BACKGROUND: Rice blast, caused by Pyricularia oryzae , is a devastating fungal disease threatening global rice production. Overreliance on chemical fungicides has raised environmental concerns and led to resistant strains, necessitating the development of sustainable alternatives. This study integrated marine microbiology and natural antifungal compounds to create eco‐friendly alternatives to chemical fungicides for disease management. RESULTS: We identified Pseudomonas aeruginosa R64 with broad‐spectrum antimicrobial activity from mangrove soil in the Mai Po Nature Reserve. The R64 fermentation extract (RFE) exhibited multifaceted inhibition of P. oryzae , suppressing mycelial growth, conidiation, conidial germination and appressorial formation, while disturbing cell wall and membrane function. It also attenuated virulence by impairing appressorial penetration and invasive growth. Further chemical analysis identified phenazines and quinolines as the primary compounds in RFE, corroborated by PCR detection of corresponding phenazine biosynthetic gene clusters. Comparative bioassays with two main bioactive components of RFE, phenazine‐1‐carboxamide (PCN) and phenazine‐1‐carboxylic acid (PCA), against P. oryzae implicated PCN as the principal antifungal effector. RFE and PCN had higher efficacy than tricyclazole in P. oryzae growth inhibition, but were less effective than isoprothiolane. Furthermore, RFE and PCN displayed lower acute ecotoxicity to an environmental indicator organism than isoprothiolane, suggesting their potential as sustainable biopesticides for rice blast management. CONCLUSION: Natural products from mangrove soil bacterium P. aeruginosa R64 inhibited key developmental and infection processes of P. oryzae , effectively reducing rice blast development. The promising disease inhibition and low ecotoxicity of mangrove‐associated bacteria highlight their untapped potential for innovative, eco‐friendly fungicide mining for sustainable agriculture. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Rice blast, caused by the filamentous fungus Pyricularia oryzae, has long been one of the major threats to almost all rice-growing areas worldwide. Metconazole, 5-(4-chlorobenzyl)-2, 2-dimethyl-1-(1H-1, 2, 4-triazol-1-ylmethyl) cyclopentanol, is a lipophilic, highly active triazole fungicide that has been applied in the control of various fungal pathogens of crops (cereals, barley, wheat), such as the Fusarium and Alternaria species. However, the antifungal activity of metconazole against P. oryzae is unknown. In this study, metconazole exhibited broad spectrum antifungal activities against seven P. oryzae strains collected from rice paddy fields and the wild type strain P131. Scanning electron microscopic analysis and fluorescein diacetate staining assays revealed that metconazole treatment damaged the cell wall integrity, cell membrane permeability and even cell viability of P. oryzae, resulting in deformed and shrunken hyphae. The supplementation of metconazole in vitro increased fungal sensitivity to different stresses, such as sodium dodecyl sulfate, congo red, sodium chloride, sorbitol and oxidative stress (H₂O₂). Metconazole could inhibit key virulence processes of P. oryzae, including conidial germination, germ tube elongation and appressorium formation. Furthermore, this chemical prevented P. oryzae from infecting barley epidermal cells by disturbing appressorium penetration and subsequent invasive hyphae development. Pathogenicity assays indicated a reduction of over 75% in the length of blast lesions in both barley and rice leaves when 10 μg/mL of metconazole was applied. This study provides evidence to understand the antifungal effects of metconazole against P. oryzae and demonstrates its potential in rice blast management.

BACKGROUND: Rice blast, caused by Pyricularia oryzae , poses a formidable threat to global rice production. Given that chemical fungicides, one of the important methods used to control rice blast, cause harm to the environment and human health, sustainable alternatives are required. This study aimed to synthesize silver nanoparticles (AgNPs) with antifungal activities against P. oryzae using mangrove soil bacteria, and to evaluate their ecotoxicity. RESULTS: In this study, AgNPs were synthesized using fermentation extracts from Pseudomonas aeruginosa R64. The resulting nanoparticles (RFE‐AgNPs), ranging in size from 4.516 to 37.031 nm, were characterized by UV–visible spectroscopy and high‐resolution transmission electron microscopy equipped with energy dispersive spectroscopy. RFE‐AgNPs at 20 μg mL ⁻¹ had strong antifungal activity against P. oryzae , inhibiting colony growth by 77%. Moreover, RFE‐AgNPs significantly hindered conidial germination, germ tube elongation and appressorium formation. In addition, microscopic observation revealed that RFE‐AgNPs treatment disrupted mycelial morphology, lipid and glycogen metabolism and the cAMP signaling pathway of P. oryzae , limiting its infection and expansion in host tissues. A pathogenicity test confirmed that RFE‐AgNPs significantly reduced disease development in barley and rice leaves. Importantly, RFE‐AgNPs displayed significantly lower toxicity to Artemia salina compared with isoprothiolane and commercial AgNPs. CONCLUSION: RFE‐AgNPs synthesized using mangrove soil bacterial natural products displayed good control efficacy on rice blast but low ecotoxicity on A. salina and had great potential to be a promising candidate for eco‐friendly biopesticides to manage rice blast disease. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.