6o 



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. 6. 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 not formed by the fungus parasitic on 

 shepherd's-purse, but are produced in considerable num- 

 bers in the tissues of many hosts. 



After passing the winter in a resting condition, the 

 oospores liberate enormous quantities of zoospores, which 

 attack seedling crucifers. 



