POTATO BLIGHT 175 



passes to coalesce with that of the female. The fertilised 

 egg (zygote) now secretes a wall (c), and remains for a 

 time in a resting condition. Eventually it germinates, 

 forming zoospores in water (F), which germinate, and 

 if the hyphae of this find a suitable seedling they pene- 

 trate it through a stoma just above the ground level. 



The hyphae of Pythium can only grow in air which 

 is nearly saturated with water vapour, but these con- 

 ditions are often realised between the stems of the 

 seedlings growing in a seed-box which is frequently 

 watered. When a seedling has been killed the hyphae 

 grow out from it and easily bridge the space to another 

 seedling, which they enter and kill, and so on till the 

 whole crop quickly becomes a rotting mass. The 

 hyphae of the mycelium are easily visible as very 

 delicate white filaments, rather like cobweb, stretching 

 between the seedlings. 



Potato Blight (Phytophthora infestans). This is a 

 fungus closely allied to Pythium, and often causes 

 enormous damage to the potato crop. Potato blight 

 first appeared in Europe between 1845 and 1850 and 

 ruined the Irish crop, causing serious and widespread 

 famine. Since then it has always been with us, but 

 its incidence is much more severe in some years than 

 in others, according to the weather. 



The mycelium ramifies in the tissues of the leaf, 

 and the diseased patches turn brown as the tissues 

 die. Under conditions favourable for the spread of 

 the disease the whole potato haulm is soon reduced to a 

 rotting mass. If the under side of a potato leaf bear- 

 ing a patch of the disease is examined with a hand 

 lens, the edge of the patch is seen to be covered with 

 very delicate white glistening threads (the hyphae) 

 which have grown out through the stomata of the 



