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Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae severely damages rice production, particularly under climate change conditions. This study detected bioactive compounds from leaf extracts of Kalanchoe pinnata (Lam.) Pers. and tested their disease-reducing effects against BLB. Methanol leaf extracts of K. pinnata were prepared by liquid-liquid extraction combined with ultrasonication. Using gas chromatography–mass spectrometry, 27 compounds were identified in the extracts. Based on their abundances and reported involvements in plant defense responses against pathogens, three compounds were selected for further tests, i.e., 5-oxotetrahydrofuran-2,3-dicarboxylic acid, dimethyl ester; ethyl linolenate and 1,3-benzenedicarboxylic acid, bis(2-ethylhexyl) ester. To have adequate amounts of these compounds for subsequent bioassays under net house conditions, their commercially available analogs were employed, i.e., dimethyl furan-2,5-dicarboxylate (FDME), water-soluble linoleic acid (LinA) and bis(2-ethylhexyl) phthalate (DEHP). Rice seeds were soaked in six different concentrations of each compound (3, 5, 10, 15, 20, and 25 ppm) for 24 h before sowing. Seed soaking with either FDME or DEHP significantly reduced BLB lesion lengths after pathogen inoculation. The effects of FDME were maintained until 21 DAI. Moreover, seed soaking using a mixture of FDME and DEHP provided similar protection as that of the chemical control using Starner 20WP.
