Partial saturation ebb and flow irrigation and silicon to suppress Pythium root rot of poinsettia

A greenhouse equipped with two flooded floors with separate reservoirs was used to determine if partial saturation versus more complete saturation of the root medium, with and without silicon addition, could suppress Pythium root rot of poinsettias. The partial saturation floor was designed to deliver and drain water quickly. The control floor operated with longer watering to achieve close to full saturation. The partial saturation floor used approximately 25% less water than the control floor over the life of a crop. Rooted poinsettias were grown on each floor in 2010, 2011, and 2014. A K2SiO3 solution was added to the root medium several times in each season. Controls received K2CO3. In 2010 and 2011, fifteen plants were artificially inoculated with Pythium aphanidermatum and placed randomly on each floor. In 2014, one half of the plants were artificially inoculated with Pythium aphanidermatum. Disease was monitored by evaluating plant weights, uptake, and retention of silicon, and by rating the root systems for percent root rot. In 2010 and 2014, disease was significantly reduced under partial saturation compared to control watering. There were some effects of silicon, but it never halted the onset of symptoms or reduced the damaging effect of disease on plant growth. There were no interactions of effects of watering and silicon on progress of disease. Plants were smaller and more compact under partial saturation than under control watering. These findings should encourage growers to adopt partial saturation on their flooded floors in order to reduce damage from disease after inoculum has been accidentally introduced. •Does irrigation with less water suppress Pythium root rot of poinsettias?•Disease was reduced under partial saturation compared to control watering.•Silicon never halted onset of symptoms or reduce damaging effects of disease.•Interactions of effects of watering and silicon did not affect progress of disease.•Plants were more compact under partial saturation than under control watering.