ROOT CROPS. 



195 



Before the rapid killing of the potato plant takes place the fungus grows 

 in the form of fine filaments or hyphse, which thread their way chiefly 

 between the cells, pressing closely against the walls, but frequently pushing 

 them in and sometimes penetrating them. It has been shown also that they 

 may send special absorbing organs (haustoria) or short branches into the 

 cells. These enable the fungus to absorb its nourishment more readily from 

 the cell contents, and are known to occur in any parts — the stems, leaves, or 

 tubers, though not constantly. 



The fungus may thus be said to pass through an incubation period, after 

 which it prepares for reproducing itself. Through the openings (stomata) in 

 the leaves on the under surface, it sends out branching filaments, called 

 conidiophores or spore bearers (Fig. 3), on the ends of which its summer 

 spores are produced. A spore is formed first at the end of a branch, 



Sections ot Potatoes affected by Late or Irish Blight (Ptitilophthora infestans). 



but later becomes pushed as : de, and remains for a time attached by a 

 slender stalk, while the branch grows on and produces a second spore. 

 This process continues many times, a new spore being formed at the end, 

 and a swelling marking the place where each previous one has been borne. 

 These branching threads and their spores form the downy mildew, best seen 

 near the advancing edge of the brown spots of the leaves. The production 

 and further development of these spores depend, as already indicated, on 

 weather conditions. 



Should the weather become dry the spores will soon perish and the 

 development of the fungus and the spreading of the disease will be checked. 

 It will thus be understood why some growers attribute the disease to the 

 weather alone, and do not recognise the fungus as the cause, and so fail to 

 take satisfactory measures to prevent the outbreak of Late Blight. 



