Several small-molecule perturbagens of the plant endomembrane system are known, but few selectively disrupt endoplasmic reticulum (ER) structure and function. We conducted a microscopy-based screen for small-molecule disruptors of ER structure and discovered eroonazole, a 1,2-4-triazole that induces extensive ER vesiculation in Arabidopsis seedlings. To identify eroonazole targets, we synthesized a clickable photoaffinity derivative and used it for whole-seedling labeling experiments. These reveal that the probe labels multiple oleosins, plant membrane proteins that stabilize ER-derived lipid droplets. Oleosin labeling is absent in an oleosin1234 quadruple mutant and reduced using an inactive analog. Cellular analyses of the ER in the quadruple mutant demonstrate that oleosins are required for normal ER structure during seed germination and suggest that perturbation of oleosin function by eroonazole underlies its effects on seedling ER structure.