Penetration and plant infection analyses of the VdSsk2 and VdSte11 mutant strains of Verticillium dahliae. (A) Cellophane membranes were plated onto minimal medium and inoculated with conidia harvested from wild-type strain XS11, the ΔVdSsk2 and ΔVdSte11 strains, and the respective complemented strains. Penetration was examined at 5 dpi. The top panels show colonies of the strain grown on the cellophane membranes covered above PDA at 5 dpi. The bottom panels show colonies grown from hyphae penetrating through the membrane at another 2 dpi after removing cellophane membrane. Bars = 0.5 cm. (B) Hyphopodia and penetration pegs formed on the cellophane membrane by the above strains. Black asterisks and arrowheads indicate hyphopodia and penetration pegs, respectively. The ΔVdSte11 strain failed to form hyphopodia and penetrate the membrane. Bar = 5 μm. (C) Infection assays of onion epidermis 48 h after inoculation of strains XS11, ΔVdSsk2, ΔVdSte11, and the respective complemented strains. Black arrows point to invasive hyphae (IH) and hyphae (HY). Bar = 20 μm.