Part L Fungi. 



A. DESCRIPTION OF FUNGOTJS DISEASES. 



1. Potato rot; late blight; downy mildew {Phytoj)hthora 

 infestans^ DeBary). — The fungus causing the common potato rot is 

 an old offender. It was undoubtedly introduced into Europe with 

 some of the early importations of the potato, and has in certain 

 years proved so destructive that famines have resulted from the 

 entire loss of the potato crop. Such occurrences eventually lead 

 to thorough study of the organism. As early as 1846, the fungus 

 causing the trouble was veiy carefully described in an English 

 publication,* and since that time other observers have given the 

 disease much attention. It has spread to all regions in which pota- 

 toes are extensively grown, so that both scientists and farmers are 

 very familiar with many of its characteristics. 



The most interesting feature connected with the fungus is 

 undoubtedly the wonderful energy whicli it exhibits, under favor- 

 able conditions, in the destruction of the potato plants. It some- 

 times spreads with such rapidity that a crop may be ruined in one 

 or two days ; and unfavorable conditions, or the total destruction of 

 the plants, formerly appeared to be the only effectual agents in pre- 

 venting or checking the spread of the dreaded disease. This rapid 

 decay of both the foliage and tubers is perhaps the most distinctive 

 of those characters which are commonly brought forward for the 

 identification of the disease. It is almost invariably accompanied 

 by a strong, disagreeable odor which is easily recognized by all who 

 have once experienced it. When large fields have been attacked, 

 the smell is particularly strong ; it then arises entirely from the 

 foliage, and is not produced by the tubers. 



The conditions which favor such rapid decay are, as a rule, not 

 generally present throughout this state. The fungus makes its 



*Rev. M. J. Berkeley, JoMrjiaZ of the Royal Horticultural Society, Vol. I. 



