Since the adoption of closed subirrigation systems many concerns have arisen regarding the spread of soilborne plant pathogens through the irrigation water. During the last four years, we have conducted surveys and controlled experiments to determinethe risks of dispersing fungi such as Pythium, Phythophthora, (root rots) and Fusarium (vascular wilts) and bacteria such as Erwinia spp. (soft rot) through these irrigation systems. In addition to determining the movement of infective propagules (namely infective spores of fungi and bacterial cells) in closed irrigation systems, the surveys conducted in commercial greenhouses (growing a variety of floricultural crops), were also aimed at measuring typical population densities of the pathogens mentioned above. The population densities in the storage tanks and levels of disease observed in the growing benches of the commercial greenhouses sampled were used lo establish disease causing thresholds.
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