114 GROW YOUR OWN VEGETABLES 



known as Phytophthora infestans, the main part of which 

 is to be found inside the attacked tissues of the potato 

 plant. The external mould is merely its reproductive 

 portion, developed by the parasite at the expense of the 

 nutrient material absorbed from the cells of the host 

 which it destroys. 



" The aerial spores of the fungus are borne on these 

 branched filaments and are exceedingly minute. They are 

 produced in vast numbers, and are scattered from one plant 

 to another and from field to field by wind or other agencies. 

 As every spore brought into contact with a damp potato leaf 

 is capable of starting a new centre of infection, it will be 

 readily understood that under favourable conditions the 

 disease spreads rapidly. The disease is seldom conspicuous 

 before the end of June or even the middle of July, although 

 careful search may reveal its presence on a small scale 

 somewhat earlier. The severity of the attack and the 

 rapidity of its spread is entirely dependent on the nature of 

 the season. In a dry season it may hardly be noticeable, 

 except to a careful observer, but in damp weather it spreads 

 readily. In a warm, damp July or August the disease often 

 spreads rapidly, and whole fields may be blackened com- 

 pletely in the course of a few days. In most seasons the 

 blight is prevalent to some extent in September and October, 

 and it usually causes somewhat premature decay of the 

 foliage. 



" The spores, which are produced chiefly on the under 

 sides of the leaves, fall to the ground, are washed down into 

 the soil by rain, and thus reach and subsequently infect 

 the tubers of the new crop. From the spores germ-tubes are 

 produced which penetrate the skin of the tuber, and from 

 them mycelium (spawn) develops within the tubers and 

 kills the cells. 



" A potato tuber infected with the blight fungus at first 



