Invasion of sugar beet leaves by Ahlburg wild type DsRed and Ahlburg ctb2 disruption strain 2-1- DsRed. A Fluorescence microscopy of leaf cross sections at 21 days after inoculation showed hyphae of the Ahlburg wild type DsRed invading mesophyll cells. B Leaf cross sections at 21 days after inoculation with the gene disruption strain did not show invasion of subepidermal layers. The hyphae were not closely attached to the surface and easily dislocated. C and D 21 days after inoculation the infection was at its peak and symptoms were prominent on the leaves infected with wild type. C Cross section through a leaf spot lesion of a leaf infected with wild type. Many hyphae colonize the entire tissue. The loss of blue autofluorescence of the plant cell walls indicates degradation of the host tissue. D Cross sections of leaves infected with the gene disruption strain showed strongly increased blue autofluorescence of the plant cell walls, but no hyphal growth. Scale bars: A and B = 100 μm, C and D = 20 μm. Abbreviations: e extracellular space, ec epidermis cell, ew epidermal cell wall, h hyphae, mc mesophyll cell.