112 HEART-ROT 



4. Can trees be infected through growing on infected 

 soil? 



In order to infect the trees they were lifted from the 

 ground and their roots were washed in running water. 

 Pieces of infected roots, or lumps of infected soil, taken 

 from pure cultures were tied to roots of the trees by means 

 of bast. Infection points were tied round with sterilized 

 moss, and the trees were carefully potted in suitable soil. 

 On March 30, 1915, twenty-four infections were made with- 

 out wounding, eighteen with slight scalpel wounds into the 

 phloem, eighteen on roots still attached to the trees which 

 had been placed in nearly boiling water for one minute in 

 order to kill them, whilst the other roots of the trees remained 

 alive, and twelve on roots which were both boiled and 

 wounded. 



None of the infections were effective where there was no 

 wound, whereas all those where the fungus had touched 

 a wound were successful. In these successful infections the 

 mycelium grew into the wood and usually reached the 

 centre of the root. The hyphae grew both upwards and 

 downwards in the wood and bark, killing and browning the 

 tissues on their way. Growth was very slow, but rather 

 more rapid in the wood than in the phloem, where it was 

 always hindered by cork layers which were made across the 

 phloem and cortex as an obstruction. The extreme slowness 

 of growth is shown by the fact that in most cases the 

 mycelium had only spread 5-6 mm. upwards and downwards 

 in the wood and about 1 mm. less in the phloem in six months. 



Infections on killed roots were invariably successful. In 

 five weeks the mycelium had spread to one and a half inches 

 in the wood, and since infections were mostly made on the 

 larger roots the hyphae soon spread to the stem and killed 

 the tree. With some of the trees, whose experimental roots 

 had been killed, I was doubtful whether enough healthy 

 roots had been left to supply water during the summer 

 months. On this account I cannot be certain that death 

 was always brought about by the fungus. But some were 

 examined as soon as the trees began to look unhealthy, 



