A new application method for the systemic fungicide metalaxyl to control poinsettia (Euphorbia pulcherrima Willd. ex. Klotzsch) root rot caused by Pythium ultimum Trow

The poinsettia cultivar “Freedom Red”, highly susceptible to Pythium ultimum was used to explore a better, more effective and environmentally sound application method of the true systemic fungicide metalaxyl. Three different rates of fungicide were used to control root rot. They were applied both as a drench and in paint/fungicide mixes as a coat to the interior of the containers. How much active ingredient was leaching-out of the containers was determined for both methods of application. The development of a disease assessment method was also described. The arbitrary and subjective qualitative scale was compared with the use of traces of the roots for quantify Disease Severity Index (DSI) using digital imaging technology.; The P. ultimum inoculum used in this study was effective in causing disease and death of poinsettia cuttings within three–four weeks after planting in the Pythium infested potting mix (LC-1). Treatments with paint alone, paint/fungicide mixes as well as the higher rates of metalaxyl were not phytotoxic to poinsettia plants. Highly significant differences in DSI were found between plants inoculated with Pythium alone and control plants grown in containers coated with paint but no fungicide. Both methods of application had similar efficacy in controlling Pythium. Results indicates that the paint/fungicide method although as effective in controlling Pythium root rot is not more environmental friendly than the traditional drenches. Metalaxyl is a highly polar and water-soluble chemical, as a consequence, it was easily eliminated from the containers.; Even though the root tracing method for estimate DSI has the advantage of being less subjective and less dependent on the experience of the evaluator it is expensive and not available to everybody. The traditional method using a subjective scale was as effective as the digital image to estimate DSI.