A revised model of fungicide translaminar activity.

Translaminar redistribution is valuable for fungicide activity but difficult to measure and predict. The translaminar activity of 38 fungicides active against cucumber powdery mildew was measured experimentally and used to develop a QSAR (Quantitative structure–activity relationship) model of translaminar movement from calculated parameters. Over 300 physiochemical parameters generated from energy-minimized 3D structures were considered and one-parameter, two-parameter, and five-parameter models were developed. The one-parameter lipophilicity model explained 39% of variability in translaminar activity in the full dataset but none of the variability in the small succinate dehydrogenase inhibitor (SDHI) set. Adding a polar surface area parameter to the lipophilicity parameter improved predictability to 52% and explained over 70% of the variability in the SDHI class. The expanded model with five physiochemical parameters explained more than 80% of the variability in overall translaminar redistribution. The three additional parameters were correlated with molecular size and reactivity. The models were validated with a Leave-One-Out method that showed excellent robustness (r2 adj = 0.83, q2 = 0.79, p < .0001) for the five-parameter model. Because the models require only calculated parameters from a 3D chemical structure, they could enable the design or selection of compounds likely to be translaminar.