36 MICROBES, FERMENTS, AND MOULDS. 



oi'dium. Brown patches appear on the upper surface 

 of the leaf, as if it had been scorched ; and in corre- 

 spondence with these there is a delicate down " like 

 the Whiteness of a slight hoar-frost " (Vaissier) on its 

 lower surface. The hyphse issuing from the mycelium 

 ramify at right angles, and these branches bear the 

 spores, as in the potato- fungus, Peronospora infestans 

 (Figs. 17, 18). These numerous spores, diffused through 

 the air, are powerful sources of contagion. 



Fig. 17. Mildew a, veitical section of a leaf, bearing tufts of Peronospo-a viticola 

 on its lower surface; b, a withered leaf, bearing the winter spores (oospuies) 

 (x 20diaui.). 



This parasite destroys the tissue of the leaf, 

 exhausts it, and finally causes it to wither and fall. 

 Those which are least affected have only diseased 

 patches. The bunch of grapes and the young 

 herbaceous shoots are rarely affected. 



In addition to the ordinary or summer spores of 

 which we have spoken, the sexual spores must be 

 noted ; the oospores, or dormant winter spores, which 



