Several types of destructive degeneration or "mosaic" diseases are carried 

 over in the tubers of diseased plants to the plants grown from them another 

 year. They are perpetuated indefinitely by clonal propagation. These 

 diseases may also be carried from diseased plants to healthy plants by in- 

 sects and particularly by plant lice. Thus an entire strain or clone of a 

 variety may become infected and if the disease becomes severe the crop 

 may be a complete failure as was the case in the test plantings shown above. 



63 



