THE LARCH CANKER 65 



canker should be more frequent in lowlands than in the 

 larch's natural home in the Tyrol. Hartig adduced three 

 reasons to account for this difference. The first is that 

 when the larch is growing in its native home, where natural 

 regeneration takes place freely, young trees are always 

 springing up among the older ones, and forests are com- 

 posed of trees of all ages. But canker, as an epidemic, is 

 nearly confined to plantations under twenty years of age, 

 and trees which are older than this are generally free from 

 attack. So in the Tyrol the canker fungus can only pick 

 out here and there the younger trees from among a much 

 larger number of older ones. (This leaves out of account 

 the frequency of canker on side branches of older trees 

 which I have particularly noticed in the Tyrol.) The second 

 reason is that in the mountains the trees are not often 

 surrounded by damp stagnant air, which is conducive to 

 the formation of fructifications of the fungus and to the 

 germination of the spores. Lastly, the early spring in the 

 lowlands causes a premature renewal of vigour. The twigs 

 become full of sap and the needles begin to appear. But 

 too often this is followed by May frosts, which cut back the 

 shoots and make a number of frost -wounds on the young 

 twigs, which reduces the vitality of the trees. In the Alps 

 summer development does not begin so early, so that the 

 trees are less likely to be cut back by frosts. 



All these causes are no doubt operative to a limited 

 extent, but their importance must not be over-estimated. 

 Indeed, larch in its native habitat is by no means free from 

 canker, and as high as 5,000 ft. in the south Tyrol I have 

 found parts of trees almost riddled with the disease, though 

 not so much on the main trunks as on .the side branches. 

 Boden (1904) also comments on the fact that trees may be 

 cankered as high as 7,200 ft. in the Alps, whereas the 

 disease is often absent from plantations in the plains. In 

 parts of the Tyrol, which I have seen, the larches do not grow 

 in dense formation, but are sufficiently scattered to allow 

 the full development of the side branches. Of course many 

 branches must die, but not, as a rule, till they have grown 



1888 



