280 DISEASES OF FIELD & GARDEN CROPS. [CH. 



traversed in every part by the spawn of the fungus. 

 During warm, humid conditions of the weather the black 

 decomposed spots are sometimes present for several days on 

 the leaves before the fungus is seen. These blotches indi- 

 cate that the putrefactive spawn of the fungus is within 

 the leaves, awaiting favourable conditions for its complete 

 development as a white bloom outside. The phenomena 

 just mentioned are accompanied by a peculiar and very 

 offensive odour well known to every person who has 

 walked through a field of potatoes suffering from disease. 

 The odour is caused by the putrescence set up in the 

 tissues of the host plant by the contact of the mycelium of 

 the potato fungus. Although the attack of disease in 

 potato plants is apparently sudden, and made on 

 apparently sound plants, yet all known facts point to 

 the probability of the existence of the fungus in a nascent 

 state during at least several weeks prior to its general 

 recognition. The belief in the extreme suddenness of 

 fungoid growths is, in many instances, a mere popular 

 delusion. The common field mushroom is supposed by 

 rustics to grow in a single night ; but it is well known 

 to careful observers that the infant mushroom exists 

 just beneath the surface of the soil in a growing state for 

 several weeks before it suddenly bursts through the earth 

 and expands its cap or pileus. We have ourselves seen 

 fields of potatoes which were apparently undiseased one 

 day, prostrate on the ground the next, and the haulms 

 blown away by the wind on the third day. This apparent 

 suddenness of the attack in the early autumn appears to be 

 well known in America ; for Professor W. G. Farlow of 

 Harvard University writes in reference to the " Potato 

 Rot." (Bulletin of the Bussey Institution, part iv. p. 319): 

 " At times its advent is so sudden that, within a few 

 hours, the potato fields change from green to brown and 

 black, and the plants which, in the morning, gave pro- 

 mise of an abundant crop, before night present a mass of 

 decaying vegetation, in which are involved not only the 



