Single 'major gene' resistance is frequently overcome by pathogens, especially the obligate biotrophs such as the powdery mildews and rusts. 'Partial' resistance has sometimes been favoured in preference as a potentially 'durable' source of resistance. The partial resistance may be sufficient to reduce epidemic development enough to stop serious yield loss. It is generally thought to be due to the additive effects of many genes, each of a small effect but together expressing effective resistance.

The pathogen is therefore at a disadvantage as it would have to accumulate genes to 'match' each of the 'resistance genes', rather than just one. The selection pressure would be low for each of the genes of small effect, so the resistance is likely to be 'durable'. This is also often referred to as 'polygenic resistance'.

Partial resistance is characteristically environmentally labile. Its quantitative nature also means that accuracy in assessment is of great importance.

At SCRI we have devised methods for accurately and objectively assessing resistance by visual [1, 6], biochemical [1, 2, 4, 5, 10] and electronic means [1]. We have looked at factors which affect resistance expression [3, 8, 9, 11, 12, 14] and the analysis of resistance data [7, 16, 18]. We also monitor the expression of resistance to Rhynchosporium secalis, a disease which relies upon quantitative resistance, under field conditions [15, 17, 19].

Modern statistical techniques have provided us with methods for mapping the genes involved in quantitative resistance. Using doubled haploid mapping populations, Quantitative Trait Loci (QTL) for R. secalis have been mapped to several places in the barley genome, and these data are being used to help isolate resistance and early signalling genes in a gene discovery programme.

The test as to whether partial or polygenic resistance is durable is whether isolates are able to overcome it. We have studied isolates and natural populations of pathogens to determine this.

Refereed publications

[1] Newton AC, 1989. Measuring the sterol content of barley leaves infected with powdery mildew as a means of assessing partial resistance to Erysiphe graminis f.sp. hordei. Plant Pathology 38, 534-540.

[2] Newton AC, 1990. Detection of components of partial resistance to mildew (Erysiphe graminis f.sp. hordei) incorporated into advanced breeding lines using measurement of fungal cell wall sterol. Plant Pathology 39, 598-602.

[3] Newton AC, 1993. The effect of humidity on expression of polygenic resistance to mildew in barley. Plant Pathology 42 364-367.

[4] Newton AC, Reglinski T, 1993. An enzyme-linked immunosorbent assay for quantifying mildew biomass. Journal of Plant Disease and Protection 100, 176-179.

[5] Newton AC, Thomas WTB, 1993. Evaluation of sources of partial resistance to mildew in barley using enzyme linked immunosorbent assay and other assessment methods. Euphytica 66, 27-34.

[6] Newton AC, Hackett CA, 1994. Subjective components of mildew assessment on spring barley. European Journal of Plant Pathology 100, 395-412.

[7] Hackett CA, Reglinski T, Newton AC, 1995. Use of additive models to represent trends in barley field trials. Annals of Applied Biology 127, 391-403.

[8] Newton AC, Dashwood EP, 1998. The interaction of humidity and resistance elicitors on expression of polygenic resistance of barley to mildew. Journal of Phytopathology 146, 123-130.

[9] Newton AC, Guy DC, 1998. Exploration and exploitation strategies of powdery mildew on barley cultivars with different levels of nutrients. European Journal of Plant Pathology 104, 829-833.

Non-refereed publications

[10] Newton AC, Thomas WTB, 1991. Use of enzyme-linked immunosorbent assays (ELISA) to assess and characterise the expression of partial resistance to mildew (Erysiphe graminis f.sp. hordei). Barley Genetics VI, 599-601.

[11] Newton AC, 1992. Expression of partial resistance to mildew in barley. Proceedings of the Eighth European and Mediterranean Cereal Rusts and Powdery Mildews Conference, Weihenstephan, Germany, Vorträge für Pflanzenzüchtung 24, 31-33.

[12] Newton AC, Harrison JG, 1994. Interaction of the Expression of Partial Plant Disease Resistance and the Environment. Scottish Crop Research Institute Annual Report for 1993.

[13] Jones ERL, Newton AC, Clifford BC 1994. Rhynchosporium of barley. United Kingdom Cereal Pathogen Virulence Survey Annual report for 1993, 58-64.

[14] Newton, A.C., Thomas, W.T.B. and Goleniewski, G. 1995. Effects of nitrogen on mildew levels and yield in major gene and partial resistance spring barley cultivar mixtures. pp.165-172 in: Integrated control of cereal mildews and rusts: towards co-ordination of research across Europe. Proc. Workshop COST817, Z?5-10 Nov 1994, E. Limpert, M.R. Finckh and M.S. Wolfe, eds. European Commission, Luxembourg, 276 p.

[15] Jones ERL, Newton AC, Clifford BC, 1995. Rhynchosporium of barley. United Kingdom Cereal Pathogen Virulence Survey Annual report for 1994, 52-60.

[16] Hackett CA, Newton AC, 1996. Improving the accuracy of field trials by modelling spatial trends with generalised additive models. Scottish Crop ResearchInstitute Annual Report for 1995.

[17] Jones ERL, Newton AC, Clifford BC, 1996. Rhynchosporium of barley. United Kingdom Cereal Pathogen Virulence Survey Annual report for 1995.

[18] Hackett CA, Newton AC, 1985. Modelling spatial trends in barley field trials using generalised additive models. Aspects of Applied Biology 43, 59-66.

[19] Jones ERL, Newton AC, 1999. Rhynchosporium of barley. United Kingdom Cereal Pathogen Virulence Survey Annual report for 1998, 67-74.