CHAPTER IV. 

 NATURAL AND ARTIFICIAL INFECTION. 



§ 10. In artificial infection we have a safe mode of distinguishing 

 whether a fungus is parasitic or not ; in other words, whether 

 it is capable of penetrating into the organs of living plants. 

 This method of investigation should always be resorted to in 

 determining the cause of disease, more especially if mycelium 

 or sporocarps of several fungi are present on the diseased 

 material simultaneously. For it not unfrequently happens that 

 the disease has made so much progress as to make it quite 

 impossible to determine whether or not any fungi present on 

 the dead remains are really the cause of disease. In many 

 cases where one finds a mycelium in living parts, it has 

 disappeared, and only sporocarps remain in portions already 

 killed. 



Injuries due to insects frequently accompany fungi on a 

 diseased plant, so that it is extremely difficult to say which was 

 the primary cause of the damage, and artificial infection must 

 be resorted to. So also with injuries from some external source 

 like drought, heat, cold, moisture, and mechanical causes. 

 Fungi appear so soon after hurtful agents like these, that it 

 becomes doubtful whether they are the cause of the death of 

 the host, or the result of it. 



Minute observations in situ of all tlie circumstances connected 

 with the attack, combined with examination of numerous speci- 

 mens and comparison with neighbouring plants, enable one, 

 after some experience, to say with a fair degree of certainty, 

 whetlier the disease in question is of fungoid origin or not. 



