Family Cruciferae 211 



With the maturity of the sporangium its cell wall 

 dissolves, liberating the swarm spores which swim 

 about for a time, then come to rest, round up, and 

 increase in size and germinate by sending out a germ 

 tube. Oospores are produced in a fashion somewhat 

 similar to Pythium. The oogonia are formed as 

 terminal spherical bodies. The antheridium de- 

 velops terminally, lying close to the oogonium. 

 The content of the antheridium is emptied into the 

 body of the oogonium (fig. 35 d) and fertilization is 

 effected. The mature oospore (fig. 35 e) is spherical 

 with a thick smooth or undulated wall and germin- 

 ates by means of a germ tube. 



Control. When the disease is present on a large 

 scale, it is useless to attempt to control it. The 

 fungus, as we have seen, is a soil parasite, hence soil 

 treatments discussed on p. 53 could not be consid- 

 ered on a large scale. As far as we know, this disease 

 attacks only radish and beet seedlings. Badly in- 

 fected fields should be devoted to other crops for 

 several years until the parasite is starved out. On 

 a small scale, infected soils may be treated with 

 formaldehyde (see p. 53), or fire (see p. 56). 

 DOWNY MILDEW, see CABBAGE, p. 194. 



WHITE RUST 

 Caused by Cystopus candidus (Pers.) Lev. 



The damage caused by white rust depends largely 

 on seasonal conditions. The disease is most preva- 



