Disease control of soilborne pathogens by biological control agents (BCAs) is often inconsistent under field conditions. This inconsistency may be partly influenced by pathogen diversity if there is a differential response among pathogen species and isolates to selected BCAs. The responses of 148 Pythium isolates obtained from soil at three forest nurseries and representative of 16 Pythium spp. were evaluated in the presence of Streptomyces lydicus strain WYEC108 in an in vitro assay. Percent growth inhibition, inhibition zone distance, mortality, and growth rate were recorded for each isolate, and data were analyzed for effects of species and isolate. Responses of three Pythium spp. (Pythium irregulare, P. sylvaticum, and P. ultimum) were further analyzed for a location (nursery) effect. Although S. lydicus inhibited all Pythium isolates, differences in percent growth inhibition, inhibition zone distance, and mortality were observed among Pythium spp. and isolates. Small but significant location effects were also noted. Growth rate also varied among Pythium spp. and isolates and was found to strongly bias percent growth inhibition and, to a lesser degree, inhibition zone distance; depending on which measure was used, slower-growing isolates appeared less sensitive (growth inhibition) or more sensitive (inhibition zone) to S. lydicus than faster-growing isolates. Results illustrate the importance of using multiple, representative pathogen isolates in preliminary BCA inhibition assays as well as accounting for the effect of pathogen growth rate on pathogen inhibition by BCAs. Future studies should take pathogen diversity into account when evaluating biological control efficacy.
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