RUBBER 603 



canker, and there appears to be no doubt that reinfection 

 of the stems takes place largely by means of spores from 

 the diseased pods. 



The external signs of canker on the stem are at first 

 very slight; the bark may be a little darker over the 

 diseased spot. In more advanced stages a red-brown 

 liquid may exude, which dries in small streaks on the 

 bark. But the disease is unmistakeable as soon as the 

 bark is scraped. Then, instead of a green layer under- 

 lying the corky bark, one meets with a black layer, and 

 if that is cut away the cortex, instead of being white, or 

 yellowish, or cleared, is found to be a dirty red, which 

 rapidly darkens to claret colour. If the whole of the 

 diseased cortex be laid bare it will usually be found to 

 be bordered by a black line. There is an earlier stage 

 than this, in which the diseased cortex is not claret- 

 coloured. When first attacked it is greyish-yellow, and 

 appears sodden. But even in this stage it is bordered 

 by a black line, and the layer immediately beneath the 

 outer brown bark is black. 



Cankered bark has a peculiar smell which soon attracts 

 boring beetles, particularly a small brown beetle about 

 the size of a shot-hole borer. Numerous specimens of 

 damage to Hevea supposed to be caused by borers are 

 sent in for examination, but in practically all cases it 

 turns out that the tree has been first attacked by either 

 canker o<r pink disease. 



The disease begins in the outer layer of the cortex and 

 gradually penetrates to the cambium, at the same time 

 extending up and down and round the stem. It may 

 penetrate completely through the cortex, and spread so 

 rapidly that the tree is dead in a few weeks. In such a 

 case the bark is usually smooth and unbroken, and does 

 not exhibit any of the phenomena which pass under the 

 name of canker in other trees. There is generally a 

 copious exudation of the brown liquid, and the tree is 

 soon riddled by borers. 



Canker at the collar of the tree is equally disastrous. 

 In that situation the disease has usually obtained a good 

 hold before it is discovered. It kills the cortex all round 

 the base of the stem, along the lateral roots, and down 

 the tap root. 



