It is obviously impossible 

 to sterilize the soil, but good drainage is useful. Since 

 the fungus may accumulate in the soil, the chief remedy 

 for such a disease as flax wilt lies in rotation of crops, 

 so that the fungus in the soil may be allowed to die off 

 between the crops of flax. It has been found necessary 

 where flax has been raised for a long number of years 

 to rotate the crop so that the flax shall not be put in 

 the soil more than once in five to seven years. The 

 reason for this is readily inferred from the life story of 

 the disease. In the case of flax wilt other methods of 

 treatment, namely, seed treatment with formaldehyde 

 and resistant varieties, are also necessary. 



Potato Scab. Potato scab is a disease of potato 

 tubers causing scabby patches on the skin. It lives and 

 accumulates in the soil in a manner somewhat similar 

 to flax wilt. Rotation of crops may therefore be nec- 

 essary in severe attacks of the disease. Ordinarily, 

 however, the disease can be prevented by treating the 

 seed potato with a corrosive sublimate solution or a 

 solution of formaldehyde, thus destroying the fungous 

 parasite on the skin of the potato. 



Rusts of Cereals Resistant Varieties. The rusts of 

 cereals are among the best known of the diseases of 

 agricultural plants. These rusts have been known since 

 the time of the Romans and undoubtedly caused losses 

 in times even earlier. Rust epidemics in recent' years 

 have been common in almost every country on the globe 

 where cereals are raised in quantity. As with the 

 smuts, so here, the disease which is to the farmer known 

 as rust of wheat, oats, barley, and so forth is really a 

 collection of diseases. There are in other words, two 



