322 



The Supplement to the Tropical Agriculturist 



layer which underlies the cork is usually 

 green, and the laticiferous layer is white, yel- 

 lowish, or clear red. Consequently, when 

 healthy bark is scraped, the first living layer 

 met with is green, and if this is cut away the 

 inner layers are found to be white, or clear 

 red, or mottled red and white. But when can- 

 kered bark is scraped, the layer immediately 

 beneath the cork is black ; and if that is cut ofl 

 the laticiferous cortex is seen to be a dirty red, 

 which changes rapidly to dirty claret-coloured. 

 Usually the discoloured patch is surrounded by 

 a black line, and it is often mottled with black. 

 I have here a painting which shows charac- 

 teristic Hevea canker in its fully developed 

 form after the outer layers of the cortex have 

 been cut away. There is an earlier stage than 

 this, in which the diseased cortex is a greyish 

 yellow colour and appears sodden. These 

 patches are usually surrounded by a dark line as 

 before. Here is some possibility of making a 

 mistake over the earliest stage. There are 

 certain types of cortex which are greyish-yellow 

 and yet are not diseased. For example, there is 

 a smooth-barked tree which a 'yields very little 

 latex and has a greyisbyellow cortex which is 

 somewhat granular ; but as that type of cortex 

 occurs over the whole tree and yet the tree 

 continues to flourish, it cannot be due to canker. 

 There should not be any difficulty in dis- 

 tinguishing that type of cortex from early stages 

 of canker, if it is remembered that the 

 latter occurs in patches, clearly distinct from 

 the general internal colour of the cortex. The 



CLARET-COLOURED STAGE OF " CANKER " 



is unmistakable, and with a little practice the 

 earlier stage is equally readily recognised. There 

 is no long interval between the two stages : 

 when I say " earlier," I only mean earlier by a 

 few days. The disease begins in the outer layer 

 of the cortex and gradually penetrates to the 

 cambium. At the same time it spreads up and 

 down and round the tree. It travels more rapidly 

 up and down than laterally, and in some cases 

 forms a patch two or three feet in length on 

 one side of the tree only. The wood beneath 

 the canker patch is discoloured, but 'chiefly by 

 fungi which follow the canker fungus. In dis- 

 eases like this, the wood appears to be poisoned 

 by the products of decay of the cortex rather 

 than destroyed by the fungus which causes the 

 disease. When the cortex and the outer layers 

 of the wood are killed right round the tree, of 

 course it dies. The diseased cortex is moist and 

 has a peculiar smell. Boring beetles are often 

 attracted by it, and we receive numerous samples 



of supposed damage caused by borers which turn 

 out to be cases of canker. It is safe to say, on 

 our present knowledge, that all the borers found 

 in the stems of Hevea follow a previous attack 

 of fungus disease of some kind, usually canker 

 or pink disease. 



Cases of " canker," such as I have just 

 described, practically never show any cracks or 

 scales on the diseased patch. The surface is 

 quite smooth and unbroken, and the first thing 

 the planter notices is that a brown liquid 

 exudes, or the stem is attacked by borers. If 

 the " canker " occurs between or along the 

 tapping cuts, ho may notice that the latex 

 does not flow. No latex exudes from can- 

 kered bark, but, of course, that is not neces- 

 sarily a sign of canker. But a tree should 

 be examined for canker whenever the latex 

 does not flow, especially if it flows from one 

 or two cuts and not irom the others. Two 

 years ago, I should have concluded my descrip- 

 tion of canker at this point. But recently 

 other appearances of the disease have been 

 noted. In the cases just described — what I 

 should regard as the normal case — the disease, 

 if untreated, progresses until the tree is killed, ; 

 and the tree dies with its cortex unbroken. 

 But I have seen several cases lately in which the 

 tree has begun to heal itself : and in those 

 cases the appearance (that is, after the heal- 

 ing process has set in) is not the same as in 

 normal canker. From experience of Hevea in 

 cacao, I have always opposed the idea that 

 there could be any extensive self healing, but 

 from what I have seen this year I must ad- 

 mit that it can occur, at least in the Low- 

 country in a season like the present. Indeed 

 most of the canker found ab this date is in 

 course of healing. The course of events in these 

 cases runs as follows : — The fungus begins its 

 attack on the outer layers of the cortex in the 

 ordinary way. It produces the usual coloured 

 patch, and penetrates, say, to half the thickness 

 of the cortex. Then it stopi, and the tree begins 

 to cut out the diseased patch by a layer of cork 

 cells. While tho disease is advancing, the symp- 

 toms are just the same as those already des- 

 cribed,— the diseased patch is claret-coloured 

 and soft. But when the tree begins to cut out 

 the dead tissue, the appearance changes. 



THE DEAD PART DRIES UP AND BECOMES BROWN 



internally, and it forms a scale which can be 

 easily removed. The inner side of the scale is 

 usually moist and covered with a soft white 

 layer. That layer is not fungus, but newly- 

 formed cork cells. Beneath the scale, the cortex 

 is healthy and yields latex, but its surface is 



