Significant Influence of EC50 Estimation by Model Choice and EC50 Type.

The effective control to 50% growth inhibition (EC50) 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 EC50. These models and definitions are generally left out of phytopathology literature. Here, we demonstrate that model choice and type of EC50 (relative versus absolute) can matter for EC50 estimation using data from Pythium oopapillum and Fusarium virguliforme. For some P. oopapillum isolates, the difference between absolute and relative EC50 was significant. Hormetic effects changed F. virguliforme EC50 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 EC50 estimation and clearly indicate which model and EC50 measure (relative versus absolute) was used. We provide guidelines for model choice and interpretation for those wishing to set up experiments for accurate EC50 estimation.