Horticulture Research International, Stockbridge House, Cawood, North Yorkshire, Y08 3TZ, UK

Background and objectives
It was the increased occurrence of persistent soilborne root disease that helped 'persuade' the salad crops industry to utilise inert substrates eg rockwool in a hydroponic environment. This change to 'sterile' growing media initially alleviated root disease problems though the oomycete fungi [1], particularly Pythium and Phytophthora spp, recurred as the industry intensified.

Most growers of hydroponic crops currently use an 'open' system (OS) of culture where excess applied nutrient solution is discarded and, as a result, pathogen spread is limited. As legislative, environmental and financial pressures have increased growers have been encouraged to adopt 'closed' systems (CS) to minimise ground-water contamination by nitrates, phosphates and pesticides by re-using the run-off solution. The perceived risk of dissemination of root pathogens in the recycled nutrient solution currently deters many growers from adopting CS. Yet, conversely there is increasing anecdotal evidence from growers who, having adopted CS (including NFT), have successfully avoided severe root disease problems. The objective is to determine if root disease in CS can be suppressed biologically and whether a sustainable approach to disease control in this unique hydroponic environment can be developed.

Results and conclusions
The potential for dissemination of pathogen propagules in CS has now been well demonstrated [2]. To minimise this spread it should be possible to disinfect the hydroponic solution prior to re-use. Extensive research, undertaken to assess the relative performance of various disinfection techniques (eg Pasteurisation, UV, Ozone, Ultra/Micro-filtration, Peroxygens) has shown that many techniques will effectively prevent the dissemination of one or more of the commonly occurring root pathogens [3]. Yet, the wider adoption of these CS is likely to be marred in some countries (in the absence of government 'grant' intervention) by the high capital investment required and the lack of an improved financial return from the crop.

In commercial-scale trials, specifically with oomycete fungi, spore dispersal in CS, as opposed to OS, is invariably rapid, yet disease development would appear, on occasions, to be suppressed. In tomato inoculated with P. cryptogea, a disease suppressive potential has been generated within the recirculating hydroponic solution and this has prevented expression of characteristic symptoms of the disease when compared with an equivalent OS. Various mechanisms, including direct microbial antagonism, indirect microbial antagonism by the liberation of antibiotics, siderophores, and biosurfactants into solution and by the stimulation of host defence systems through systemic induced resistance, have been proposed to account for the effect.

If a technologically simple, yet sustainable, approach to root disease control within this specialised environment is to be achieved then a much clearer insight into the importance and function of specific microbial components in the root zone must be gained. If components of the rhizosphere microflora are responsible for generating a suppressive potential in CS then the use of 'active' (eg Heat, UV, Ozone), as opposed to 'passive' (eg Filtration), disinfection technology is untenable. Instead, the research effort should concentrate on improving our understanding of the microbial processes occurring within the root zone aiming to harness, and possibly enhance, the overall suppressive activity. The effective integration of 'passive' disinfection technology eg slow sand, or other, filtration techniques may offer good long-term potential in this respect.

1. Stanghellini ME, Rasmussen SL, 1994. Plant Disease 78, 1129-1138.
2. McPherson GM, 1995. Med. Fac. Landbouww. Univ. Gent, 60/2b, 371-379.
3. Runia WTh, 1995. Acta Horticulturae, 382, 221-229.