56 DISEASES OF CROPS. 



fertilized parents, exposing them to infection, ruthlessly 

 destroying aU that suffer, saving those ■which resist best, 

 and repeating the process in successive seminal genera- 

 tions. . . . There is no great improbability in a new 

 variety of potato arising which would resist the fungus 

 completely, or at least much better than any existing 

 variety. With respect to the cross-fertilization of two 

 distinct seedling plants, it has been ascertained that the 

 offspring thus raised inherit much more vigorous consti- 

 tutions and generally are more prolific than seedJings 

 from self-fertilized parents.'' (2) Destroy common bitter- 

 sweet, henbane, and similar plants belonging to the same 

 natural order as the potato, as Peronospora infestans also 

 infests these as well as the potato. (3) Burn all infested 

 haulms of the previous potato crops. (4) When the crops 

 are first attacked, a good plan is to remove all infested 

 stems and leaves. (5) " When cut sets are used at plant- 

 ing, the cut surface should perhaps be allowed to heal or 

 dry before planting ; or, if this is not convenient, the cut 

 surfaces might be quickly passed over a hot iron." (6) 

 "Potatoes should be stored in perfectly dry, airy places, 

 in positions where light is not entirely excluded. Pota- 

 toes should never on any account be stored in heaps, or 

 in the damp holes in the ground termed 'pies ' " (Smith). 

 (7) The " earthing-up " of potatoes has a tendency to 

 lessen the attacks of Peronospora infestans. 



Cure. — (1) The author has shown {Journal Chemical 

 Society, 1886, p. 119, and Chemical Neivs, vol. 53, p. 255) 

 that iron sulphate destroys P. infestans, by acting upon 

 the cellulose walls of the mycelium of this fungus, but 

 does not injure the cellulose of the host-plants.^ A good 



' The celluloBe of parasitic fungi is most probably an isomeric 



