Research Station for Floriculture and Glasshouse Vegetables, P.O. Box 8, 2670 AA Naaldwijk, The Netherlands

Background and objectives
The development from growing in border soil to soilless cultures has not resulted in the disappearance of soilborne diseases. Most root-infecting pathogens also occur in these new cultivation systems. Recirculation of the nutrient solution implies a risk for the dispersal of pathogens. Some fungal pathogens such as Pythium and Phytophthora are easily transmitted via recirculation water as is Olpidium spp., the vector of several viruses. Viruses belonging to the tobamogroup can be transmitted via the recirculation water as well as plant pathogenic nematodes like Pratylenchus vulnus. To exclude any risk of dispersal of plant pathogens the recirculation water has to be disinfected before re-use. Disinfection methods for recirculation water were investigated in the Netherlands to establish the options for growers for disinfection against the specific plant pathogens.

Materials and methods
Disinfection equipment was installed in a glasshouse with a closed cultivation system. Recirculation water from a vegetable crop was collected and infected artificially with plant pathogens. Fusarium oxysporum was test pathogen for fungal diseases, although in some additional experiments the recirculation water was infected with a Phytopthora sp. Tomato mosaic virus was the test pathogen for the group of tobamoviruses and Radopholus similis was the nematode tested. The infected recirculation water passed the disinfection equipment, after which the infectivity of the suspensions was tested in comparison with untreated infected recirculation water. The infectivity of the pathogens was established in bioassays, although selective media were used as well for the detection of the fungal pathogens.

Results and conclusions
Some disinfection methods proved to be effective against all plant pathogens tested. A heating system on the basis of heat exchangers is effective against all plant pathogens. Ultra-violet radiation (UV) is another option, although different pathogens require different UV-doses. Ozonation also can be used for a complete disinfection, but for disinfestation against nematodes UV-treatment is economically more feasible.

Other disinfection methods turned out to be effective only against a selective group of pathogens. Slow sand filtration and lava filtration are effective against Pythium spp. and Phytophthora spp. These methods are not sufficiently effective against the other pathogens tested. UV-radiation can also be applied selectively against fungal pathogens or nematodes. For different reasons some methods are not options for practical use, like membrane filtration and iodination.

1. Runia WT, 1995. Acta Horticulturae 382, 221-229.
2. Runia WT, Amsing JJ, 1996. ISOSC Proceedings, 381-393.