HEART-ROT 121 



operating which might account for the death of some of the 

 roots before they became attacked by the fungus ? Also 

 what is the frequency of heart-rot in conifers planted on 

 heaths ? Where the soil is peaty the aeration of the sub- 

 soil must be very poor, and heart-rot might be expected. 

 I have had no opportunity of investigating these points, 

 and foresters' notes on the subject would be of great value. 



Extreme caution is necessary when attempting to prophesy 

 the effect of different types of subsoil on the frequency of 

 heart-rot. In one part of Terringham Wood the subsoil is 

 a hard strong clay, which must be far inferior in porosity 

 to that in other parts of the wood. And yet in this portion 

 several of the larch trees were sound. I thought that 

 this must be a case in which the theory failed, until I 

 had one of the trees grubbed up and found that the 

 roots were all superficial. The subsoil had proved too 

 hard for them, and consequently the tree had been saved 

 from infection. 



Two points of interest are involved in this theory of the 

 method of infection. First, the roots which grow vertically 

 downwards are not those which are most- likely to come in 

 contact with the diseased roots of other trees, so that 

 Hartig's contact theory can hardly apply. It seems much 

 more probable that the roots become infected either directly 

 by spores and conidia or through the soil being penetrated 

 by mycelium. The other point is the depth of the soil at 

 which infection takes place. When a root is killed because 

 its lower extremities lack sufficient oxygen, it does not 

 necessarily follow that only those lower extremities die. 

 Unless the upper part of the root has sufficient vigour to 

 send out branch roots into the upper layer of soil (and I have 

 never noticed such branches in roots with primary rot), 

 the upper part will cease to receive the full supply of water 

 and nutriment, and will probably die. In one case 1 found 

 a layer of resin across the wood of a vertical root with 

 primary rot at a point less than a foot below the surface of 

 the soil, in fact quite near the rootstock, and this probably 

 represented the upper limit of death before infection. 



