Infection mechanisms of Botrytis species: pre-penetration and pre-infection processes of dry and wet conidia

Light and electron microscope studies of the infection mechanisms of two species of Botrytis, B. cinerea and B. fabae, axe described. Conidia were inoculated dry or in the presence of aqueous glucose onto glass and Vicia faba leaves and incubated in a moist chamber. In all cases, dry-inoculated conidia germinated rapidly to produce short germ-tubes only. In contrast, in the presence of aqueous glucose, conidia germinated to produce long germ-tubes which extended across the substratum. Low temperature SEM of B. cinerea conidia confirmed these results and showed further that in the presence of aqueous glucose, the germ-tubes were enclosed by an extensive sheath of fibrillar-like material. No such matrix material was found to be associated with the conidia or germ-tubes of dry-inoculated conidia. Immunofluorescence microscopy and immunogold labelling of ultrathin sections of B. fabae-infected bean leaves were carried out using the Botrytis-specific monoclonal antibody, BC-KH4. In every case antibody-binding was observed at the surface of both dry and wet conidia. Furthermore, when conidia were inoculated in the presence of aqueous glucose, antibody-binding was also located throughout the fibrillar-like matrix material associated with conidia and germ-tubes. At the TEM level, traces of amorphous matrix material were found to be associated with the short germ-tubes of dry-inoculated conidia at the site of penetration of the leaf surface. This amorphous matrix material also immunolabelled with BC-KH4. Low-temperature SEM of the macroscopic lesions formed on infection of bean leaves by Botrytis species also revealed that these lesions corresponded to clearly defined areas of collapsed epidermal cells.