Partial saturation under ebb and flow irrigation suppresses Pythium root rot of ornamentals

The timing of ebb and flow cycles for flooded floor irrigation systems was manipulated to alter the water content of the root medium. Ornamental species were grown under the different irrigation regimes to determine if the spread and severity of Pythium root rot was affected by altered sub-irrigation delivery practices. One floor delivered and drained water quickly to achieve a partial saturation of the root medium (approximately 25% less soil water) so that minimal leachates drained from the pot. The other floor operated more slowly as a standard commercial delivery system allowing the pots to imbibe more water. During 2007–2009, poinsettia, geranium or chrysanthemum crops were grown on each floor in five separate experiments. For each experiment, 15 plants were artificially inoculated with Pythium aphanidermatum or Pythium ultimum and set on the floor in a line immediately adjacent to a row of drainage ports. Disease was monitored by evaluating foliar symptoms and by rating the root systems for percent root rot. In three of the five experiments, disease was significantly reduced under partial saturation as compared to the standard saturation. In some instances, root infection as evidenced by re-isolation of the pathogen on agar, was less on plants receiving partial saturation than on standard saturation. Plants were smaller, more compact and of higher horticultural value under partial saturation than with standard saturation. These findings should encourage growers to adopt partial saturation management practices when using ebb and flow irrigation in order to reduce damage from disease after Pythium inoculum is present.

► We manipulated irrigation cycles of ebb and flow greenhouse floors.
► Potted ornamentals inoculated with Pythium spp were healthier on partially saturated floors.
► Horticultural quality was increased under partial saturation.
► Growers should adopt partial saturation when using ebb and flow irrigation.