176 PARASITIC FUNGI 



leaf (Fig. 18, F). Minute white bodies can sometimes 

 be seen on the threads. These are the conidia, like 

 those of Pythium, cut off from the ends of the hyphae 

 (Fig. 18, E, F, A). They are formed in great abundance 

 in damp air and are easily carried by the wind from 

 one plant to another. In water, for instance in a 

 raindrop or dewdrop on a leaf, the contents of the 

 conidium break up into zoospores (Fig. 18, B, C), as 

 in Pythium, and each of these eventually germinates 

 to form a hypha (D), which infects the leaf by 

 growing in through a stoma or by piercing the cuticle. 

 The conidia may also be carried down by rain into 

 the soil, where they infect the young potato tubers 

 which are growing beneath the surface. Whether 

 the fungus can survive the winter in the soil and rein- 

 feet the seed tubers in the spring is not certainly 

 known. One means of transmission of the disease 

 from season to season is by mycelium carried over in 

 certain of the tubers used for " seed." Such tubers 

 are not sufficiently diseased to be detected, but in 

 the early summer some of them may give rise to 

 diseased shoots on which air-borne conidia are formed. 

 These act as centres of infection to surrounding plants. 

 Phytophthora appears on the leaves of the potato 

 plant in the damp south-west, for instance in southern 

 Ireland, Cornwall and the Isle of Wight, as early as 

 the end of May. From these regions it develops suc- 

 cessively north-eastwards across the country. It usually 

 appears in Cambridgeshire and Lincolnshire for instance 

 great potato-growing counties about the end of 

 July or the beginning of August, but its spread and 

 the severity of the disease depend very largely on 

 the weather. A warm, nearly saturated atmosphere, 

 with constant south-westerly winds, are the conditions 



