314 DISEASES OF FIELD & GARDEN CROPS. [CH. 



but prevention or palliation may be considered fairly 

 within reach, and this prevention can only be attained 

 by skilful culture and perfect winter storage. 



Sometimes growers keep their potatoes in enormous 

 underground heaps called "pies ;" in these positions the 

 tubers frequently heat and rot ; in other instances diseased 

 potatoes are interbedded in dunghills, or dug into the 

 ground ; in all such cases the best means have been taken 

 for successfully propagating the disease. From all such 

 positions many millions of conidia of the potato fungus are 

 dispersed each June, whose special mission is to devastate 

 potato crops. The warmth and moisture of " pies " and 

 manure-heaps are the exact conditions required by resting- 

 spores for their maturation. 



To prevent the annual recurrence of the potato mur- 

 rain it is in the highest degree necessary to destroy the 

 material which is undoubtedly swarming with myriads of 

 disease germs. This destruction should be effected by 

 burning, or, where burning is not practicable, deep burial 

 might be resorted to. No more fatal mistake can be made 

 by potato growers than leaving dead stems, leaves, and 

 tubers about in their fields, especially after a potato crop 

 has suffered from disease. 



When cut sets are used at planting, the cut surface 

 should perhaps be allowed to heal or dry before planting, 

 or, if this is not convenient, the cut surfaces might be 

 quickly passed over a hot iron. It frequently happens 

 when diseased sets are used that the produce grows in a 

 healthy manner, with no trace of the murrain. There can 

 be little doubt of the existence of perennial spawn in 

 some of these examples, if not of resting-spores ; but in 

 some instances it would appear that neither spawn or 

 spores work much mischief direct from the seed tuber 

 when buried. In some instances both mycelium and 

 oospores must be dead. 



Any cure of the murrain in invaded potato plants is 

 quite hopeless, for in this disease the substance of the 



