Control of root zone pH is not effective in preventing Pythium aphanidermatum disease in cucumber

Pythium aphanidermatum is the most wide-spread root pathogen in greenhouse cucumber. Although environmental condi-tions are known to affect the disease severity and many in vitro studies have documented these effects, only few in vivo experiments have been conducted, mainly focusing on the effect of temperature. In the present study, the impact of the pH in the root environment was investigated. Mature cucumber plants were grown in aerated nutrient solution at pH values of 4.0, 5.5 and 7.0, and inoculated with P. aphanidermatum. Experiments were carried out at temperatures of 20, 25 and 30°C. No clear effect of root zone pH on mycelium density in the roots measured by means of an indirect ELISA, and growth reduction of inoculated cucumber plants could be observed, probably due to different responses of the mycelium, oospores and zoospores of P. aphanidermatum to pH.