Abstract
Pythium root rot (Pythium spp.) is a common disease of greenhouse-grown poinsettias (Euphorbia pulcherrima) that can cause serious plant loss or reduction in plant quality. Application of effective chemical fungicides to poinsettia plants has reduced losses due to Pythium; however, development of resistance to these fungicides is a legitimate concern, as well as the environmental implications of using chemical pesticides. In this study, a group of products of biological origin and known biocontrol agents were evaluated for their efficacy to control pythium root rot of poinsettia. These products and organisms were compared to metalaxyl (Ridomil), a fungicide commonly used to reduce losses to Pythium. The results showed that two products based on two different species of Streptomyces, Mycostop and Actino-Iron, were as effective as metalaxyl at reducing the symptoms associated with pythium root rot when artificially inoculated with Pythium ultimum var. ultimum compared to the control plants. Many roots remained functional throughout the duration of the experiments and the overall appearance and number of bracts of commercial quality of the plants were similar for the three treatments mentioned above. In an additional experiment, Mycostop was tested in combination with a single application of metalaxyl either at 3, 7, or 11 weeks after transplanting. Plants inoculated with P. ultimum var. ultimum and treated with metalaxyl either on week 3 or 7 after transplanting in combination with two applications of Mycostop, had greater fresh root weight than those only treated with metalaxyl at week 11 or the chemical control (three applications of metalaxyl). However, there was no significant difference in the number of bracts or the bract diameter between plants treated with metalaxyl at weeks 3 or 7 followed by Mycostop and those plants treated with the fungicide alone. A reduction in the amount of fungicide used to control pythium root rot can be achieved when used in combination with a biocontrol agent without compromising the health of poinsettias.
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