Schematic diagram of the gravel treatments in the field trials and columns with mesh bag samples (closed horizontal bars) containing infested leaf disks. Cylindrical holes each held two columns of different lengths (a 10 cm inner column and a 20 cm outer column) to allow sampling at different times with minimal disturbance. The columns were filled with field soil or gravel consistent with the surrounding substrate treatment, and mesh bags with leaf disk inoculum were inserted at 0 cm, 5 cm, and 15 cm depths below the soil or gravel surface.