Curious biological phenomena occur where land and sea alternate at low and high tides. Large crop-like populations of the perennial sea lavender (Limonium vulgare) appear and disappear over the years. Such vegetations are of great beauty, especially at bloom, and form a tourist attraction. Despite the extreme physical conditions they teem with life, among which are fertilising insects and fungi which cause plant disease. This book describes how long-lived sea lavender populations come and go, and how the fungal diseases "rust" and "mildew" may contribute to their going. The temporal and spatial distributions of rust and mildew epidemics are described. The annual cycle and the perennation of the rust (Uromyces limonii) are studied in detail. The unusual but important roles of sea water in the dispersal of the host plant and of its rust are sketched. The possible function of differential host resistance and of differential rust virulence is indicated. Data on the biology of sea lavender mildew (Erysiphe limonii) suggest that mildew and rust differ in lifestyle. Tolerance of sea lavender plants to rust and mildew is heritable and important, but does it last forever? Some theoretical implications for botany and phytopathology are tentatively discussed, especially the role of equilibrium or homoeostasis. However, the main focus of the book is on the facts collected in the salt marshes of the barrier island Terschelling, in the north of the Netherlands.This book with about 100 tables and some 100 figures provides new information of importance to botanists, plant pathologists, population biologists and general ecologists.