Enabling adaptation to water scarcity: Identifying and managing root disease risks associated with reducing irrigation inputs in greenhouse crop production – A case study in poinsettia

Abstract

Greenhouse and nursery growers consume up to 19 thousand gallons of water per acre per day. Implementation of water conservation technologies to mitigate water resource depletion is imperative but recent studies indicate that associated drought stress may enhance losses from root diseases. The overall objective of this study was to evaluate and manage disease risks associated with reducing irrigation in commercial production of a containerized annual nursery crop (poinsettia), and to improve water-use efficiency without increasing losses from root disease. Results from a two-year commercial greenhouse trial of poinsettias indicate that minor reductions irrigation application vs. the standard practice (grower irrigation) resulted in 9.4% water savings without affecting Pythium root rot (Pythium aphanidermatum) (P = 0.13), although pathogen recovery from roots was 20% higher under reduced irrigation. When poinsettias were inoculated with P. aphanidermatum in controlled greenhouse experiments, plant health and pathogen infection incidence were unchanged when substrate volumetric water content (VWC) was reduced from 45% to 35%, with a 13.8% reduction in irrigation volume. However, incidence of plant decline increased by 50% under 25% VWC and shoot growth was significantly lower (P < 0.0001) compared to plants under higher VWC. When a plant growth promoting bacteria (Companion – Bacillus subtilis GB03) was added to infected plants, shoot growth was significantly increased under the 45% VWC treatment (P < 0.05). These studies indicate that irrigation inputs can be decreased by an estimated 23.2% in the commercial greenhouse without affecting Pythium root rot. Below a minimum irrigation threshold (25% VWC), plant–pathogen dynamics may be altered to favor root disease development. Addition of biological products may serve to mitigate the negative effects of reducing irrigation inputs. To the authors’ knowledge, this is the first commercial-scale study to evaluate biotic risks of sensor-network-based water use adaption; this study provides foundational information for similar studies in other greenhouse and nursery crops.