Inoculum density and infection efficiency of conidia and conidiophores of isolates of Pyrenophora tritici-repentis

Glass slides and wheat leaves were inoculated with conidia and conidiophores of Pyrenophora tritici-repentis isolates to compare the density (no./cm(2)) 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 times more lesions than did conidiophores, and differences among the isolates for lesion incidence were significant (P less than or equal to 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 tritici-repentis and should lead to improved identification of resistance to tan spot.