16 THE RICE WORM (tYLENCHUS ANGUSTUS) AND ITS CONTROL 



the vaporisation of the morning dews affects the readings, and that midday or 

 afternoon records would give lower relative readings in the early months of 

 the year. 



It has already been pointed out that Tylenchus angustus lives and feeds 

 towards the top of the rice plant. To reach this position it must be able to 

 mount the plant, emerging from the water at its base and climbing up the 

 parts above water. This it cannot do, as explained above, unless the sur- 

 rounding air is at a point approaching saturation with water vapour. The 

 same condition is necessary to allow it to spread from one plant to another. 

 No doubt when the boro paddy is first transplanted, some of the worms should 

 be able to make their way between the folds of the leaf bud at the water level 

 and reach a position suitable for feeding and even for multiplying. It is 

 probable that there is some injury to the crop at this stage (the mere so since 

 the second crop of aman shoots has been found infected in January and 

 February), but it would not attract much attention ; the symptoms, as 

 already pointed out, are not very definite on seedling plants ; and the injury 

 would most likely be attributed to the check received during transplantation. 

 Once the rice shoots had grown well above the water, further multiplication 

 of the worm would cease, and further infection of the susceptible parts would 

 become impossible, as its migration would be prevented by the dryness of the 

 air ; those worms that had failed to get out of the water would die after a 

 month or two. Thus the characteristic attacks as the crop ripens would fail 

 to develop. 



This explanation of the immunity of the boro crop has been considerably 

 strengthened by observations made in 1918 on the second gro\vth of infected 

 aman. A field on the Dacca Experiment Station was found to have the young 

 shoots and dwarf ears that grew from the stubble of diseased winter rice in 

 January (after harvest in December) heavily infected, while those that grew 

 in February entirely escaped. The infected and clean shoots were often 

 within a few inches of one another, yet the worms were unable to reach the 

 latter, though, when immersed in water, they were found to be highly motile. 

 The February attacks observed in NoakhaU are probably correlated with the 

 higher humidity of that district. 



The same factor serves to explain the remarkable differences that have 

 been observed in the length of time between infection and the appearance of 

 the disease, according to the season of the year and the method of infection 

 employed. In field inoculations carried out during the monsoon, the first 

 symptoms may be observed in about 8 days where the worms are directly 



