Lr19 Resistance in Wheat Becomes Susceptible to Puccinia triticina in India

Abstract

Lr19, a resistance gene originally transferred from Agropyron elongatum to wheat (Triticum aestivum L.), has remained effective worldwide against leaf rust (Puccinia triticina Eriks.) except in Mexico (1). This report records a new pathotype of P. triticina virulent on Lr19 from India. From 2003 to 2004, 622 wheat leaf rust samples from 14 states were subjected to pathotype analysis. Samples were established on susceptible wheat cv. Agra Local, and pathotypes were identified on three sets of differentials following binomial nomenclature (3). Virulence on Lr19 (Agatha T4 line) was observed in approximately 2% of samples. These samples were picked from Lr19 (NIL), cvs. Ajit, Lal Bahadur, Local Red, Lok1, and Nirbhay from Karnataka and Gujarat states. All Lr19 virulent isolates were identical. The reference culture is being maintained on susceptible wheat cv. Agra Local and has also been put under long-term storage in a national repository at Flowerdale. From 2004 to 2005, this pathotype was detected in 6.3% of samples from central and peninsular India. There is no wheat variety with Lr19 under cultivation in India, however, it is being used in wheat breeding programs targeted at building resistance against leaf and stem rusts. NIL's Lr19/Sr25 (LC25) and Lr19/Sr25 (82.2711) were also susceptible to this isolate, whereas Lr19/Sr25 (spring accession) was resistant. The new isolate, designated as 253R31 (77-8), appears to be close to the pathotype 109R31 (4) with additional virulence for Lr19. The avirulence/virulence formula of pathotype 253R31 is Lr9, 23, 24, 25, 26, 27+31, 28, 29, 32, 36, 39, 41, 42, 43, 45/Lr1, 2a, 2b, 2c, 3, 10, 11, 12, 13, 14a, 14b, 14ab, 15, 16, 17, 18, 20, 21, 22a, 22b, 30, 33, 34, 35, 37, 38, 40, 44, 48, and 49. To our knowledge, this is the first report of virulence on Lr19 from two states of India. On international rust differentials, it is designated as TGTTQ (2), and is different from CBJ/QQ (1), the other isolate reported virulent on Lr19 from Mexico. The Mexican isolate is avirulent on Lr1, 2a, 2b, 2c, 3ka, 16, 21, and 30 to which the Indian isolate is virulent. However, both isolates are avirulent on Lr9, 24, 26, 36, and Lr42. Among the wheat cultivars identified during the last 6 years, HD2824, HD2833, HD2864, HI1500, HS375, HUW 510, HW 2044, HW 5001, Lok 45, MACS 6145, MP4010, NW 2036, PBW 443, PBW 498, PBW 502, PBW 524, Raj 4037, UP 2565, VL 804, VL 829, and VL 832 and lines of wheat possessing Lr9, Lr23, Lr24, and Lr26 showed resistance to this pathotype. PBW 343, which occupies more than 5 million ha in India, is also resistant to this pathotype along with PBW 373. An integrated strategy using a combination of diverse resistance genes, deployment of cultivars by using pathotype distribution data, slow rusting, and adult plant resistance is in place to curtail selection of new pathotypes and prevent rust epiphytotics. References: (1) J. Huerta-Espino and R. P. Singh. Plant Dis. 78:640,1994. (2) D. V. Mc Vey et al. Plant Dis. 88:271, 2004. (3) S. Nagarajan et al. Curr. Sci. 52:413, 1983. (4) S. K. Nayar et al. Curr. Sci. 44:742, 1975.