Root Disease Incidence and Severity in Nursery Crops with Soil Moisture Sensor-based Automated Irrigation

Conventional (e.g. timer-based) irrigation control in commercial ornamental production is based largely on grower intuition and experience with a crop. When in doubt, growers typically irrigate excessively out of precaution. Elevated soil moisture content has been shown to predispose crops to root pathogen infection and thereby increase crop losses. Losses from disease for some specialty crops can approach 30%, significantly impacting growers’ profitability.  Preliminary data and observations from studies conducted in commercial nurseries have suggested a relationship between the use of soil moisture sensor based automated irrigation and a reduction in disease-related mortality. In this study, an irrigation controller, connected to dielectric moisture sensors, was used to maintain substrate volumetric water content (θ) when θ dropped below a programmed threshold. Three irrigation treatments were examined, with the first two treatments maintaining θ at 0.20 m³·m^-3 and 0.30 m³·m^-3, respectively. The third treatment sought to replicate the soil moisture profile of conventional irrigation patterns by allowing θ to dry to 0.20 m³·m^-3 before an irrigation event that raised θ to 0.30 m³·m^-3. Three species (Rhododendron sp. ‘Roseum Elegans’, Kalmia latifolia ‘Sarah’, and Pieris japonica ‘Prelude’) were grown under each irrigation treatment. Half of the plants in each irrigation treatment were inoculated with Phytophthora nicotiana and growth and mortality were tracked throughout the trial. Variations in mortality were observed among treatments and only noted in inoculated plants.