INFECTION OF BEECH LEAVES (FAGUS-SYLVATICA) BY THE ENDOPHYTE DISCULA-UMBRINELLA (TELEOMORPH, APIOGNOMONIA-ERRABUNDA) - LOW-TEMPERATURE SCANNING ELECTRON-MICROSCOPY STUDIES

The sequence of events leading to infection by Discula umbrinella of l eaves taken from axenically grown beech seedlings was studied at the u ltrastructural level. On the host surface, even under optimal conditio ns and when contact with the host is established, only approximately 7 0% germination can be observed. In hanging drops containing malt extra ct, germination is only approximately 10%, suggesting that abundant nu trient supply alone is not enough to promote germination. Incubation o f conidia in leaf extracts, on the other hand, results in the inductio n of germination even if no contact with an appropriate support is est ablished. Between 16 and 24 h after infection four stages of germinati on can be observed on beech leaves: (i) ungerminated conidia; (ii) ger minating conidia producing long superficial hyphae; (iii) conidia with distinct, appressorium-like swelling at the end of the germ tube; and (iv) germinated conidia with a halo around the tip of the germ tube. Transmission electron microscopy shows that subcuticular penetration i nto the host occurs at places where halos are seen. Apparently only ge rmination with formation of appressorium-like structures and halos lea ds to successful penetration. Application of conidia to the abaxial si de of the host leaf results in the formation of structures similar to those observed on the adaxial surface. Conidial germ tubes penetrate t he cuticular layers of the outer edges of the guard cells, but direct penetration through the stomata was rarely observed. Appressorium or h alo formation, observed only on the host surface and never on other su bstrates, suggests host-specific induction of penetrating structures. The mode of penetration observed for D. umbrinella bears some similari ties with that described for some biotrophs and pathogens, suggesting affinities between endophytes and pathogens.