xvi.] WHITE-RUST DISEASE OF CABBAGES, ETC. 93 



Cruciferous plants in the spring owe their infection 

 with white rust to the zoospores germinating upon the 

 seed-leaves. No one, of course, has seen such extremely 

 minute objects as zoospores with the unaided eye, so no 

 one has ever seen them naturally transferred from the 

 germinating oospores on the wet ground, to the young 

 seed-leaves of cabbages, cauliflowers, and other cruciferous 

 plants. It is said that the zoospores cannot effectually 

 , germinate and form mycelium upon and in leaves and 

 stems of cruciferous plants unless the latter are very 

 young. But as cruciferous weeds infected with Cystopus 

 are extremely common, the oospores must occur in pro- 

 fusion in all districts every spring. No doubt the little 

 motile zoospores are carried through moist air by currents 

 of wind, and distributed in every direction throughout 

 the country. 



Alternation of crops must tend to diminish white rust. 

 Cabbages, cauliflowers, etc., should not be grown for two 

 years in succession where white rust has prevailed. 

 Cruciferous weeds should be gathered together and burnt, 

 especially when they exhibit the well-known white 

 sprinkling of the white-rust fungus. No cabbage, cauli- 

 flower, turnip, or mangel refuse should be allowed to 

 remain in a decaying state throughout the winter in the 

 fields, for in those positions not only the white-rust 

 fungus, but the putrefactive mildew of the cabbage tribe 

 and the fungus of club-root hibernate. Clean and intelli- 

 gent farming will greatly reduce the attacks of these two, 

 as well as of many other pests. 



