Albert Howard 377 



(e) By complete cutting back in the cold weather when the root 

 regeneration of surface-rooted types is difficult probably on account of 

 low soil-temperatures. In December, 1918, 641 healthy well-developed 

 August-sown plants were cut back when over five feet high. The following 

 February, 162 of these were badly wilted, 185 were partly wilted and 

 294 were normal. A number of root washings were made and in all cases 

 wilt was found to be associated with the practical absence of root re- 

 generation. The plants which developed wilt were those which had their 

 laterals near the surface, the deeper rooted plants producing normal 

 growth. Thus the monsoon results are reversed during the cold weather. 

 In the cold season, the factor which checks root regeneration is ap- 

 parently low soil-temperature. This affects surface-rooted plants much 

 more than deep-rooted types. The plants were kept under observation 

 till April by which time a remarkable change had taken place. The rise 

 in the soil-temperature in March caused the wilted plants to recover, 

 root regeneration took place and the growth became normal. 



(/) By waterlogging slowly from below during the rains, by closing 

 the drainage openings of lysimeters. At the beginning of the rains of 

 1918, indigo was sown in two sets of lysimeters. These were air-tight 

 cemented tanks, 1/1000 of an acre in area, four feet high, built above the 

 ground level and provided with drainage openings which could be closed 

 at will. In one set, alluvial soil exceedingly rich in available phosphate 

 (0-318 per cent.) from the Kalianpur Farm near Cawnpore was used, in 

 the other light Pusa soil was employed. The latter, when analysed by 

 Dyer's method, gives very low figures for available phosphate (0-001 per 

 cent.). In both soils the indigo in the lysimeters with free drainage 

 escaped wilt altogether. When the drainage openings were closed and 

 waterlogging from below took place all the plants were wilted in both 

 Kalianpur and Pusa soil. Wilt in the Kalianpur soil (rich in available 

 phosphate) was much worse than in Pusa soil (low in available phosphate). 

 The growth in Kalianpur soil was much slower than in Pusa soil. 



These experiments establish the cause of wilt. The disease results 

 from damage to the fine roots and nodules under circumstances where 

 root regeneration is difficult or impossible. 



As has been stated above wilt generally makes its appearance during 

 the latter half of the rainy season. During this period it is often the 

 cause of low yields of indigo. The agency w r hich brings about wilt during 

 this period has been found to be defective soil-aeration caused by the 

 upward rise of the ground water, combined with the destruction of the 

 porosity of the surface soil by heavy rain. This interferes with soil- 

 Ann. Biol, vii 25 



