Are Primary Germ Tubes of Conidia Unique to Erysiphe graminis?

Abstract

Conidia of Erysiphe graminis f. sp. hordei and E. graminis f. sp. tritici produce primary germ tubes that breach epidermal walls of host plants several hours before appressoria are formed. The present study, using micromanipulation in the scanning electron microscope, examines whether or not some other Erysiphe species, E. pisi, E. cichoracearum and E. polygoni also develop primary germ tubes or similar structures. For comparison, E. graminis f. sp. hordei and E. graminis f. sp. agropyri were also examined. Conidia of E. cichoracearum produced only a single germ tube during the initial 16hr after inoculation, and only a few per cent of E. pisi and E. polygoni conidia had more than one tube. Furthermore, penetration pores were only observed below appressorial tubes of these species. By contrast, conidia of E. graminis f. sp. hordei and E. graminis f. sp. agropyri produced both short primary germ tubes which breached epidermal walls of host plants and appressoria that penetrated and developed haustoria. From these results and the literature, we conclude that primary germ tubes may be unique to and thus represent a characteristic feature of E. graminis development.