2017 ASHS Annual Conference

Pythium is a common disease of poinsettia, causing millions of dollars in annual economic loss and expenses in the U.S. ornamental plant industry. Our study compares Pythium incidence between poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) grown in a peat-based substrate of different volumetric water contents (VWC). Sensor Controlled Irrigation (SCI) was used to control substrate water content and included a slight-deficit irrigation treatment. Deficit irrigation is a method of managing water use for a variety of purposes, including increasing the efficiency of plant water use. However, in some cases, root stress caused by deficit irrigation may promote disease incidence. This 9-week study took place at the University of Maryland Research Greenhouse complex. Poinsettia plants underwent three SCI treatments where substrate VWC (and matric potential) were maintained at 45% (near saturation = > -10 kPa), 35% (container capacity = -15 kPa) and 25% (slight deficit = -40 kPa). We inoculated one-half of the plants in each irrigation treatment with P. aphanidermatum. Additionally, we treated one- half of the plants from each treatment combination with Companion (Bacillus subtilus) to create 12 treatment combinations for this study. We assessed plant height and disease incidence weekly. For the final harvest at the end of 9 weeks, we measured shoot dry weight and fresh root weight, along with Pythium disease incidence. At the end of the study, disease incidence in all inoculated plants, regardless of Companion treatment, was 28%, 36% and 42% in the 45%, 35% and 25% VWC treatments, respectively. Pythium recovery in inoculated plants was higher in the 25% VWC treatments, regardless of Companion inoculation. Interestingly, plant shoot dry weights and plant height were not different among all treatment combinations. Root fresh weight was significantly lower in 25% VMC, but this may be an artifact of low moisture content in root tissue. Root dry weights may have shown no differences. The weekly disease assessments generated an Area Under the Disease Progress Curve (AUDPC) across all 12 treatments. Square root transformed data revealed that disease progression was significantly higher only with plants treated with 25% substrate VWC and inoculated with Pythium and Companion. Based on this research, higher risk may be presented by employing deficit or near-deficit irrigation management to disease prone species like poinsettia. However, we performed Pythium inoculations under well-watered conditions. It is possible that inoculation under lower substrate VWCs may have suppressed Pythium.