54 SCIENTIFIC REPORTS OF THE AGRICULTURAL RESEARCH 



Experiments carried on at Pusa have thrown light on 

 several of the anomalies in the parasitism of the eelworm 

 which causes nfra. It was previously known that the 

 spring crop, called boro rice, and also the transplanted 

 autumn and winter rice, ordinarily escape the disease under 

 natural conditions, though they can be readily inoculated 

 artificially. As the boro fields are often surrounded by 

 severely infected fields of winter (aman) rice, they are cer- 

 tainly frequently contaminated by diseased stubble from 

 the latter, and it was not easy to understand why they are 

 not attacked. It has now been conclusively proved that 

 the worm cannot migrate so as to reach the upper parts of 

 the rice plant, where alone it can feed, in dry air, but does 

 so readily when the humidity of the air approaches satura- 

 tion point, even though there is no film of free water on the 

 plants. It has also been found that worms preserved dry 

 retain their vitality and are able to renew active motion 

 after at least eight months, while those kept in moist air 

 remain alive for about four months, and those immersed in 

 water for only one to two months. The boro fields are sub- 

 merged in water from the time the crop is transplanted 

 into them in December-January, until near harvest. Worms 

 set free into this water from previously infected stubble, 

 will die unless they can migrate from the water within a 

 couple of months. While the seedlings are small, no doubt 

 some can migrate to them at or near the water level, but the 

 injury caused to rice plants at this stage of growth is slight. 

 As the plants get bigger, only certain parts near the top of 

 the shoot (above the upper joints of the stem and the young 

 ear chiefly) remain susceptible to attack, as only these parts 

 have unthickened cell walls. But the worm is unable to 

 reach them, owing to the low air humidity between Febru- 

 ary and April, and so the plants escape the serious damage 

 that results from an attack on the shoot while the ears are 

 developing. The crop is harvested in April, before the 

 humidity rises enough to allow any worms that may still 

 remain alive on the lower parts of the plants to migrate to 

 the top of the shoot. It has also been found that the worm 



