ACCIDENTS, DISEASES, PAKASITES. 



191 



subsoil, forming basins in which water remains stagnant., 

 are those where the disease develops with the greatest in- 

 tensity. Heat seems to play a secondary part. 



Pourridie" is due to the development of the roots of fungi 

 of different species ; Dematophora necatrix (R. Hartig)., 



Fig. 157. Dematophora Necatrix. 

 a, Mass of white filaments. 



Fijr. 158. Fructifications of 

 Dematophora A'ecatrix. 



Agaricus melleus (L.) and Roesleria hypogcea (Thum. anct 

 Pass). These often grow on parts of the tissues already 

 injured. 



Dematophora Necatrix. The roots of vines affected 

 by Dematophora necatrix (Figs. 154 to 158) show,, 

 between the bark and the wood, felty patches of mycelium 

 penetrating the medullary rays, or stuck against the bark. 

 If the conditions of humidity are suitable the fungus will de- 

 velop white fluffy filaments round the roots, assuming later 

 on a felt-like appearance penetrating the whole soil. These 

 filaments become mouse-grey, and, later on, brown. If the 

 surrounding soil is saturated with water, abundant fructifi- 

 cations appear (Fig. 159), giving a special velvety appearance 

 to the parts attacked. 



