Plant Sanitation. 



522 



[December, 1909. 



,1 angle tree. It is known to occur on 

 rotting logs in up-country jungles, and 

 it has been found on the roots of Croton 

 lacciferum (Keppitiya), but in the latter 

 case it had attacked the plant in the 

 same way as it attacks tea ; the Croton 

 cannot at present be regarded as the 

 starting point of the disease. It would 

 be of assistance if, when this disease has 

 been identified on the estate, the planter 

 would collect and forward all the fungi 

 found on the jungle stump nearest to the 

 diseased bushes, as well as some of its 

 decaying roots- Every effort should be 

 made to get rid of this disease in the 

 new clearing ; it has proved extremely 

 difficult to eradicate in one instance, and 

 has taken an annual toll of the bushes 

 for several years. 



It is well-known that the stumps of 

 Grevilleas afford a starting point for 

 our commonest tea root disease, caused 

 by Ustulina zonata, and it is by no 

 means rare to find groups of half a dozen 

 dead tea bushes round a large percent- 

 age of these stumps on up-country 

 estates. It has now been demonstrated 

 that Albizzia stumps also become centres 

 of the same disease ; and where large 

 Albizzias are felled a similar loss of tea 

 bushes may be expected. This propa- 

 gation of root disease is practically 

 inevitable, so long as large trees which 

 must be afterwards felled are planted 

 among tea. In this respect Albizzias are 

 worse than Grevilleas, because of the 

 larger area occupied by the roots and 

 base of the stem. The average Grevillea 

 stump can be extracted with compara- 

 tively little labour, but one would 

 hesitate before advising the extraction 

 of Albizzia stumps. Of course, the 

 decay of stumps is brought about by 

 fungi in all cases, and it is a matter of 

 chance whether the fungi which settle 

 on any individual stump are harmless 

 or injurious species. It is therefore 

 quite possible to fell either Grevilleas 

 or Albizzias without any subsequent 

 development of root disease. But the 

 risk must be borne in mind, and either 

 tree should be up-rooted where practic- 

 able. Some day we may arrive at the 

 conclusion that it is unwise to plant, 

 among tea, species which will grow into 

 huge trees which must be felled later, 

 or at least that it is unwise to allow 

 them to grow so large. It may be noted 

 that the root disease which develops 

 from Grevillea and Albizzia stumps has 

 not been known to attack Hevea. 



The original "canker" of Hevea has 

 not been much in evidence during the 



last three or four years, but several 

 cases were notified during the prolonged 

 rainy season of this year, and the subject 

 is being reinvestigated. It has been 

 determined that in many cases, probably 

 in the majority of cases, the death of 

 the bark in patches after the tree has 

 been scraped before tapping is due to 

 " canker," sometimes assisted by deeper 

 scraping than should be allowed. But 

 the most general symptoms exhibited 

 during the current year differ com- 

 pletely from those previously recorded. 

 The disease attacked the renewing bark 

 on the surface which was being tapped. 

 The bark showed numerous vertical 

 black lines, and on cutting it out these 

 lines were found to extend into the 

 wood. These black Jines may be found 

 on the cambium before they are evident 

 externally. The bark round these lines 

 decays, leaving a narrow vertical wound. 

 Sometimes adjacant patches coalesce, 

 and the whole of the renewing bark 

 decays. In most cases the disease does 

 not extend downwards as rapidly as the 

 bark is excised during tapping, and it 

 is therefore possible to continue tapping 

 although the tree is diseased. With the 

 advent of drier weather the disease 

 stops and the bark renews over the 

 wounds, but as it has to grow in from 

 the edges of the vertical wounds, the 

 renewed, bark is rough. Except for this 

 rough bark, there is no permanent 

 injury to the tree. 



The cause of Hevea canker cannot be 

 said to have been determined. Four 

 organisms have been found fairly re- 

 gularly in the diseased tissue. Two of 

 these are in all probability only sapro- 

 phytic and are not being considered at 

 present ; the other two are a bacterium 

 and a Nectria. The evidence of the 

 diseased tissue seems to point to the 

 bacterium as the cause, since it is always 

 found in advance of the hyphae of the 

 Nectria, and the discolouration of the 

 wood is identical with that in the bac- 

 terical cultures. But inoculations with 

 both these organisms have so far been 

 unsuccessful. The bacterium has been 

 isolated and grown in pure cultures, but 

 an attempt to produce "canker" by 

 inoculating tapped surfaces with bac- 

 teria from these cultures has proved a 

 failure up to the present. In the case 

 of the Nectria, the ripe spores were 

 caught as they were ejected from the 

 fructification, and these were proved 

 capable of germination by sowing them 

 in culture solutions ; but no success has 

 yet followed the inoculation with these 

 spores. 



