230 DISEASES OF CULTIVATED PLANTS 



White root rot. This disease is very prevalent in vine- 

 yards, orchards, etc., on the Continent, but is fortunately 

 somewhat rare in this country. It is caused by Rosellinia 

 necatrix. Prill, and Del. ( Dematophora necatrix, Hedwig). 

 One of the marked peculiarities of this pest is its power of 

 becoming parasitic on a great variety of plants belonging to 

 widely separated Orders ; in fact, it may be stated broadly 

 ttiat it attacks every plant with which it comes in contact. 



Hartig enumerates the following as having been destroyed 

 by the fungus : vines, fruit-trees, potatoes, beans, beet, young 

 maples, oaks, beeches, pines and spruces. As in Armillaria 

 melted^ the mycelium of the fungus under consideration 

 travels underground, and when it comes in contact with the 

 rootlets of a plant it kills them, and gradually works its way 

 into the tissues of the larger branches of the root. In the 

 case of large plants, the mycelium, after travelling along the 

 tissues of the root up to the base of the trunk, bursts 

 through the cortex in the form of a snow-white, fluffy 

 mycelium, which again traverses the ground and spreads 

 until it comes in contact with another root. During the 

 progress of the disease numerous minute sclerotia are 

 formed, in the cortex of the diseased roots ; and if such 

 roots happen to be exposed to the air, these sclerotia burst 

 through the bark and give origin to groups of minute 

 bristle-like, dark-coloured conidiophores which bear numerous 

 conidia at their tufted tips. 



A second kind of fructification sometimes occurs on 

 decaying roots, under the form of minute black conceptacles, 

 or pycnidia, As previously stated, the mycelium is at first snow- 

 white, but the older and exposed portions soon change to a 

 smoky brown colour, and develop pear-shaped swellings at 

 intervals throughout their length. According to Viala these 

 swellings under certain conditions gradually become globose, 

 and are capable of emitting mycelium which forms a new 

 plant. 



The highest or ascigerous stage of fruit has been dis- 

 covered by Viala, appearing only on trees that have been 

 dead for a long time and much decayed. 



Owing to the subterranean habit of the fungus, spraying is 

 out of the question, as is also a cure when the mycelium is 

 once established in the roots. The soil should be well 

 drained, as the fungus thrives best in sodden soil. If the 

 disease appears, affected plants should be isolated by digging 



