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 distinct growth stages (GS) over 30 h following inoculation. The growth stages were ungerminated conidia (GS1), germinated conidia, having produced a germ tube (GS2), germlings where the germ tube had forked (GS3), germlings with a multi-lobed germ tube (GS4), germlings with a single hypha (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 germlings with abnormally long germ tubes (GS10), which did not develop hyphae. Conidia germinated rapidly, with a quarter of conidia producing germ tubes by 2 h after inoculation (hai). Most germlings produced multi-lobed appressoria, which showed considerable variation in structure. Haustoria, although often difficult to visualize, were first seen 4 hai, and the first hyphae 14 hai, growing from the body of the conidium. Subsequent hyphae developed from both the body of the conidium and from the appressorium.