Disease resistance. Eyespot.

Contributed by Dipak K. Santra, Kim Kidwell and Kimberly Campbell

Background information

Strawbreaker foot rot disease, which is caused by the necrotrophic fungus Pseudocercosporella herpotrichoides (Fron) Deighton (teleomorph Tapesia yallundae Wallwork & Spooner), is a significant yield-limiting disease of wheat in North and South America, Europe, Australia, New Zealand and Africa. In the United States the disease is the most common in the Pacific Northwest. This disease is also called eyespot because of its characteristic elliptical lesions on basal leaf sheaths and culms.

Eyespot is the most severe disease where wheat is grown continuously and the climate is cool and moist. Severity is greater on winter than on spring wheat. In years when eyespot is severe, it costs growers millions of dollars in fungicides and yield losses. Additional complications are the increasing costs of fungicide application, the stricter regulations for their application, and the evolution of new strains of the pathogen that are resistant to commonly used fungicide (1). Use of cultivars resistant to eyespot disease is the most economic and environmental friendly way to minimize economic loss of wheat. Therefore, incorporation of genes for resistance to eyespot is an important aim of varietal improvement.

Pictures of eyespot disease infected wheat field, symptom on stem and the elliptical lesion are shown in the "Eyespot of wheat" web page prepared by the Plant Pathology Department of the University of Nebraska, Lincoln.

Resistance Genes

A number of eyespot resistance genes are known in wheat germplasm. The first and the most effective one is Pch1, originally found in Triticum ventricosum (2n=4x=28, genomes DN, 2). This single major gene was transferred to hexaploid wheat first crossing A. ventricosa with Triticum persicum (AABB) to obtain fertile amphidiploid and then this wheat was crossed with a common wheat, Marne, to develop VPM-1 (= Ventricosa x Persicum x Marne). Thus, hexaploid cultivar VPM-1 became a ready source for the Pch1 gene for further transferring to other common wheats. Pch1 is located on chromosome 7DL.

A second source of resistance was found in French cultivar Cappelle-Desprez. A single gene, Pch2 confers resistance at seedling stages (3). A polygenic component in `Cappelle-Desprez' confers resistance at adult stage (unpublished results). The Pch2 gene is located on chromosome 7AL. A third resistance gene Pch3 was identified in Dasypyrum villosum (2n = 14, genome VV) (4). It is located on the long arm of chromosome 4V. The fourth source of resistance to eyespot disease was identified in Triticum tauschii (syn. Ae. squarrosa, 2n = 14, genome DD) (5). However, the chromosomal location of resistance gene (s) in T. tauschii is unknown.

Pch1 is actively being used in both Europe and the USA because it is more effective in limiting disease development than Pch2 and is present in several hexaploid wheat varieties. However, neither the Pch1 nor the Pch2 confer complete resistance by itself (6). The breeders can further enhance resistance in wheat adopting different strategies. A first strategy is to pyramid Pch1 and the Pch2 into a single variety, but no study doing this has been reported so far. A second possible strategy is to transfer into wheat other major alien genes for resistance (for example Pch3 or the gene from T. tauschii) that can be later combined with Pch1 Although no such report is published yet, progress is being made in this direction (Tim Murray, personal communication). A third strategy is to create genotypes with an increased copy number of the gene Pch1 (7).

Methods

Resistance to eyespot, specially gene Pch1, is associated with morphological, biochemical and DNA markers that can be useful for mapping, assessing diversity in wild populations and marker assisted selection in wheat breeding programs. In the methods section you can find two different protocols for Pch1.

Although, the endopeptidase marker Ep-A1b and a few RFLP markers were reported to be linked with the Pch2, none of these markers were closely linked to this gene (8,3). Therefore marker assisted transfer of the Pch2 gene has not yet been reported.

References

1. Resistance to benzimidazole fungicides in the cereal eyespot pathogen, Pseuudocercosporella herpotrichoides, in the U.S. Pacific Northwest 1984 to 1990. Murray TD. In: Plant Disease,1996, 80:19-23. [abstract]

2. Genome origin of Triticum cylindricum, Triticum triunciale, and Triticum ventricosum (Poaceae) inferred from variation in repeated nucleotide sequences: a methodological study. Dubcovsky J, Dvorak J In: American Journal of Botany, 1994, 81:1327-1335

3. Identification of an RFLP interval containing Pch2 on chromosome 7AL of wheat.de la Peña RC, Murray TD, Jones SS. In: Genome, 1997, 40: 249-252. [abstract]

4. A new source of resistance to Pseuudocercosporella herpotrichoides, cause of eyespot disease of wheat, located on chromosome 4V of Dasypyrum villosum. Murray TD, de la Peña RC, Yildirim A, Jones SS In: Plant Breeding, 1994, 113:281-286. [abstract]

5. Resistance to stripe rust and eyespot diseases of wheat in Triticum tauschii. Yildirim A, Jones SS, Murray TD, Cox TS, Line RF In: Plant Disease,1995, 79:1230-1236. [abstract]

6. Identification of resistance to Pseudocercosporella herpotrichoides in Triticum monococcum. Cadle MM, Murray TD, Jones SS. In: Plant Disease, 1997, 81:1181-1186. [abstract]

7. Isolation of a chromosomally engineered durum wheat line carrying the Aegilops ventricosa Pch1 gene for resistance to eyespot.. Huguet-Robert V, Dedryver F, Roder MS, Korzun V, Abelard P, Tanguy AM, Jaudeau B, Jahier J. In: Genome, 2001, 44:345-349. [abstract]

8. Linkage relations among eyespot resistance gene Pch2, endopeptidase Ep-A1b and RFLP marker Xpsr121 on chromosome 7A of wheat. de la Peña RC, Murray TD, Jones SS. In: Plant Breeding, 1996, 115:273-275. [abstract]

9. Registration of 'Madsen' wheat. Allan RE, Peteson CJ, Rubenthaler GL, Line RF, Roberst DE. In: Crop Science, 1989, 29:1575-1576.

10. Registration of 'Hyak' wheat. Allan RE, Peteson CJ, Rubenthaler GL, Line RF, Roberst DE. In: Crop Science, 1990, 30:234.

11. Amplified fragment length polymorphism-derived microsatellite sequence linked to the Pch1 and Ep-D1 loci in common wheat. Groenewald JZ, Marais AS, Marais GF. In: Plant Breeding, 2003, 122: 83-85. [abstract]

12. Association of an isozyme locus and strawbreaker foot rot resistance derived from Aegilops ventricosa in wheat.. McMillian DE, Allan RE, Roberts DE. In: Theoretical and Applied Genetics, 1986, 72:743-747.