(a) Plant material and Phytophthora infestans inoculation

Seeds of S. arcanum were surface sterilized using approximately 5% NaOCl (30 s), washed 3×3 min in sterile H₂O, plated on 1.2% H₂O agar and incubated in dark for 3 days (16 h/8 h with 18°C/15°C). Afterwards, the seeds were transferred to a 16 L (166 ± 17 µmol quanta m⁻² s⁻¹) : 8 D regime. Nine days post sterilization (dps), seedlings were transferred to 0.5% Murashige & Skoog medium [27] with 1% sucrose.

The isolate, IPO-C, of P. infestans was grown on rye-sucrose-agar plates (with 100 µg ml⁻¹ ampicillin, 10 µg ml⁻¹ amphotericin B and 20 µg ml⁻¹ vancomycin; [28]) at 18°C in the dark. Zoospores were isolated and leaflets of S. arcanum were inoculated at 28 dps as described in de Vries et al. [29]. Three biological replicates (three to four seedlings each) were sampled per treatment and time-point (0 hpi, 6 hpi, 24 hpi, 48 hpi, 72 hpi and 96 hpi).