FUNGI: DOWNY MILDEWS. 



I8 5 



Downy Mildews.^ 1 



397. The downy mildews make up a group of plants which are closely 

 related to the water moulds, but they are parasitic on land plants, and some 

 species produce very serious diseases. The mycelium grows between the 



Fig. 204. 



Fertilization in Peronospora alsinearum; tube from an theridium carrying in the 

 sperm nucleus in figure at the left, female nucleus near; fusion of the two nuclei 

 shown in the two other figures. (After Berlese.) 



cells of the leaves, stems, etc., of their hosts, and sends haustoria into the 

 cells to take up nutriment. Gonidia are formed on threads which grow 

 through the stomates to the out- 

 side and branch as shown in figs. 

 198-201. The gonidia are borne 

 on the tips of the branches. The 

 kind of branching bears some re- 

 lation to the different genera. 

 Fig. 200 is from Peronospora 

 alsinearum on leaves of ceras- 

 tium; figs. 197 and 199 arePlas- 

 mopara viticola, the grape mil- 

 dew, while figs. 198 and 202 are 

 from Phytophthora infestans 

 which causes a disease known as 

 potato blight. The gonidia of 

 peronospora germinate by a germ 



Fig. 205. 

 Ripe oospore of Peronospora alsinearum. 



tube, those of plasmopara first 



form zoogonidia, while in phy- 



tophthora the gonidium may either germinate forming a thread, or each 



gonidium may first form several zoogonidia, as shown in fig. 20^. 



398. In sexual reproduction oogonia and antheridia are developed on the 

 mycelium within the tissues. Fig. 204 represents the antheridium enter- 



