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Relation Between Two Measures of Disease Expression in Barley-Ustilago hordei Interactions. J. V. Groth, Former Graduate Student, Department of Botany, University of British Columbia, Vancouver 8, Canada, Present address: Department of Plant Pathology, University of Minnesota, St. Paul 55108; C. O. Person(2), and T. Ebba(3). (2)(3)Professor, and former Graduate Student, respectively, Department of Botany, University of British Columbia, Vancouver 8, Canada, (3)Present address: Provisional Military Government, Institute of Agricultural Research, P.O. Box 2003, Addis Ababa, Ethiopia. Phytopathology 66:1342-1347. Accepted for publication 30 April 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1342.

Disease expression was examined in 12 barley cultivars inoculated with 21 Ustilago hordei dikaryons. Two parameters were measured: (i) the proportion of smutted plants (among-plant disease severity) and (ii) the proportion of smutted heads on smutted plants (within-plant disease severity). Sufficient plants were inoculated to ensure that 300 plants were available for measuring among-plant disease severity and another group of 30 plants was available for measuring within-plant disease severity. Data were divided into two sets according to degree of plant tillering to eliminate the effect of a negative regression of within-plant disease severity on number of tillers per plant. Strong correlations (r = 0.734 and 0.767) were found for among-and within-plant disease severity, respectively. Both variables were found to be largely genetically determined. The correlation suggests identity of genes which condition each of the disease reactions, either in the host or pathogen. Some nonidentity may be indicated by cultivars and dikaryons which differ in only one of the measures, when examined in relation to a single dikaryon and cultivar, respectively.

Additional keywords: disease resistance, virulence, pathogen fitness, covered smut of barley.