290 DISEASES OF FIELD & GARDEN CROPS. [CH. 



before the tubers are reached by the putrefactive spawn 

 of the fungus, but in other instances the attack is so 

 sudden and so highly destructive that the whole of the 

 potato plants above ground in a large field will be 

 destroyed in a day or two. The disease doubtlessly 

 starts at first from a few centres only ; there it remains 

 for a brief time more or less unobserved. The fungus, 

 however, possesses such wonderful powers of spore pro- 

 duction and rapid growth, especially when the air is 

 moist and the temperature ranges from 60 to 70 Fahr., 

 that in a few days one fungus growth will become ten 

 thousand. This growth goes on in a constantly increas- 

 ing ratio until at length the great flood of disease seems 

 to almost suddenly cover the potato fields. When the 

 attack is not violent it is obvious that a good plan is to 

 remove all the tainted potato stems and foliage before the 

 spawn reaches the tubers ; but, on the other hand, if the 

 stems are removed before the tubers are ripe, injury must 

 accrue to the crop, as the starch which is subsequently 

 stored up in the tubers is formed in the leaves. Still, it 

 is better to have a poor or partial crop than none at all. 

 Some cultivators advise the growing of more early ripening 

 potatoes, as such varieties now commonly escape the 

 murrain ; but there can be little doubt that if a change 

 could be brought about in the general habit of the potato 

 plant it would be followed by an exactly corresponding 

 change in the habit of the parasite. Late ripening varie- 

 ties are, moreover, wanted, and their cultivation cannot 

 be dispensed with. 



When the fungus spawn reaches the tuber it decom- 

 poses the cells and corrodes the starch. In bad cases the 

 tubers are soon reduced to a mass of putrefaction. In 

 mild cases the spawn of the fungus hibernates and be- 

 comes perennial, as was first pointed out by the Rev. 

 M. J. Berkeley in vol. i. of the Journal of the Royal Horti- 

 cultural Society for 1846. Mr. Berkeley writes (p. 26) 

 in reference to the fungus growing from ripe, harvested, 



