'the potato disease' 



699 



Although it has often been customary to associate most tuber 

 destruction with the same cause as that which destroys the 

 leaves and stems, it is certain that this view is in many cases 

 incorrect if not in all. 



Cause. — The fungus causing the disease is known as Phyto- 

 phthora infestans de Bary. The silky bloom seen around the 

 dead patches on the underside of the leaves consists of the 

 branched hypha: of the parasite. The unseptate mycelium 

 permeates the tissues of the leaf and feeds upon the substances 

 therein. In dry weather it vegetates within 

 the leaf and does not produce reproductive 

 organs, but in damp weather the mycelium 

 gives rise to branched hyphae which make 

 their exit chiefly through the stomata of 

 the leaves (Fig. 234). Upon the tips of 

 these hyphje, sporangia, and conidia are 

 borne singly, which, after reaching their 

 adult size, drop oif or are pushed aside 

 by the growth of a portion of the hypha 

 immediately below them. Each sporangium 

 is of oval form with a colourless membrane f.g. 234.-Hyph^of/'^j;<«. 

 (c. Fig. 235); when kept in water for an 4^/i",T sS^mtrcfT^S 

 hour or two the protoplasm within divides ^=„^J ^conWk™^ dniaS 

 into 5 or 10 zoospores which escape from about 100 diameters.) 

 the apex of the sporangium (Z?, Fig. 235). The zoospores {z) are 

 provided with two cilia by means of which they are able to 

 swim about in rain-drops or dew-drops on the leaves of the 

 plants After swimming for a few minutes they lose their cilia, 

 round themselves off {a), and soon develop a thin germ-tube 

 (t) which penetrates into any potato-leaf on which they may 

 occur ; once inside the tissues of the leaf, the germ-tube develops 

 into a new mycelium. 



The conidia are in all respects similar to the sporangium in 

 shape and size, but they produce germ-tubes directly (C, Fig. 235). 



