2018 ASHS Annual Conference

Biofilms are ubiquitous on the inside surface of irrigation pipes. Biofilm in biological filters remove plant pathogens and nutrients from recirculated and wastewater. The objective of this project was to determine if biofilms in irrigation pipes could affect plant pathogens and nutrients from the nutrient solution and therefore affect plant health or quality of poinsettia (Euphorbia pulcherrima cv. Classic Red). The experimental design was a blocked split-plot design in which the initial presence or absence of biofilm in pipes was the main factor, and inoculation with Pythium aphanidermatum was the sub-factor. The experimental unit consisted of a drip irrigation line with 5 and 6 plants for experimental run 1 (R1) and 2 (R2), respectively. Each block had two polyvinylchloride (PVC) main lines of which one had a pre-established biofilm and the other was made of new unused pipes. The main lines were split in two lines of which one was inoculated with a 5-day old P. aphanidermatum and the other had no pathogen. Bacteria attached to the surface, disease incidence, solution and media pH and electrical conductivity, relative chlorophyll content (SPAD), and plant height were measured weekly. Plant height, SPAD, total bract area, total leaf area, and shoot dry weight were measured at harvest. The amount of bacteria attached to the pipe was used as indicator of biofilm presence in the pipes. Interaction between biofilm and Pythium was observed only for disease incidence by week, and SPAD at harvest in the first experimental run. Disease incidence was 58% and 23% in plants inoculated with Pythium compared with 0% in the controls for R1 and R2, respectively. The plants that grew with solution coming from pipes with the pre-established biofilm were 2.7 cm taller, had 4.1 higher SPAD units, and weighed 7.6 g (dry mass) more than the plants irrigated with pipes with no initial biofilm in R1. Overall plants with P. aphanidermatum were smaller and had higher disease incidence compared with the control. Plants with P. aphanidermatum were 6.7 cm shorter, had 8.8 lower SPAD units, 2,352 cm^-2 smaller bract area, 2,263 cm^-2 smaller leaf area, and weighed 26.4 g (dry mass) less than plants without P. aphanidermatum. These results suggest that irrigating plants with pipes that contain biofilm may slightly benefit overall plant health.