Pseudomonas chlororaphis and organic amendments controlling Pythium infection in tomato

Postma, J.; Nijhuis, E.H.


To create more resilient growing systems for greenhouse crops, the efficacy of different organic amendments alone and in combination with the biocontrol strain Pseudomonas chlororaphis 4.4.1 was tested to suppress Pythium disease in tomato plants. Four independent greenhouse experiments were performed with young tomato plants in potting soil. Inoculating the pathogen Pythium aphanidermatum to the potting soil prior to sowing resulted in significant losses of tomato plants; i.e. 94–98% healthy plants in pathogen-free control compared to 43–68% healthy plants when the pathogen was added. P. chlororaphis 4.4.1 inoculations increased the number of healthy plants in the potting soil up to 80% on average; soil and seed treatment were both effective. Numbers of P. chlororaphis in the rhizosphere had increased 4 to 100 fold 3 weeks after its inoculation (qPCR detection). All compost types reduced Pythium disease in potting soil resulting in 80–95% healthy plants. Animal bone char was not effective against P. aphanidermatum, whereas with plant-based biochar there was an effect, although not significantly different from the control treatment. Phosphorous and potassium uptake by the plants were elevated by the different organic amendments. These results demonstrate the potential of the organic amendments to enhance sustainability of growing media such as potting soil by increasing its disease suppressiveness, the re-use of nutrients and replacement of peat by using organic amendments. In addition, inoculating the growing medium or tomato seeds with the biocontrol strain P. chlororaphis 4.4.1 enhanced Pythium control in the susceptible growing media.