100 



FUNGOID DISEASES 



and with early sowing. It often occurs in company 

 with white rust. 



Symptoms. — i\ grey white mould appears on the 

 under side of the leaves ; the latter assume at the 

 same time a sickly appearance, shrivel up, and 

 when rain comes, rot with a strong characteristic 

 odour. 



Cattse. — Peronospora pm^asitica causes this dis- 

 ease. Its hyphse grow within the plant (compare 



true mildews) and draw nour- 

 ishment from the cell contents, 

 thereby preventing normal 

 development. Aerial hyphse 

 are sent out through the 

 stomata, which branch in a 

 tree-like manner, and form, 

 at the tips, colourless con- 

 idia (Fig. 29) which fall off 

 easily and are capable of im- 

 mediate ofermination. Rest- 

 ing spores (oospores) with a 

 thin yellowish outer wall are 

 formed, on the mycelium 

 within the plant, as a result of sexual union. These 

 develop the following spring. 



Preventive Measures. — On account of the wide- 

 spread nature of the fungus, little can be done. 



1. Keep down all cruciferous weeds. 



2. Sow swedes later in the dry warm districts, 



Fig. 29. — Conidiophore of 

 Pcronospora parasitica, 

 which causes fa se mildew 

 of swedes, growing 

 through a breathing pore ; 

 notice the detached con- 

 idia. (Magnified. ) (After 

 Frank.) 



