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| Suppression of Pythium and Phytophthora in irrigation systems | |
| As part of a move to cut costs and conserve resources in sustainable agriculture interest in re-circulating re-cycled irrigation water has greatly increased. Comparisons between ‘active’ (e.g. ultra violet light) and ‘passive’ e.g. slow sand filtration (SSF) treatment were made in several countries in an EU-funded MIOPRODIS project, but no consistent differences in disease suppression were found. This contrasts with the evidence found at Horticultural Research Institute’s centre at Efford, UK. In both systems there was no spread of disease from infected plants to the rest of the system but the incidence and severity of symptoms in these infected plants were between 40 and 75% greater in an active ozonation-treatment than in the SSF- treated beds. SSF involves a combination of physical filtration and microbial activity within the sand to eliminate unwanted plant pathogens. Sample cores taken from 3-year-old HRI Efford sand beds removed 80% of applied the Phytophthora spores that passed through ‘quarry-fresh’ sand as a control. These sand cores were 20 cm deep and a mature SSF sand column of this depth would remove between 70 and 100% of the microbes. Sand columns of no less than 50 cm are recommended. Whilst SSF give the most reliable and consistent suppression they are in many ways the most difficult to assess. More recent work was undertaken on the suppression of Pythium and Phytophthora in SSF systems, re-circulating and run-to-waste hydroponic feed solution and peat. One important conclusion is that the composition of the micro-flora is different each time a filter is set up. Many different groups of microbes are able to interact in the sand to produce the right conditions for effective SSF. This explains why these SSF systems are so robust and versatile over a wide range of temperatures, mineral and organic matter concentrations. In all three systems, mixed microbial populations seemed to be responsible for suppression and no single organism or group of organisms stands out as responsible. “Grower” Tim Pettitt (Eden Project) & Professor John Whipps, University of Warwick www.warwick.ac.uk |
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