PAEASITIO FUNGI AND MOULDS. 33 



O'idium. — Oidium, or Urysiphe 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: Phylloooera vastatrix. 



The oidium, the white disease or meunier, was 

 equally destructive ia 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. 13). These 

 spores escape, as fine dust, difiuse 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 hyphss, and 



