36 MICROBES, FERMENTS, AND MOULDS. 
oidium. 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 hyphe 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, vertical section of a leaf, bearing tufts of Peronospo a viticola 
on its lower surface; b, a withered leaf, bearing the winter spores (oospo:es) 
(x 20 diam.). 
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 
