ELIMINATION OF ROOT-INFECTING PATHOGENS IN RECIRCULATION WATER FROM CLOSED CULTIVATION SYSTEMS BY ULTRA-VIOLET RADIATION

The development from growing in border soil to soilless cultures has not resulted in the disappearance of soil-borne diseases. Most root-infecting pathogens also occur in these new cultivation systems. Some pathogens such as Pythium, Phytophthora, cucumber green mottle mosaic virus and tomato mosaic virus are easily transmitted in recirculation water as is Olpidium, the vector of several viruses. To exclude any risk of dispersal of plant pathogens the water has to be sterilised before re-use. Rainwater collected from the glasshouse roof can also be contaminated with pathogens. When this rainwater is used for irrigation water it should be disinfected first.

Currently water disinfection by heating or ozonisation is applied on some 450 nurseries.

Initially ultra-violet(UV) radiation was not effective against plant pathogens. Only at a low capacity of 18 l h^-1 and a high UV-dose of 430 mJ cm^-2 could fungal spores be eliminated in nutrient solution. A low capacity however implies a poor turbulence in the water, resulting in varying UV-doses in the different water layers. Two new UV-installations with high capacities were tested for their efficacy against fungal and viral plant pathogens. One installation was equipped with a high-pressure and the other with a low-pressure mercury vapour lamp. In both installations a sandfilter first removed organic particles from the water. A UV-dose from the high-pressure lamp of 28 mJ cm^-2 reduced the infectivity of conidia of Fusarium oxysporum f.sp lycopersici by 90 % resulting in a 99.9 % reduction at 84 mJ cm^-2. The low-pressure lamp completely eliminated conidia of Fusarium at a UV-dose of 70 mJ cm^-2. Tomato mosaic virus was reduced by 99 % using a UV-dose of 100 mJ cm^-2 in both installations.

Ultra-violet radiation can be effective against plant pathogens providing the dose under controlled conditions is sufficient. Numerous growers use UV-radiation for the disinfection of water in closed cultivation systems.