BGN 13: Erysiphe graminis resistance in the Hordeum murinum complex BARLEY GENETICS NEWSLETTER, VOL. 13, II. RESEARCH NOTES
Giles & Barrett, pp. 78-82

II. 40. Erysiphe graminis resistance in the Hordeum murinum complex.

Barbara Giles(1) and John Barrett(2) Department of Genetics, University of Cambridge, England.

(1) Present address: Cytogenetics Section, Ottawa Research Station, Research Branch, Agriculture Canada, Ottawa Ontario, Canada K1A OC6.
(2) Present address: Department of Genetics, The University of Liverpool, P. O. Box 147, Liverpool, L69 3BX, U.K.

As part of an assessment of genetic variation in the wild barley Hordeum murinum (Giles, 1981), a range of populations were screened for resistance to Erysiphe graminis f. sp. hordei, using isolates from different sources.

(1) Twenty, one-week-old plants from each of twenty-five populations (Table 1) were exposed in a green-house to three isolates of E. graminis originally isolated on cultivated barley in the U.K., carrying a total of 5 virulence factors (vg, va6, vv, vAs and vTyra; pathogenic on Barley Mildew Resistance Groups 2, 3, 4, 5 and 7 respectively). Plants were scored two and four weeks after exposure.

Table 1. Populations used in screen of resistance to Erysiphe graminis f. sp. hordei.

(2) Detached leaf segments from twenty individuals from each of four populations (Table 1) were inoculated in a settling tower (M. S. Wolfe, pers. comm.) with one of two isolates derived originally from a bulk sample from Israel. Isolate 1 had been isolated on cv Kwan and was known to infect a range of different resistance groups; Isolate 2 was isolated from the bulk sample on cv Omer (M. S. Wolfe, pers. comm.). Leaf segments were scored one week after inoculation.

Reactions were recorded on a scale from 0 (full resistance) to 4 (severe infection). Resistance reactions were further classified as Necrotic (N) and/or Chlorotic (C).

The preponderance of necrotic resistance reactions in Experiment 1 at both assessment dates (Table 2), prevent any formal statistical analysis, since the expectations in all other classes are very small. Moreover, since the majority of the reactions are of the "resistant" type at both assessment dates (95% at two weeks and 100% at four weeks) lumping data to increase expectations does not facilitate analysis. Inspection of the data show that only two populations (V and Y) exhibit a susceptible reaction after two weeks and even these two populations did not show evidence of continued infection after four weeks (a formal test of heterogeneity is not possible (a) because expectations are small and (b) since the two scores are based on the same plants, the assessments are not statistically independent).

Table 2. Reactions of 25 populations of Hordeum murinum to Erysiphe graminis (Experiment 1) isolated from cultivated barley (populations A,B...G,...Y as in Table 1).

When the four populations were tested with the isolates of Israeli origin (Table 3), the data indicate that differences between populations exist but again analysis of the crude data is not possible under the assumptions of heterogeneity x2. However, by lumping together all of the "resistant" reactions, the data can be analysed, (X210 = 37.57, p < 005).  Partitioning of this x2 indicates that the major source of heterogeneity is due to differences between populations (Table 3). However, it must be noted that in this experiment assessments were made only one week after inoculation and that it is possible that the differences observed may have been reduced if assessments had been made after a longer time interval (cf. Experiment 1).

Table 3. Reactions of four populations of Hordeum murinum to two different races of Erysiphe graminis of Israeli origin (Experiment 2).

In summary these data show that:
(l) when exposed to powdery mildew of agricultural origin, Hordeum murinum appears to show high levels of resistance but that there appears to be variation between populations both in the type of resistance and timing of its expression during plant development.

(2) when exposed to isolates of predominantly wild origin, Hordeum murinum exhibits higher levels of susceptibility and significant differences exist between populations in the expression of resistance/susceptibility. A contributory factor to this observation may be that isolates derived from cultivated barley may have been selected for performance on cultivated barley, whereas the Israeli isolates may have been exposed to a wider range of host species and hence may have a wider host-range (Wahl et al., 1978). Seed of the Hordeum murinum populations used in these experiments is available from (1) Plant Gene Resources of Canada, Ottawa Research Station, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6, and (2) Seed Bank, Royal Botanical Gardens, Kew, England.

Acknowledgements:
We would like to thank Dr. M. S. Wolfe and the staff of the Department of Plant Pathology and Entomology, especially Peter Minchin and Susan Slater, at the Plant Breeding Institute, Cambridge, for the facilities and advice they generously provided. The authors also gratefully acknowledge the financial support provided by grants from the Perry Foundation, Boreham, Chelmsford, U.M. (B.E.G.) and the Agricultural Research Council (J.A.B.).

References:
Giles, B. E. 1981. Morphological and biochemical variation in Hordeum murinum. Ph.D. Thesis. University of Cambridge.

Wahl, I., N. Eshed, A. Segal and Z. Sobel. 1978. Significance of wild relatives of small grains and other wild grasses in cereal powdery mildews. In "The Powdery Mildews" (D.M. Spencer, ed.), pp. 83-100. Academic Press, London and New York.

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