PARASITIC FUNGI AND MOULDS. 33 
Oidium. — Oidium, or LHrysiphe Tuckeri — so 
called from the name of the vine-grower by whom 
it was first described—has been longest known to 
us among these parasitic fungi. It belongs to the 
group of Ascomycetes, and appears to have reached 
us from America in 1845, in which year it was first 
observed in England. Thence it passed over to 
France. In 1847 it was noticed in the neighbourhood 
of Paris; and afterwards, in 1850-1851, in the south 
of France, where for twenty-five or thirty years it 
raged with such intensity as to threaten for some 
years the almost complete destruction of the vine- 
yards, a destruction which is now taking place under 
the attacks of another parasite, belonging in this 
instance to the animal kingdom: Phylloxera vastatrix. 
The oidium, the white disease or mewnier, was 
equally destructive in the vineyards of Madeira, so 
that it was necessary to uproot all the vines, and 
replace them by sound plants which were incapable 
of bearing grapes for some years. 
The oidium appears on the grape in the form 
of greyish filaments, terminating in an enlarged head, 
which contains an agglomeration of spores, not free 
or in a chaplet, as in Aspergillus (Fig. 18). These 
spores escape as fine dust, diffuse themselves in the 
air,and spread the disease afar with extreme facility. 
If a spore lodges on a vine-leaf under favourable 
conditions of moisture and warmth, it soon germinates, 
penetrates the epidermis by means of its hyphe, and 
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