82 DISEASES OF TREES 



the sporophores that may be in process of development upon 

 them. 



In summer the mycelium that vegetates on the surface of the 

 ground produces gonidia on the whorled branches of gonidio- 

 phores, and these, by being carried on the skins of mice, &c., 

 may originate new centres of infection. But besides these, black 

 spheroid perithecia, about the size of a pin-head, are produced 

 either on the surface of the diseased oaks themselves or on 

 the surface of the ground in their neighbourhood (Fig. 27, &). 



It is probable that the spores which are formed in the pere^ 

 thecia do not as a rule germinate and reproduce the disease 

 till the following year. 



Generally speaking, it is only in wet years that the parasite 

 does much damage. It may be combated by digging trenches 

 round the diseased spots in the seed-bed so as to isolate them. 

 One should avoid transferring diseased plants from the seed- 

 bed to the plant-bed. 



Rhizoctonia violacea, which kills saffron and lucerne, has not 

 yet been scientifically examined in its different stages of develop- 

 ment, and it remains to be determined whether this parasitic 

 mycelium belongs to a form related to one of the foregoing fungi 

 or not. Fuckel's statement that this mycelial form belongs to the 

 fungus Byssothecium circinnans appears so utterly improbable 

 that it is not worth while to take further notice of it. On the 

 other hand, I feel called upon to describe here the following 

 important parasite of the vine. 



DEMATOPHORA NECATRIX.* THE VINE-ROOT FUNGUS 1 



Amongst the numerous enemies of the vine, the root-fungus, 

 D. necatrix, occupies a prominent position. The disease which it 

 induces is known as Wurzelpilz, Weinstockfaule, Pourridie de la 

 Vigne, Pourriture, Blanc des Racines, Blanquet, Champignon 

 blanc, .Aubernage, Mai nero, and Morbo bianco, and is dis- 



1 R. Hartig, Dematophora necatrix: n. sp., Untersuchungen aus dem 

 Forstbot. Institut zu Munchen, III., 1883. 



* [Viala has published a very thorough investigation of this disease and 

 the devastations it causes in the south of France, &c. : "Monographic dit 

 Pourridie des Vignes et des Arbres fruitiers," 1891. ED.] 



