But different water sources carry different levels of risk, with even river water showing occasional contamination spikes. "With rainwater run-off it's the material that collects in the gutters and downpipes, and on the roofs themselves if not kept clean, that can be a source of Pythium and Fusarium," he warned.
Water recirculation "isn't a problem if it's kept clean, but if not can quickly spread disease around your crop", he said, adding that to deal with these risks "there are different forms of treatment appropriate for difference situations, many available off the shelf, with more upcoming", though "some things are too fiddly to do on a large scale and can be a drain on staff time".
Ultraviolet systems such as Priva's Biolux "have gone from strength to strength" but these and oxidation "mean you kill all your friends in the water too, leading a power vacuum", he said. "You can artificially fill it again with things like Trichoderma, but this is something that requires a lot more investigation."
Meanwhile, "any turbidity can interfere with ultraviolet" with performance falling off when less than 60 per cent of the light penetrates through 10mm of water, necessitating pre-filtration.
Ozone treatment "is very effective and like ultraviolet has no downstream residuals", he said. "It's widely used in the drinks industry but has never really taken off in horticulture, so prices haven't come down much and there isn't the depth of technical support." Peroxide similarly avoids problematic by-products "though there isn't much good data on dosing and exposure times", he said.
Heat treatment "was a hot topic in the 1990s and was widely adopted in the Netherlands, but has possibly had its day due to the energy cost", he added. Slow sand filtration "can be built in the nursery so isn't promoted so much".
He pointed out: "Cleaning the sand bed is the least popular job on the nursery," but the addition of China clay "avoids the formation of a biofilm layer on the sand, so a layer at the top will greatly reduce the frequency with which it needs cleaning". CGA technical officer Derek Hargreaves added: "Slow sand filtration is just a tank and a few bits of pipe. No one makes any money on it but that doesn't mean it's not as good."