Litchi downy blight, caused by Peronophythora litchii ( P. litchii ), is a devastating disease to litchi fruit. The objective of this study was to investigate the effects against P. litchii of arachidin-2 (ACD2), a naturally occurring secondary metabolite from peanuts. ACD2 inhibited the mycelial growth and sporangia germination with scanning electron microscopy showing deformed mycelia and sporangia in-vitro . ACD2 also suppressed the expression of pathogenic genes. Metabolomic analysis exhibited that ACD2 treatment significantly reduced levels of 17 metabolites, including nicotinamide adenine dinucleotide (NAD) and adenosine triphosphate (ATP), while increasing cyclic adenosine monophosphate (cAMP) content. Molecular docking suggested that ACD2 binds strongly to alternative oxidase (AOX), inhibiting the enzyme. Moreover, pyruvate, an AOX activator, counteracted the ACD2's effects on ATP content, hydrogen peroxide level, mycelial growth and leaf lesion. ACD2 reduced disease severity and inhibited browning via the pathogen-induced litchi fruit in-vivo . These findings suggested that ACD2 is a promising biocontrol agent against litchi downy blight. © 2025 Elsevier Ltd.