The diseases and pests of the rubber tree . outer layer decayed and the black linebecame evident externallv. Subsequently it extended upthe tapping surface, and broadened out laterally, so as toform a narrow vertical wound. The lines occurred in vary-ing numbers on the same tapping cut, but all parallel toone another (Fig. 13). If they were distant from oneanother, narrow yertical wounds resulted, but when theywere close together the adjacent wounds spread laterally 120 THE RUBBER TREE and coalesced, so that in some cases a continuous woundwas produced, wMcli extended right across the tappingc
The diseases and pests of the rubber tree . outer layer decayed and the black linebecame evident externallv. Subsequently it extended upthe tapping surface, and broadened out laterally, so as toform a narrow vertical wound. The lines occurred in vary-ing numbers on the same tapping cut, but all parallel toone another (Fig. 13). If they were distant from oneanother, narrow yertical wounds resulted, but when theywere close together the adjacent wounds spread laterally 120 THE RUBBER TREE and coalesced, so that in some cases a continuous woundwas produced, wMcli extended right across the tappingcut. If tapping was continued, the wounds followed thetapping cut downwards as fresh inner cortical tissues wereexposed, and in some cases the lines extended into the un-tapped cortex below the cut. When the rains ceased thedecay stopped. The resulting damage depends on the sizeof the wounds. The cortex is destroyed down to the wood,and the cambium is killed over the diseased areas. Newgrowth, therefore, can only take place from the edge of. Fig. 13.—Black thread (Oeylon): early stage. Natural size, the woimd, and hence each wound becomes bordered by aswollen callus. In many cases the consequence is a seriesof narrow parallel wounds which soon heal; but whereadjacent lines have coalesced a broad wound is formedwhich cannot be healed over for some years. It was notice-able, ia the earliest cases of this disease, that it was worston the tapping cuts which were newly opened during therains of the North-east Monsoon. No organisms capableof reproducing the disease were found in the decaying similar disease occurs in Burma, and has been investi-gated by Dastur, who describes it as follows. The naked ir PHYTOPHTHORA DISEASES 121 tissues laid bare by tapping become disfigured by the appear-ance of vertical, slightly depressed black lines. These followthe tapping cut as it is continued down the stem, and extendin through the cambium into the wood. The blackening ofthe tissues
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