November 30, 2005
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes, December 2005 [edited] <http://www.apsnet.org/pd/searchnotes/2005/PD-89-1360A.asp>


Lr19 resistance in wheat becomes susceptible to Puccinia triticina in India
S. C. Bhardwaj, M. Prashar, S. Kumar, S. K. Jain, and D. Datta, Regional Station, Directorate of Wheat Research (ICAR), Flowerdale, Shimla-171 002, Himachal Pradesh, India. Plant Dis. 89:1360, 2005; published on-line as DOI: 10.1094/PD-89-1360A. Accepted for publication 29 Sep 2005.

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 3 sets of differentials following binomial nomenclature (3). Virulence on Lr19 (Agatha T4 line) was observed in approximately 2 percent 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 percent 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 1st report of virulence on Lr19 from 2 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.

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[The report that the Lr 19 resistance gene for leaf rust no longer confers resistance against the fungus is cause for concern. Leaf or brown rust caused by _Puccinia triticina_is one of the most important cereal diseases worldwide. Yield losses may reach 40 percent in susceptible cultivars, but it is generally in the range from 1-20
percent. The global leaf rust population varies in virulence and this variation may result from one or more factors. Development of genetic resistance to rust is the most efficient, cost-effective and environment-friendly approach to prevent the losses caused by rust epidemics. The use of cultivars with single-gene resistance permits the selection of mutations at a single locus to render the resistance effective in a relatively short time.

However, due to selection pressure and evolution, new virulent races of the fungus appear, which increase the need to develop durable resistance. Hence the use of combinations of genes, irrespective of whether they are major or minor, has been suggested as the best method for genetic control of leaf rust. This can be achieved by pyramiding effective resistance genes, but they are difficult to monitor in the field for expression of individual resistance genes against the background of other resistance genes. With the advent of molecular marker technology it is now possible to tackle such complex problems.

DNA-based molecular markers have several advantages over the traditional phenotype markers. They can be used for marker-assisted selection (MAS) to improve the efficiency of selection in plant breeding because the environment does not affect the expression of molecular markers. These markers can also be detected at all stages of plant growth, whereas phenotypic markers often can only be identified at a specific growth stage. Many resistance genes against leaf rust in wheat have been introgressed from wild relatives. The leaf rust resistance genes Lr19 and Lr24 have been introgressed from _Agropyron elongatum_. Scientists are to test the validity of the molecular markers and the correspondence between the molecular and the resistance tests for the leaf rust resistance genes Lr19 and Lr24 in some lines, and the use of combined tests to permit the identification of relevant resistance lines for pyramiding resistance genes.

Links:
<http://www.ars.usda.gov/is/AR/archive/may01/leaf0501.htm?pf=1S>
<http://www.nappfast.org/casestudies_files/wheat_rust percent20.pdf>
<http://www.cdl.umn.edu/tritname.html>
<http://www.oznet.ksu.edu/historicpublications/Pubs/SC068.PDF>
- Mod.DH]

[see also in the archive:
Leaf rust, Septoria spp., wheat - Kazakhstan: corr. 20050825.2509
Leaf rust, Septoria spp., wheat - Kazakhstan 20050823.2488
2001
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Brown rust, wheat - Russia (E. Siberia) 20010724.1442
2000
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Leaf rust, wheat - Australia 20001215.2187
1999
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Crop diseases - Canada (Manitoba) 19990816.1417
Leaf rust, wheat - Canada 19991015.1833]