210 REPRODUCTION OF THE DOWNY MILDEWS 



of entire vineyards before the method of kilHng the fungus by 

 spraying the plants with copper salts was discovered. In both 

 of these pests and in allied genera the sporangia-bearing hyphae 

 are produced in such numbers as to cause a downy, mould-like 

 appearance on the leaves, thus accounting for their popular name, 

 Downy Mildews. 



In a related genus, Albugo, the sporangia are formed by the 

 repeated cutting ofif of the tips of the hyphae, as shown in Fig. 



Fig. 132. 



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. 



