IS DAMPING-OFF, CAUSED BY PYTHIUM, LESS OF A PROBLEM IN HYDROPONICS THAN IN TRADITIONAL GROWING SYSTEMS?

Pythium is commonly found in hydroponic growing systems where it can live as a harmless saprophyte growing externally on roots. It was found that the ability of a zoospore-producing isolate to infect and rot the roots of lettuce plants could be increased by adding sucrose to the inoculum. A similar result was achieved when sucrose was applied to the plants via the leaves or roots. It is reasoned that the sucrose treatments stimulated root exudation, thereby providing more microbial substrate at the infection sites along the roots. This extra energy increased the virulence and therefore the inoculum potential of the isolate, enabling it to infect and successfully overcome the resistance of the plant.

It is reasoned that hydroponic systems of the recirculating type are normally poor providers of microbial substrates. The organic matter content of natural soils is lacking and root exudates are carried off by the flow and greatly diluted in the nutrient solution. The rhizosphere effect is thus greatly weakened. Furthermore, because of competition by other microorganisms for the sparse supplies of energy etc., Pythium is prevented from building up a sufficiently high inoculum potential to enable it to successfully infect the host.

It follows that one approach to the management of Pythium disease problems in hydroponic systems is to ensure that the fungus remains in its saprophytic phase. This might be achieved by the establishment of microbial populations selected for their ability to preferentially use up the substrates necessary for the virulence of Pythium. Alternatively, it might be possible to achieve the same result by removing these materials from the nutrient solution by means of filtration.

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