Can visual phytotoxicity data from nontarget terrestrial plant studies be used in the European Union environmental risk assessment?

An evaluation of the reproducibility, reliability, and relevance of visual phytotoxicity scores assigned by 23 assessors on five plant species commonly tested in regulatory-required nontarget terrestrial plant studies for registration of plant protection products in the EU was undertaken. Significant variability in visual phytotoxicity scores was observed among the assessors, resulting in generally low reproducibility, as indicated by poor interclass correlation values. Assessors were consistent in their scoring only when highly severe effects were observed on the tested plants, whereas substantial disagreement in scores was recorded in the presence of moderate and slight effects. The implications of the possible use of visual phytotoxicity scores in the EU regulatory environmental risk assessment were evaluated, particularly in relation to the theoretical derivation of the Effect Rate resulting in 50% effect (ER50) based on this parameter, which may vary significantly based on the assessor's judgment. These findings highlight the limitations of visual assessments in accurately gauging phytotoxicity around the 50% effect level and underline the need to base regulatory risk assessments on objective and reproducible quantitative measures. A comparison of median visual phytotoxicity scores with ER50 derived from dry weight measurements indicates that, in 80% of the species/herbicide combinations in our evaluation, the two metrics aligned, suggesting that visual assessments may not provide additional relevant information for the environmental risk assessment beyond quantitative biomass data. We propose that, if visual phytotoxicity scores have to be considered in the EU regulatory environmental risk assessment, the establishment of a standardized framework for visual phytotoxicity assessment is needed to ensure evaluations are reproducible and relevant to ecological risk management decisions. Without such standardization, current practices may lead to inconsistent regulatory outcomes for the registration of plant protection products in the EU.