Enhanced Resistance to Leaf Fall Disease Caused by <i>Phytophthora palmivora</i> in Rubber Tree Seedling by <i>Sargassum polycystum</i> Extract

The brown seaweed (<i>Sargassum polycystum</i> C. Agardh-Sargassaceae) extract was examined for its bioelicitor properties in the rubber tree seedling (<i>Hevea brasiliensis</i> (Willd. ex A.Juss.) M&#252;ll.Arg. - Euphorbiaceae) and its application to reduce the leaf fall disease caused by <i>Phytophthora palmivora</i> (Edwin John) Butler, 1917 (Peronosporaceae). The major purpose of this study was to apply this seaweed extract (SWE) to improve the disease resistance in rubber tree seedling compared to a chemical fungicide (1% metalaxyl). After foliar spraying of SWE solution, two antioxidant enzymes, catalase (CAT) and peroxidase (POD) and systemic acquired resistance (SAR)-triggered enzyme, &#946;-1,3-glucanase (GLU), were analyzed. Both secondary metabolites, a phytoalexin scopoletin (Scp) and a signaling molecule salicylic acid (SA) were measured by high performance liquid chromatography (HPLC). Both SWE- and metalaxyl-treated plants had a close disease index (DI)-score which were 16.90 &#177; 1.93 and 15.54 &#177; 1.25, respectively, while the positive control sprayed with <i>P. palmivora</i> showed DI-score of 29.27 &#177; 1.89 which was much higher than those treated with SWE or fungicide. CAT, POD, and GLU were increased in rubber tree leaves treated with SWE solution. Furthermore, Scp and SA were significantly increased in SWE-treated leaves. Enhanced systemic acquired resistance induction, 2.09 folds of SA accumulation, was observed in the distal area comparing to the local area of SWE application. In conclusion, the positive effects of SWE elicitation from these studies revealed that SWE could be used as an alternative biocontrol agent for foliar spraying to enhance the defense responses in rubber tree seedling against <i>P. palmivora</i>.