224 PHYTOPHTHORA SP. ON HEVEA BRASILIENSIS 



does not cause a true canker on Hevea ; the diseased bark is usually smooth 

 and outwardly appears to be sound ; the fungus forms no open wounds and 

 therefore he considers the popular name canker to be a misnomer. Again 

 a cankered tree may be killed and the bark destroyed "without the occurrence 

 of any roughness or open wounds." " Black Thread " causes a true canker ; the 

 diseased renewing bark in advanced cases completely decays and falls off 

 leaving a big gaping wound and in the case of a minor attack the renewal of 

 the bark is rough with vertical swollen ridges of wound tissue. However 

 serious be the attack of " Black Thread," it has not been known to kill the 

 tree. Hevea canker is found even when the stem has acquired a thick outer 

 brown bark while " Black Thread " is confined on the stem solely to the 

 tapping surface. In the case of Hevea canker, therefore, there is generally no 

 outward indication of the disease while the presence of the Burma disease can 

 always be spotted immediately on its appearance. In some cases, however, 

 the cankered bark exudes a reddish or purplish liquid, which gives an outward 

 symptom of the diseased condition of the bark : in the Burma disease no 

 such discoloured exudation has been observed, but there is an exudation of 

 white latex from the vertical cracks on the diseased parts. From Fetch's 

 accounts Hevea canker does not appear to be a wound parasite on the stem, 

 but " Black Thread " is, as is shown by inoculation experiments and by the 

 characters of this disease in nature. 



During the prolonged rains of 1909 and 1910 Petch^ found in Ceylon the 

 decay of the renewing bark on the tapped surfaces. The decay was first 

 indicated by the appearance of vertical black lines, just above the tapping cut. 

 This decay was found to extend to the wood ; the bark on the diseased area 

 rotted and left long narrow wounds extending to the wood. The progress of 

 the decay stopped when the rains ceased and the wounds healed up but the 

 renewal of the bark was rough with vertical swollen ridges of wound tissue. 

 This description by Fetch of the disease of the tapping surface in Ceylon very 

 much resembles that of " Black Thread " of Burma; but there is one point 

 of minor difference ; according to Fetch in some cases the decay travelled 

 downwards and involved the untapped bark ; Rutgers^ also has found in Java 

 that the disease penetrates downward in the old bark ; in Burma, however, 

 " Black Thread " has never been observed to involve the untaj^ped bark. 

 Fetch considers the decay of the renewing bark on the tapped surface due 



1 Fetch, T. Cacao and Hevea Canker. Circs. & Agr. Jour. Roy. Bot. Card., Ceylon, V, 

 No. 13, 1910, p. 166. 



* Rutgers, A. A. L. and Arcns, P. Diseases of Hevea Brasiliensis in Java. Internationaal 

 Rubber-Congres met Tentoonstelling, Batavia, 1914, p. 5. 



