Significant Influence of EC Estimation by Model Choice and EC Type.

The effective control to 50% growth inhibition (EC) is a standard statistic for evaluating dose-response relationships. Many statistical software packages are available to estimate dose-response relationships but, recently, an open source package ("drc") in R has been utilized. This package is highly adaptable, having many models to describe dose-response relationships and flexibility to describe both hormetic relationships and absolute and relative EC. These models and definitions are generally left out of phytopathology literature. Here, we demonstrate that model choice and type of EC (relative versus absolute) can matter for EC estimation using data from Pythium oopapillum and Fusarium virguliforme. For some P. oopapillum isolates, the difference between absolute and relative EC was significant. Hormetic effects changed F. virguliforme EC distributions, leading to higher estimates than when using four- or three-parameter log-logistic models. Future studies should pay careful attention to model selection and interpretation in EC estimation and clearly indicate which model and EC measure (relative versus absolute) was used. We provide guidelines for model choice and interpretation for those wishing to set up experiments for accurate EC estimation.