THE PREVALENCE OF FOMES ANNOSUS. I 1 5 



4. Douglas fir in America and Scots pine in this country 



occur naturally on glacial sands and gravels where 

 both species utilise mycorrhiza to a large extent. 

 Although the soils are frequently acidic, it is sug- 

 gested that the symbiosis enables the trees to with- 

 stand the acids. Both species are at times badly 

 attacked on the more fertile clays and clay-loams. 

 In such badly aerated localities, which are unfavour- 

 able to the mycorrhiza, they would have to rely more 

 on their own root-hairs, so that they would probably 

 be more easily damaged by the soil acids. ^ 



5. On soils of a more or less impermeable nature, it is quite 



common to find that groups of trees around old stumps 

 of a former crop, are badly attacked. This seems to 

 be due, not merely to infection coming from the old 

 stumps but also to a local acidity in the soil caused 

 by the decay of these stumps. The nature of the soil, 

 too, hinders the removal of the acids. 



6. On all types of soil, where there is an over-accumulation 



of dead vegetable matter, the fungus is likely to be 

 present. This is especially the case on soils from 

 acid igneous rocks, such as granite, where the bases 

 have been leached out, as in the glacial sand and 

 gravels of north-east Scotland. There the Scots pine 

 seems to be less liable to attack than larch or spruce, 

 especially on poorer soils where it might have the 

 assistance of mycorrhiza. 



7. It is very striking how heart-rot seems to appear in young 



and middle-aged woods some time after the canopy has 

 been closed. There is no doubt that the practice of 

 very close planting, combined with a policy of no 

 thinning, has resulted in an excess of raw humus, 

 rendering the soil sour and so damaging the rootlets 

 and paving the way for attack by the fungus. 



Although the economic damage done is of the greatest 

 importance, there is a silvicultural aspect which is equally 

 important. In many young woods, especially where there is 



^ Scots pine is very often attacked in this country, but the great production 

 of resin at the collar confines the decay to the roots. Thus groups of Scots 

 pine may be seen quite dead, although the parts above ground all appear to 

 be free from disease. The roots, however, have been destroyed by root fungi. 



