V. J. F, SHAW 127 



In the case of the tobacco the results of the field experiments were not so 

 obvious. At Birowlie in two successive seasons the larger quantity of 0. cernua 

 appeared in one particular plot, although this plot received a heavy appli- 

 cation of sodium nitrate in each season. This suggested that the incidence of 

 "tokra" in a field was dependent upon factors, other than the application of 

 nitrate, such as the amount of " tokra " seed present and the resulting degree 

 of infection of the soil. The results of the experiments in the " new plot " 

 at Pusa in 1916-17 confirm this view. A reference to Text-figure 5 will 

 show that in this plot the 0. cernua occurred chiefly in the western half 

 of the field and was not materially influenced by the nitrate. The two 

 original tobacco plots, A and B, gave results which superficially did not 

 agree with this. In these plots the amount of 0. cernua was less in the 

 areas which had received the nitrate. This was most marked in the case of 

 the plot B which had not had any manure in the two previous seasons, and 

 received an appHcation of nitrate upon half its area for the first time in the 

 season 1916-17. In this plot the unmanured area contained about 30 plants 

 of 0. cermia to every tobacco plant and the manured area contained about 15 

 plants of 0. cernua to each tobacco plant. But a proportion of 15 "tokras" 

 to each tobacco plant in the manured area appears, in the writer's opinion, 

 too large to justify the claim of any curative properties against "tokra" 

 for nitrate of soda. Moreover it must be pointed out that in the two plots 

 A and B in the season 1916-17, the amounts of " tokra " in the two unmanured 

 areas differed from one another nearly as much as when the comparison was 

 made between one of the areas which had received nitrate and one which 

 had not received any {see page 115). If applications of sodium nitrate 

 were likely to prove a preventive against " tokra " a result would surely 

 have appeared in the pot cultures, where the application of 1 oz. to each 

 pot was at the late of several tons to the acre. In this case, however, 

 0. cernua on tobacco was almost quite uninfluenced by the nitrate and 

 the most favourable result obtained with other crops was a delay in the 

 appearance of the " tokra." 



Chili saltpetre therefore can hardly be advocated as a cure for " tokra." 

 In a valuable crop, such as tobacco, and in a district where labour is as cheap 

 as it is in Bihar, much may be done to keep these pests down by hand-weeding, 

 the " tokras " being uprooted before they have formed mature seed which could 

 infect the soil for the succeeding crop. With a crop such as mustard it might 

 be possible to cultivate early varieties, which ripen before the "tokra " has 

 matured its seed, and by ploughing immediately after harvesting to bury the 



