THE EARLY DEVELOPMENT OF ERYSIPHE-PISI ON PISUM-SATIVUM L

Erysiphe pisi, the powdery mildew pathogen of Pisum sativum, followed a developmental sequence that allowed the identification of 10 distinc t growth stages (GS) over 30 h following inoculation. The growth stage s were ungerminated conidia (GS1), germinated conidia, having produced a germ tube (GS2), germlings where the germ tube had forked (GS3), ge rmlings with a multi-lobed germ tube (GS4), germlings with a single hy pha (GS5), germlings with two (GS6 and 7) or three (GS8 and 9) hyphae, one of which may have formed from the appressorium (GS7 and 9), and g ermlings with abnormally long germ tubes (GS10), which did not develop hyphae. Conidia germinated rapidly, with a quarter of conidia produci ng germ tubes by 2h after inoculation (hai). Most germlings produced m ulti-lobed appressoria, which showed considerable variation in structu re. Haustoria, although often difficult to visualize, were first seen 4 hai, and the first hyphae 14 hai, growing from the body of the conid ium. Subsequent hyphae developed from both the body of the conidium an d from the appressorium.