Ultrastructural and cytochemical studies on the infection process of Sclerotinia sclerotiorum in oilseed rape

Stem rot, caused by Sclerotinia sclerotiorum (Lib.) de Bary, is one of the major diseases of oilseed rape worldwide. The infection process of S. sclerotiorum in leaves and stems and the alterations of cell wall components in the infected host tissues were examined by electron microscopy and cytochemical labelling techniques. One day after inoculating (dai) leaves and stems with fungal grown agar disks, dense mycelial networks were usually formed on the inoculated tissues. Then, the infection cushions of different size were developed. Mucilage produced by the pathogen covered mycelium and infection cushions. Hyphae forming infection cushions were often flattened and increased in diameter. After removing the infection cushions, the diameters of the numerous pores through which penetration pegs had entered the cuticle of leaves and stems were small and had almost the same diameter. Penetration of leaves and stems occurred 2 and 3 dai, respectively. Small changes in cuticle were observed. After penetration, hyphae of the pathogen extended between the cuticle and epidermal cell walls as well as inside the epidermal cell walls. Then inter- and intra-cellularly spreading hyphae were observed in the hemi- and ultra-thin sections by light- and transmission electron microscopy, 5 dai. Hyphae also colonized xylem and phloem. During colonization marked alterations in the host tissues were detected, including disorganization of cytoplasm, cell organelles, disintegration of cell walls and collapse of host cells. The enzyme- and immunogold-labelling investigations showed obvious degradation of cellulose, xylan and pectin in the host cell walls of infected tissues. The degradation of cell wall components suggests that the pathogen may secrete cell wall degrading enzymes (Cwdes) such as cellulases, xylanases and pectinases during infection and spreading in the oilseed rape tissues.