350 The Spraying of Plants. 



and also a rotting of the tubers. No distinct discoloration s appear 

 upon the leaves, as is the case with fungous diseases, but the 

 entire plant is unhealthy and dies prematurely. Tubers fre- 

 quently show discolored patches on their surface before decay- 

 ing; a soft rot results. No remedy is known, except rotation. 



Scab (Oospora scabies, Thax.). Description. Potatoes very 

 commonly suffer from the attacks of a fungus which causes the 

 skin of the tubers to become rough or scurfy (Fig. 81), the 

 injury often penetrating to a considerable depth. The life 

 history of the fungus is not yet well understood, but it is 



FIG. 81. Potato scab. 



known that the disease may be communicated to new tubers by 

 unclean seed, and that barnyard manure, lime, or ashes may 

 have a tendency to increase the disease. Soil in which scabby 

 potatoes have been produced also appears capable of infecting 

 later crops. One kind of scab is caused by an insect. 



Treatment. Land in which the scab fungus is found should 

 not be planted to potatoes, and only clean fertilizers or unin- 

 fected manure should be applied. Scabby seed may be cleaned 

 by soaking it for an hour and a half in a solution of corrosive 

 sublimate, using one ounce of the poison in eight or nine gal- 

 lons of water. This may be done either before or after the 

 potatoes are cut, but the tubers must not again be brought in 



