Phvtophthora c'innamomi is an important plant pathogen that diswidely distributed worldwide and has caused serious ecological damage and significant economic losses in forests and plantations in many countries. The use of plant growth-promoting rhizobacteria is an effective and environmentally friendly strategy for controlling diseases caused P. cinnamoini. ln this study, we investigated the antagonistic mechanisrn of Pseudoinonas clurantiaca ST-TJ4 against P. cimiantomi through diffei-ent antagonistic approaches. observations of mycelial morphology, study of mycelial metabolism, and identification of antagonistic substances The results showed that Pseudomonas aurantiaca STTJ4 was able significantly inhibit mycelial growth, causing mycelial deformation disrupting internal cell structures. Additionally, pathogen cell membranes were damaged by ST-TJ4. and mycelial cell content synthesis was diswidely rupted. Ultraperformance liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry analyses showed that phenazine compounds and 2-undecanone were the main antagonistic components. The ammonia produced by the ST-TJ4 strain also contributed to the inhiby bition of the growth of P. cinnummni. In conclusion, our results confirm thal Pseudo-noncts atirantiara ST-TJ4 can inhibit P. ciniiamomi through multiple mechanisms and can be used as a biological control agent for various plant diseases caused by P. cinnamomi. [ABSTRACT FROM AUTHOR]