222 REPRODUCTION OF THE DOWNY MILDEWS 



does not appear until the latter part of July. It only becomes of 

 serious importance in sultry weather, when a short period of 

 rain will result in the formation of the sporangia and whole 

 fields will be devastated in a few days. 



Fig. 132. 



Fig. 133. 



Fig. 132. Asexual reproduction of the mildew: A, hyphae of Plasmo- 

 para emerging from a stoma and bearing numerous sporangia. B, enlarged 

 view of sporangium of Peronospora germinating on a dry leaf. In this case 

 the sporangium behaves as a spore sending out a hypha that will penetrate 

 the tissues of the leaf. C, sporangium of Phytophthora germinating in the 

 water and forming zoospores. D, zoospore enlarged. E, zoospore has come 

 to rest and is forming a tube that will penetrate the tissues of the leaf as 

 in the case of B. 



Fig. 133. Reproduction of the white rust, Albugo: A, asexual stage, 

 showing several erect hyphae forming spores. B, sexual stage which is 

 characteristic of the Peronosporales in general — 0, gametangium contain- 

 ing a single female gamete which is being penetrated by tube from male 

 gametangium, an. — After Wager. 



Very similar in character is the grape blight, Plasmopara viti- 

 col'a. This disease causes a browning of the leaves and the stunt- 

 ing of the fruit, which become brown or gray, and, finally, the 



