120 PLANT DISEASES 



ceptacles called pycnidia, which produce in their interior 

 numerous spore-like bodies called stylospores. 



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 swell- 

 ings, under certain conditions, gradually become globose, 

 and finally become free, and are capable of emitting a 

 mycelium which forms a new plant. 



Finally, the highest or ascigerous condition of fructifica- 

 tion has been discovered by Viala ; the perithecia are com- 

 paratively large, black, and surrounded by the bristlelike 

 conidiophores already described. This last form of fruit 

 is rare, appearing only on trees that have been dead for 

 a long time, and much decayed. 



Preventive Means. — 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 places that are sodden with stagnant water. If the 

 disease appears, affected plants should be isolated by 

 digging a narrow trench about a foot deep round the 

 trunk at such a distance as to include the main roots ; and 

 if roots are cut through, they should be followed and 

 removed. All the removed soil should be thrown inside 

 the trench. Trees that have been killed should be re- 

 moved, and special care should be exercised in removing 

 the stumps to get out all the roots possible, as if these are 

 allowed to remain they will form centres of infection from 

 which the mycelium will rapidly spread. 



Weeds growing near to infected plants should also be 

 removed and burned. A method that has proved benefi- 



