60 PLANT DISEASES 



tion of dense cushions of conidia produced in chains 

 beneath the epidermis; finally the conidia become free, 

 rupture the epidermis, and appear on the surface as a fine 

 white powder. On germination the conidia produce 

 zoospores which eventually germinate, and enter into the 

 tissue of seedlings through the stomata. 



./'\ 



Fig. b. Cystopus candidus. i, diseased leaf of cabbage; 2, 

 chains of conidia, x 350 ; 3, conidium germinating and producing 

 zoospores, x 300 ; 4, ripe oospore, x 300. 



Oospores are only produced by the fungus parasitic 

 on shepherd's-purse when parts of the flower are 

 attacked. 



After passing the winter in a resting condition, the 

 oospores liberate enormous quantities of zoospores, which 

 attack seedling crucifers. 



