AND COLLEGE, PUSA, FOR 1915-16 49 



the numbers of " tokras " occurring in plots to which sodium 

 nitrate had been added were compared with the numbers 

 of " tokras " in plots which had not received any nitrate. A 

 difficulty which renders the results of the first season's work 

 on these lines somewhat inconclusive is that the number of 

 " tokras " appearing in a plot will depend, apart from any 

 influence of artificial manures, on the amount of " tokra ' : 

 seed which the plot contained. This factor appeared in 

 many cases to mask any effect which might have been due 

 to the addition of sodium nitrate. The work will be con- 

 tinued for another year in order to eliminate this source of 

 error but the results obtained to date do not lend any sup- 

 port to the view that sodium nitrate will be found a specific 

 remedy for " tokras." 



(3) Black thread of rubber. Work on the " black 

 thread ; disease of Hevea in Burma was continued. A 

 popular account was published as a bulletin of the Burma 

 Department of Agriculture and a more technical memoir 

 is in the press. The disease first breaks out soon after the 

 rains begin and completely disappears after the close of the 

 monsoon. It is not fatal to the tree but does much damage 

 by attacking the tapped area of the bark. Infected areas 

 do not yield latex and severely diseased trees have to be 

 excluded from the tapping round. In 1914 the loss of 

 rubber on one estate was 2 — 3,000 lb. and in 1915, 8 — 9,000 

 lb., there being 12,000 trees affected in 1914 and 42,000 in 

 1915, out of 77,000 in tapping. The cut surface of the 

 tapped bark becomes marked by vertical cracks, from which 

 latex occasionally exudes; sometimes a thick cushion of 

 coagulated latex forms below the renewing bark, causing 

 the latter to bulge out and ultimately fall off so as to leave 

 an open wound. The renewal of the bark is irregular, 

 masses of callus appear on the cut surface and further tap- 

 ping is hindered. 



The cause of this damage is a species of Phytophthora, 

 differing from the well-known canker fungus, Phytoph- 

 Ihora Faberi, to which it had been attributed in the Dutch 

 Indies. It seems to occur in Ceylon also but has been there 



