ROOT DISEASES IN HYDROPONICS -- THEIR CONTROL BY DISINFECTION AND EVIDENCE FOR SUPPRESSION IN CLOSED SYSTEMS
G M MCPHERSON
Horticulture Research International, Stockbridge House, Cawood, North Yorkshire, Y08 3TZ, UK
Background and objectives
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
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.