| Summary: | Over the years, <i>Eucalyptus</i> plantations have rapidly expanded in Sarawak, Malaysia, accounting for 19% of the total forest plantation area. In a routine forest health surveillance conducted in 2022 at Sarawak, Malaysia, tree stands of <i>Eucalyptus urograndis</i> (<i>Eucalyptus grandis × Eucalyptus urophylla</i> hybrid) were detected with symptoms of stem canker disease caused by Chrysoporthe infection. Given the limited information on the chemical control of Chrysoporthe stem canker disease, there is a growing need to develop effective chemical control strategies to protect and cure Chrysoporthe infection on <i>E. urograndis</i> trees. Therefore, this study aimed to identify the causal pathogen of this stem canker disease in 7-year-old <i>E. urograndis</i> trees in Sarawak, Malaysia, and evaluate the efficacy of various fungicides as curative or protectant treatments on canker infection using artificial inoculation methods. Fungal isolates were first collected and subjected to molecular identification and pathogenicity analysis. Then, in vitro efficacy tests were evaluated using five licensed fungicides: thiram, prochloraz manganese chloride, copper hydroxide, dimethomorph, and mancozeb. Subsequently, the performance of these fungicides was assessed through preventive and curative field experiments on 10-year-old <i>E. urograndis</i> trees using the artificial inoculation technique. Based on the morphological and phylogenetic analysis of the <i>ITS1/ITS4</i>, <i>β-tubulin 2</i> (<i>BT2</i>), and the combined <i>ITS1/ITS4</i> and <i>BT2</i> sequences extracted from 20 fungal isolates, <i>Chrysoporthe deuterocubensis</i> was identified as the causal pathogen of the canker disease, with isolate CHRY18 recording the highest virulence. The in vitro efficacy tests showed that prochloraz manganese chloride achieved 100% inhibition against <i>C. deuterocubensis</i> at 1.0 mg/mL. In the preventive experiment, thiram significantly inhibited <i>C. deuterocubensis</i> infection, yielding the shortest lesion length (19.40 mm) compared to the non-treated control (47.48 mm) at 20 weeks post-inoculation. In the curative experiment, a significant reduction of 54.7% in lesion length was observed in inoculated symptomatic trees after 20 weeks of post-fungicide treatment with copper hydroxide. In conclusion, this study demonstrated prochloraz manganese chloride, thiram, and copper hydroxide as effective chemical controls of <i>C. deuterocubensis</i> stem canker on <i>E. urograndis</i>.
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