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. 



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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. 



