xxxvi.j POTATO DISEASE, II. PASSIVE STATE. 329 



crops. Professor Farlow has stated, and no doubt cor- 

 rectly, that Professor De Bary never made the statements 

 attributed to him by a few of his friends in this country. 

 It ought not to be difficult on well-conducted farms 

 to keep the fields clear from rotting potato refuse. The 

 working men and boys should be taught, as a rule of 

 the first importance, that all potato refuse should be 

 scrupulously gathered together and either burnt or 

 deeply buried. Stones are always gathered together on 

 farms by boys and girls. When vegetable refuse is in- 

 corporated with dung, this material by its warmth and 

 moisture keeps the germs of nearly every known plant 

 disease alive and in good condition through the winter 

 for a renewed burst of vitality in the spring or early 

 summer. It has been said that when potatoes are grown 

 near chemical works they are frequently free from disease, 

 as sulphurous acid gas or some other gaseous impurity 

 proves fatal to the fungus of the murrain. 



