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Inoculum Density and Infection Efficiency of Conidia and Conidiophores of Isolates of Pyrenophora tritici-repentis . C. K. Evans, Research Associate, University of Minnesota, Department of Plant Pathology, 495 Borlaug Hall, 1991 Upper Buford Circle, St. Paul 55108-6030. R. M. Hunger, Professor, and W. C. Siegerist, Senior Agriculturist, Department of Plant Pathology, Oklahoma State University, Stillwater 74078-9947. Plant Dis. 80:505. Accepted for publication 8 January 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0505.

Glass slides and wheat leaves were inoculated with conidia and conidiophores of Pyrenophora tritici-repentis isolates to compare the density (no./cm2) of propagules on nonhost and host surfaces. Regression functions of the density of each propagule form on glass slides, as a function of the inoculum concentration, overestimated the density of each propagule form on wheat leaves by three to four times. Subsequently, conidia and conidiophores of three isolates were inoculated at equal rates of propagule density on wheat cultivars TAM 105 (susceptible) and Red Chief (resistant) to compare lesion incidence resulting from the different forms of propagule. Conidia caused 26 limes more lesions than did conidiophores, and differences among the isolates for lesion incidence were significant (P 0.05). Finally, the infection efficiency of the three isolates was determined utilizing their conidiophores and conidia in separate inoculum suspensions. Infection efficiency was determined from the slope of the regression of lesion incidence as a function of the density of propagules per unit area of inoculated leaf. Infection efficiency for conidia of the isolates ranged from 0.91 to 0.55 whereas infection efficiency for their conidiophores was not significantly different from zero or was extremely variable. Results indicate that studies of epidemiological parameters of P. tritici-repentis are more precise when based on estimates of conidial density on host leaf surfaces, and when conidiophores are excluded from inoculum suspensions. The estimates of infection efficiency should prove useful in the identification of virulent isolates of P. tritici-repentis and should lead to improved identification of resistance to tan spot.