Population dynamics of Pythium aphanidermatum in cucumber grown in closed systems.

Foot and root rot in cucumber, caused by Pythium aphanidermatum (Edson) Fitzp., is an economically important disease in soilless culture systems. Nevertheless, very few data are available on the populations of this pathogen. Therefore, two detection methods, nested PCR (polymerase chain reaction) and plating on a selective medium after concentration of samples, were optimised and evaluated. With both methods very low concentrations of P. aphanidermatum could be detected; i.e. the detection limits were around 0.05 CFU/ml nutrient solution. In addition, real-time quantitative PCR using a Molecular Beacon probe was designed and tested. The potential and limitations of the different detection methods are discussed. With these different detection techniques, the population dynamics of P. aphanidermatum in a cucumber crop was followed. The impact of different disinfection treatments was studied in a greenhouse experiment with a cucumber crop growing on rockwool slabs in 12 independent closed systems. The nutrient solution was recirculated without disinfection (control), after UV-irradiation (250 mJ/cm2), or after slow sand filtration treatment. Part of the crop was inoculated with an isolate of P. aphanidermatum. The non-inoculated part could only become infected through the recirculated nutrient solution. Disease symptoms (stem rot, wilt, and root rot) and the yield loss were recorded in addition to the population dynamics of the pathogen. Very clear differences in the spread of the pathogen and in disease symptoms were measured between the systems with and without disinfection. UV-irradiation and slow sand filtration were both effective in removing the pathogen and protected the crop from disease symptoms. Correlation indices between the final yield and the different measurements during the experiment were calculated.