A reproducible greenhouse assay using a fava bean model system to assess DMI fungicide-induced plant stunting in strawberry

Fungicides are essential tools in modern crop production but may also cause unintended off-target effects on plant growth and physiology. Demethylation inhibitor (DMI) fungicides are widely used for disease management in strawberry production, but some have been associated with stunting of strawberry plants in nurseries, raising concerns that repeated applications after planting could negatively affect plant growth and yield. The objectives of this study were to establish a rapid model system for assessing DMI-induced growth inhibition and to evaluate potential interactions of this chemical class with agrochemical mix partners. Three different plant species (fava bean, soybean, and basil), were examined. Fava bean provided the most consistent and reproducible results that matched field observations and were therefore selected for subsequent experiments. Repeated applications of Propi-Star EC (Propi-Star; propiconazole) caused significant stunting and increased stem diameter among all experiments, whereas other formulated DMIs (difenoconazole, flutriafol, myclobutanil, mefentrifluconazole, and triflumizole) showed no significant stunting effects. All Propi-Star-containing tank mixtures induced plant stunting, but none (including the phosphonate ProPhyt) enhanced the stunting response compared with Propi-Star alone. ProPhyt caused leaf phytotoxicity in fava beans, with more damage per leaf (estimated % leaf affected) observed in the solo treatments compared to mixtures with other agrochemicals. This observation was not explained by differences in spray suspension, pH, or electrical conductivity. In conclusion, fava beans were used in a reproducible greenhouse system to assess DMI-induced plant stunting, with responses consistent with reported field observations in strawberry production.