Effect of nutrient management practices on leaf reddening of Bt cotton (Gossypium hirsutum L.) under irrigated conditions U.N. SANTHOSH*, SATYANARAYANA RAO, B.K.DESAI, A.S.HALEPYATI AND B.G.KOPPALKAR Department of Agronomy, University of Agricultural Sciences, Raichur-584104 *Email: santhu2032@rediffmail.com ABSTRACT : Among the treatments, LRI was significantly reduced with higher NPK fertilizers and also with foliar sprays of MgSO 4 +KNO 3 , MgSO 4 +19:19:19 NPK and MgSO 4 thrice combined with initial soil application of MgSO 4 @ 25 kg /ha. The extent of reduction in LRI with 125 and 150 per cent RDF over 100 per cent RDF, was 15.4 and 30.1 per cent, respectively at 135 DAS and 11.3 and 23.8 per cent at harvest, respectively. Whereas 3 foliar sprays of MgSO 4 +KNO 3 and MgSO 4 +19:19:19 along with soil application of MgSO 4 during flowering, boll formation and boll development stages, reduced the LRI by 18.7 and 16.2 per cent at 135 DAS and 12.7 and 11.5 per cent at harvest growth stages, respectively as compared with control (2.40 and 2.59). The leaf chlorophyll, nitrogen and magnesium contents were increased at all the stages, with higher fertilizer levels and foliar application of nutrients. Key words: Foliar sprays, leaf reddening, soil application Cotton is an important commercial fibre crop of India and contributes 85 per cent of raw materials to textile industries. In India the annual area under this crop was 11.16 m ha, with annual production of 31.20 m bales and average productivity of 494 lint kg/ha (Hosmath et al.,2012). Bt cotton is intensively cultivated on black clay soil under irrigated areas of Tungabhadra and Upper Krishna Command areas (TBP and UKP). The area under cotton in these command areas has been increased distinctly over the past decade and occupied an area of more than 1.6 lakh ha. Average cotton yield is around 20 q/ha, which is less than the potential yield. The maximum yield potential of Bt cotton has been not tapped due to monocropping, declined in soil fertility, delayed sowing, imbalanced nutrition, attack of sucking pests and physiological disorders mainly leaf reddening of leaves. Among the production constraints, leaf reddening has become major physiological problem in Bt cotton cultivation. Hosmath et al., (2012) observed that Bt cotton genotypes recorded significantly higher red leaf index (1.82) as compared to non Bt cotton genotypes (1.63). The malady is observed during peak flowering and boll development stages or later on but occasionally during squaring to flowering stages. The yield losses due to reddening of leaves may be as high as 60-75 per cent. Usually reddening occurs after boll formation stage where the losses could be 15-25 per cent depending upon severity of the disease. The principle cause of reddening as reported, is due to nitrogen and magnesium deficiencies. Increased in nitrogen along with phosphorus and potassium increased the supply of nitrogen to leaf and reduced the formation of anthocyanin at the cost of chlorophyll. It showed the importance of plant nutrition in management of leaf reddening in cotton (Upperi and Kuligoud, 2011). A field experiment was conducted to study the effect of nutrient management practices on chemical composition and leaf reddening of Bt cotton during 2011-2012 at Main Agricultural Research Station, Raichur. The soil of experimental site had pH of 8.32. The available N, P and K respectively were 208.9, 25.4 and 139.5 kg/ha, whereas Mg was 8.40 (C mol/kg). There J. Cotton Res. Dev. 29 (1) 71-75 (January, 2015)