298 Studies in Forestry [CHAP. xm. 



the canker. If the cankerous spot finally extend all round the 

 stem the tree dies off above the diseased ring, though otherwise 

 the struggle between the disease and the vitality of the stem 

 may often be continued for decades. But, in .large cankers, 

 the mycelium is apt to penetrate through the medullary rays 

 into the heart-wood of the stem, and to assume such propor- 

 tions as to interfere considerably with the circulation of the 

 sap, in which case the tree begins to sicken and die. 



Dry situations and warm sunny exposures are unfavourable 

 to the maturing of the sporophores ; whilst humid localities 

 favour the ripening of the spores. The disease is also of more 

 frequent occurrence in pure crops of Larch, and whenever the 

 canopy is close. The best preventive measures are therefore 

 to be found (i) in careful choice of the soil and situation most 

 suitable to Larch, and (2) in its growth in mixed forests only 

 along with broad-leaved trees, if possible, in preference to 

 other conifers. Remedial measures are practically confined to 

 the removal of all cankerous individual stems during the 

 operations of thinning. 



B. On Broad-leaved Trees: 



i . Nectria ditissima,Tu\., the Canker of the Beech. Cankerous 

 places on Beech-stems may be occasioned by tree-lice (Lachnus 

 and Chermes\ or by frost, or by sun-burn, or by fungoid 

 disease; but in the latter case the disease is recognizable 

 through the small white Gom'dm-pustules which first of all 

 appear, to be followed afterwards by numerous dark-red 

 globular sporophores on the cankerous spots. 



Although occurring also on Oak, Ash, Hornbeam, Alder, 

 Lime, Maple, and Sycamore, as well as on Apple and Cherry 

 trees in orchards, this disease chiefly attacks the Beech. It is 

 not confined to any particular period of the development of 

 the tree, being met with in mature crops, as well as in very 

 young thickets, and sometimes on the one-year-old shoots, or 

 sometimes on the bark of much older portions of the stem. 



