PLATE XH— POTATO-DISEASE FUNGUS (Phytophthora infestans). 



(Principally after De Bar;/.) 



The Potato-disease Fungus was formerly known as Peronospora, but the fuller investigation of its history has caused it to be placed 

 in the genus Phytophthora. Although its life-history has been traced to a certain extent, yet, as in the case of Rust of Wheat and 

 other parasitic fungi, a satisfactory mode of dealing with the disease has not yet been found. 



It is but fair to add that some consider this fungus-growth as a consequence and not as a cause of the disease. Ihey maintain 

 that a fungus cannot establish itself upon a living plant, until that plant has become enfeebled; and further, that before any appearance 

 of the disease in the potato could be detected by the eye or microscope, it was possible to reveal it by a simple chemical test This 

 was done by using a minute borer and taking a thread of the potato bored out and placing it in a flask with milk in a warm closet If 

 the milk curdled in a very short time, the potato was found to be diseased; and if healthy, no curdling took place. The diseased potato 

 soon showed signs of decay and of premature germination, and so the actual disease is supposed to be antecedent to the appearance 

 of the fungus. 



Fig. 1. Diseased Leaf of Potato. 



The infected parts of the leaf turn black. 



FigS. 2 and 3. Hypha bearing Stylo-gonidia. .... . 



The filament bores its way through the tissues of the plant, absorbs and appropriates their nutriment, and 

 gradually traverses the whole plant Eventually it puts forth hypha? through the stomata of the leaf, which branch 

 and bear capsules styled Stylo-gonidia. 

 Figs. 4, 5, and 6. The contents of the Stylo-gonidium break up into separate portions (usually six), which escape by rupturing 



the wall. 

 Fig. 7. Each Zoogonidium possess a pair of cilia, and through the medium of rain or dew may find their way from one plant 

 to another and thus infect a whole field. 



Fig. 8. Zoogonidium germinating. . ... ., , , , 



The inner membrane protrudes as a filament, penetrating the epidermis, and begins to ramify through the 



underlying tissue. 



Life History— The fungus traversing the potato-plant bears aerial hyphae with Stylo-gonidia, the contents of which break up into 

 Zoogonidia. These motile Zoogonidia, on reaching the epidermis of a potato-plant, germinate to form a uncellular filament 

 which branches among the tissues and becomes like the parent-form. This is the non-sexual mode of multiplication, but a 

 sexual process has not yet been observed. 



